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A pore-scale investigation of microplastics migration and deposition during unsaturated flow in porous media
Abstract
Microplastics are ubiquitous in the natural environment and have the potential to endanger the natural environment, ecology and even human health. A series of microfluidic experiments by using soft lithography technology were carried out to investigate the effect of flow rate, particle volume fraction, particle size and pore/throat ratio on microplastics migration and deposition at the pore scale. We discovered a range of deposition patterns of the spherical microplastics from no particle deposition, to discontinuous particle layer, and to continuous particle layers in the retained liquid in the pores, depending on the particle size and volume fraction. Several metrics, including air saturation, probability of particle detainment, expansion ratio and thickness of residual liquid, were quantified to examine the role of various parameters on particle migration and retention of microplastics. At low flow rates (Q = 0.05 μL/min), microplastics migration and deposition were sensitive to changes in particle volume fraction, particle size and pore/throat ratio. In contrast, at high flow rates (Q > 5 μL/min), the migration and retention of particles were mainly controlled by strongly channelized air invasion pattern, while the particle volume fraction, particle size and pore/throat size have only secondary influence. At intermediate range of flow rates, microplastics migration and deposition were dramatically impacted by flow rate, particle volume fraction, particle size and pore/throat ratio. This work improves the understanding of the mechanisms of particle migration and retention in porous media and can provide a reference for more accurate assessment of the exposure levels and times of microplastics in soil and groundwater systems.
... Immediately following injection, no n-Fe 0 particle is attached to the surface of the mineral grains within the aquifer, and the particles are able to move within the aquifer, through the pores [192], when the pore throat diameter is less than the n-/Fe 0 particle diameter ( Figure A15). Obviously, following the initial high velocity injection, the particle velocity will slow, and as it slows, there will be a tendency for the entrained particles to settle and become trapped within the pores. ...
... When H 2 gas generation occurs, the gas bubbles aggregate [101], reduce the particle density [101], and can block pore throats [192], thereby reducing the permeability. n-Fe 0 particles will tend to adhere to the exterior of the gas bubble [101,192]. Experiments [142] using air have demonstrated that the air bubbles can close (block) the pore throats in coarse quartz sand and clays ( Figure A16). Immediately following injection, no n-Fe 0 particle is attached to the surface of the mineral grains within the aquifer, and the particles are able to move within the aquifer, through the pores [192], when the pore throat diameter is less than the n-/Fe 0 particle diameter ( Figure A15). ...
... Experiments [142] using air have demonstrated that the air bubbles can close (block) the pore throats in coarse quartz sand and clays ( Figure A16). Immediately following injection, no n-Fe 0 particle is attached to the surface of the mineral grains within the aquifer, and the particles are able to move within the aquifer, through the pores [192], when the pore throat diameter is less than the n-/Fe 0 particle diameter ( Figure A15). Obviously, following the initial high velocity injection, the particle velocity will slow, and as it slows, there will be a tendency for the entrained particles to settle and become trapped within the pores. ...
Polluted aquifers can be decontaminated using either ZVI (zero valent iron) permeable reactive barriers (PRB) or injected ZVI. The placement of ZVI within the aquifer may take several decades to remediate the contaminant plume. Remediation is further complicated by ZVI acting as an adsorbent to remove some pollutants, while for other pollutants, it acts as a remediation catalyst. This study investigates an alternative aquifer decontamination approach to PRB construction or n-Fe⁰ injection. The alternative approach reconstructs the potentiometric surface of the aquifer containing the contaminant. This reconstruction confines the contaminant plume to a stationary, doughnut shaped hydrodynamic mound. Contaminated water from the mound is abstracted, decontaminated, and then reinjected, until all the water confined within the mound is decontaminated. At this point, the decontaminated mound is allowed to dissipate into the surrounding aquifer. This approach is evaluated for potential use in treating the following: (i) immiscible liquid plumes; (ii) miscible contaminant and ionic solute plumes; (iii) naturally contaminated aquifers and soils; and (iv) contaminated or salinized soils. The results indicate that this approach, when compared with the PRB or injection approach, may accelerate the decontamination, while reducing the overall amount of ZVI required.
... For such size fractions, understanding the behavior of plastic particulate transport in geologic porous media under two phase immiscible flow presents considerable challenges, as MPs/NPs retention and release within individual pores is strongly coupled with the given hydro-chemical conditions met therein. These complexities are pertained to the relative strength of adhesive forces and torque resistance, as well as the hydrodynamic and torques applied (Bergendahl and Grasso, 2000;Shen et al., 2007;Molnar et al., 2011;Dong et al., 2018;Liu et al., 2020;Koutnik et al., 2022;Liang et al., 2022;He et al., 2023). Among these, hydrodynamic forces emerge as a dominant factor promoting fines mobility, which can be related to increasing seepage velocity at the pore scale (Bradford et al., 2011;Zhao et al., 2019;. ...
... Geometries with smaller pore aspect ratios, such as Geo-A (illustrated in Fig. 3), exhibit higher initial retention values. The observation that smaller pore to throat ratios enhance clustering and sieving during breakthrough retention is supported by He et al. (2023), who showed that retention mechanisms in microfluidic experiments are strongly influenced by pore-throat geometry, with smaller ratios increasing particle retention. By the end of the saturation phase, geometries with smaller initial throat sizes and lower pore connectivity, such as Geo-C, retain a greater proportion of MPs, around 25 %, largely due to accumulation in low-connectivity pores. ...
... While the transport and retention of particles in porous media are generally controlled by the relative size of the pores and particles (Han et al., 2020), multiphase flow conditions introduce additional complexities due to the presence of liquid-liquid or air-liquid interfaces (Han et al., 2020). During two-phase flow, particles can be retained in porous media through a variety of mechanisms, such as attachment at interfaces (Zevi et al., 2005), film straining (He et al., 2023;Wan & Tokunaga, 1997;Wu et al., 2021), bridging (Lin et al., 2021a), and filtration (Auset & Keller, 2006;Johnson & Hilpert, 2013). Furthermore, the presence of an immiscible fluid can induce particle agglomeration via adhesive forces, facilitating pore clogging (R. Zhang et al., 2023). ...
Plain Language Summary
Particle transport during tow‐phase flow is frequently involved in various natural processes and engineering applications, ranging from water infiltration to oil recovery. Therefore, understanding particle transport behaviors and their impacts on two‐phase flow in porous media is essential. Here, by using confocal microscopy we directly visualize particle transport and aggregation at the pore scale under various particle sizes, grain sizes, and flow rates. The results show that particles tend to aggregate at the wetting‐nonwetting interfaces or the solid‐wetting‐nonwetting triple point at high flow rates and smaller pore sizes. Furthermore, we observe that particle aggregation in the narrow regions of the wetting phase can facilitate the breakup of liquid clusters into smaller ones, leading to a dramatic increase in the total number of wetting liquid clusters. This indicates that particle aggregation in porous media can reduce the wetting phase connectivity by inducing liquid breakup, which subsequently reduces the relative permeability of fluids. These findings improve our understanding of the coupled processes of two‐phase flow and particle transport and thus have broad implications for both geological and engineering systems.
... Chen et al., 2020). The visualization of the microplastics is done based on their size and configuration (He et al., 2023). Supposedly in the case of large microplastics, visual inspection is done whereas for smaller microplastics dissection microscope is preferably used. ...
The increasing demand for plastic in our daily lives has led to global plastic pollution. The improper disposal of plastic has resulted in a massive amount of atmospheric microplastics (MPs), which has further resulted in the production of atmospheric nanoplastics (NPs). Because of its intimate relationship with the environment and human health, microplastic and nanoplastic contamination is becoming a problem. Because microplastics and nanoplastics are microscopic and light, they may penetrate deep into the human lungs. Despite several studies demonstrating the abundance of microplastics and nanoplastics in the air, the potential risks of atmospheric microplastics and nanoplastics remain unknown. Because of its small size, atmospheric nanoplastic characterization has presented significant challenges. This paper describes sampling and characterization procedures for atmospheric microplastics and nanoplastics. This study also examines the numerous harmful effects of plastic particles on human health and other species. There is a significant void in research on the toxicity of airborne microplastics and nanoplastics upon inhalation, which has significant toxicological potential in the future. Further study is needed to determine the influence of microplastic and nanoplastic on pulmonary diseases.
... Zhang, Manga, et al., 2022), hydraulic fracturing (Baldini et al., 2018;Jalali et al., 2018;Parisio & Yoshioka, 2020;Salimzadeh et al., 2020;Tong & Mohanty, 2016), borehole drilling (Elkatatny et al., 2020;She et al., 2020;M. B. Wang, Guo, & Chen, 2020), particulate contamination (Flury & Aramrak, 2017;He et al., 2023;Wu et al., 2021), and enhanced oil recovery (Aben et al., 2017;Baldini et al., 2018;Jasinski & Dabrowski, 2018;Liang et al., 2016;Woodworth & Miskimins, 2007). Fluids and particles can mix to form suspensions, and the flow dynamic of suspension in rock fracture is complex. ...
Plain Language Summary
Suspended particle flow and clogging in rock fractures are involved in many subsurface engineering applications and natural processes. Water‐wet particles dispersed in oil are cohesive and tend to agglomerate, clogging flow channels of crude oil to resist further recovery. Despite being common and important, the effect of liquid cohesion on particle clogging has been overlooked in previous studies. Here, we conduct visualized experiments of dilute suspension flow in a rough fracture. We, for the first time, find a dramatic enhancement of clogging by a tiny amount of additional immiscible wetting liquid, even at a weight percentage as low as 0.1%–0.5%. We obtain an experimental phase diagram of clogging patterns in the space of wetting liquid content and flow rate. The transitions of clogging regimes are explained through the combined control of clogging behavior by the suspension composition and hydrodynamic condition. We propose a theoretical model of agglomerate size to quantify the effect of capillary cohesion induced by the additional wetting liquid. The experimental and theoretical results improve our understanding of suspension flow and clogging behaviors and pave the way for the possibility of controlling particle transport and clogging processes in various applications by adjusting multiphase liquid composition and flowrate.
... Colloidal particles, e.g., mineral precipitates, pathogenic bacteria and engineered nanoparticles (ENPs), exist widely in natural environment [1][2][3]. During the last decade, the environmental behaviors of the ENPs, e.g., microplastics, carbon nanotubes, and nano-zero-valent iron (nZVI) have been studied in detail [4][5][6][7]. Colloid transport in porous media is a complex process exhibiting multi-scale characteristics from interfacial scale to pore scale, and to Darcy scale and field scale [8]. To predict colloid transport, stochastic or statistical models [9][10][11][12], colloid filtration theory (CFT) and the corresponding upscaling procedures [13,14] have been developed. ...
Microplastics (MPs) have become pervasive pollutants in terrestrial ecosystems, raising significant ecological risks and human health concerns. Despite growing attention, a comprehensive understanding of their quantification, sources, emissions, transport, degradation, and accumulation in soils remains incomplete. This review synthesizes the current knowledge on the anthropogenic activities contributing to soil MP contamination, both intentional and unintentional behaviors, spanning sectors including agriculture, domestic activities, transportation, construction, and industry. Furthermore, it examines the spatial distribution, accumulation, and abundance of MPs across various land use types, alongside a critical assessment of existing quantification methodologies. While the predominant metric for MP quantification is particle number concentration, integrating mass and area concentration enhances the ability to compare pollution levels, assess fluxes, and conduct risk analyses. Additionally, the review explores the transport behavior of MPs in soil, distinguishing between external mechanisms (abiotic factors: wind, leaching, and runoff, biotic factors: soil bioturbation and food chain interactions), and internal mechanisms that are impacted by the characteristics of MPs themselves (e.g., shape, color, size, density, surface properties), soil properties (e.g., porosity, pH, ionic strength, organic matter and mineral content), coexisting substances, and soil structural dynamics. The study of MP transport in soil remains in its early stages, with substantial gaps in knowledge. Future research should focus on integrating number, mass concentration, and area concentration for the more holistic quantification of MP abundance, and prioritize the development of more accurate and efficient methodologies. In addition, the investigation of MP transport and degradation processes under varying environmental conditions and soil management practices is critical for addressing this emerging environmental challenge.
Urban stormwater is both a major source and a mode of transport for microplastics in the environment. Black-box field studies have found that bioretention cells, a type of low impact development (LID), consisting mainly of engineered porous media, are effective systems for capturing microplastics. However, the mechanisms of how microplastics are transported, removed from stormwater, and fragmented within bioretention cells are unclear. Additionally, the impacts of microplastics on the hydrology and microplastic removal performance of bioretention cells remain unclear. This study assesses tools to model microplastic removal using LID and reviews the literature on the mechanisms of microplastic filtration in porous media. None of the evaluated stormwater tools were found to be well-suited to model microplastic removal via bioretention. We identified 74 studies that, at times, misinterpreted “all microplastics” as colloids. We recommend using a combination of two models to evaluate the full spectrum of microplastic sizes. Currently, the best-suited models are HYDRUS and the cake-layer model described in Li and Davis (2008a), which can be adapted for this purpose. More column studies are needed to parametrize these models that use the full range of polymer types and morphologies of urban stormwater microplastics.
Infiltration and retention of microplastics in porous media are important for understanding their fate in environments and formulating treatment measures. Given porous media opacity, knowledge is usually obtained indirectly by monitoring microplastic concentration in the effluent and measuring microplastic distribution after removing grains in layers. In this study, real-time visualization of infiltration and retention of microplastics in porous media under vertical water flow is performed using an improved reflective index matching method, considering the different shapes and densities of microplastics and size ratios between microplastics and grains. The spherical microplastics have the largest infiltration depths, with trajectories closest to vertical and accompanied by long acceleration durations and low deceleration frequencies. The cylindrical microplastics deviate from vertical and have stronger transverse oscillations and more frequent decelerations, while the flaky microplastics have the most significant transverse displacements. The infiltration depth can be improved by reducing the size ratio between microplastics and grains and increasing the vertical flow rate, while the density of microplastics has a relatively limited effect. Sliding and rotating of microplastics after collision with grains are observed, responsible for deceleration and transverse displacements. Different retention patterns are found, with the number of types being inversely proportional to the number of principal dimensions of the shape.
Microplastics in urban runoff undergo rapid fragmentation and accumulate in the soil, potentially endangering shallow groundwater. To improve the understanding of microplastic transport in groundwater, column experiments were performed to compare the transport behavior of fragmented microplastics (FMPs ∼1‐µm diameter) and spherical microplastics (SMPs ∼1‐, 10‐, and 20‐µm diameter) in natural gravel (medium and fine) and quartz sand (coarse and medium). Polystyrene microspheres were physically abraded with glass beads to mimic the rapid fragmentation process. The experiments were conducted at a constant flow rate of 1.50 m day⁻¹ by injecting two pore volumes of SMPs and FMPs. Key findings indicate that SMPs showed higher breakthrough, compared to FMPs in natural gravel, possibly due to size exclusion of the larger SMPs. Interestingly, FMPs exhibited higher breakthrough in quartz sand, likely due to tumbling and their tendency to align with flow paths, while both sizes (larger and smaller relative to FMPs) of SMPs exhibited higher removal in quartz sand. Therefore, an effect due to shape and size was observed.
Management aquifer recharge (MAR) technology is widely applied to solve seawater intrusion caused by groundwater overexploitation in coastal areas. However, MAR creates an important pathway for microplastics (particle size< 5 mm) to enter groundwater. To explore the clogging potential of microplastics in aquifer media, a series of laboratory-scale column experiments were conducted in this study. The hydraulic conductivity of porous media and deposition amount of microplastics were investigated under different experimental conditions. In our study, most of the microplastics were intercepted in the sand column’s surface layer. The difference of particle size in porous media greatly influence the clogging development. The hydraulic conductivity of the aquifer media decreased as the microplastic particle size decreased. When the particle size of microplastic was larger than 300 mm, most of the microplastics deposits on the surface of the porous media, forming a “microplastic accumulation layer”. Microplastics are affected by particle size, flow shear stress and preferential flow during migration. The migration ability of microplastics increased significantly with the increase of hydraulic head difference and decreased with the increase of sand column depth. The bacteria microorganisms are projected to be a new biological control strategy in conjunction with MAR. The study of clogging risk of microplastics particles in porous media during artificial recharge provides novel and unique insights for the management and control of microplastic pollution in groundwater systems.
Soil and groundwater contamination has always been a global concern. Contaminants are migrated and transformed in the soil and groundwater environments, which in turn pose potential environmental risks to humans. This paper describes four typical contaminants, including heavy metals, polycyclic aromatic hydrocarbons, microplastics, and perfluorinated and polyfluoroalkyl substances. Based on a systematic summary of the sources, hazards, and migration behaviors of these four contaminants, various existing remediation methods are analyzed, and the advantages and disadvantages of different methods are discussed. Finally, the future research prospects of soil and groundwater remediation are described, and the significance of the study of contaminant migration and remediation in the subsurface environment is emphasized. This research can help to provide theoretical and technical support for the study of contaminant migration and removal in soil and groundwater environments, and further improve the removal rate in actual contaminant sites.
Migration of microplastics in porous media is an important, yet overlooked phenomenon because most microplastic research has focused mainly on microplastic behavior in aquatic environments. Here we review experimental advances of microplastic migration in porous media, with emphasis on factors influencing microplastic migration. We observed that microplastic migration is influenced by environmental factors and microplastic properties. The effect of microplastic surface charge and functional groups, and of soil organisms on microplastic migration is unclear. Research at the macro-scale, higher than 1 m, predominantly starts with field sampling, and then carries out measurements or mathematical modeling to explore migration patterns. At the meso-scale, below 1 cm, studies often employ filled sand columns as proxies for porous media to generate breakthrough curves and retention profiles. At the micro-scale, below 1 mm, visualization of microplastic migration in pores is done by lab-on-a-chip devices to build transparent micromodels. Current research predominantly relies on industrially produced regular spherical microplastics, with limited focus on macro- and micro-scale studies.
Groundwater is the primary source of water that occurs below the earth's surface. However, the advancement in technology and the increasing population, which lead to the discharge of contaminants such as microplastics (MPs), have an adverse impact on the quality of groundwater. MPs are ubiquitous pollutants that are widely found throughout the world. The maximum abundance of MPs is 4 items/L and 15.2 items/L in groundwater at the specific location of China and USA. Various factors can affect the migration of MPs from soil to groundwater. The occurrence of MPs in water causes serious health issues. Therefore, taking appropriate strategies to control MP contamination in groundwater is urgent and important. This review summarizes the current literature on the migration process of MPs from soil to groundwater along with possible methods for the remediation of MP-polluted groundwater. The main objective of the review is to summarize the technical parameters, process, mechanism, and characteristics of various remediation methods and to analyze strategies for controlling MP pollution in groundwater, providing a reference for future research. Possible control strategies for MP pollution in groundwater include two aspects: i) prevention of MPs from entering groundwater; ii) remediation of polluted groundwater with MPs (ectopic remediation and in-situ remediation). Formulating legislative measures, strengthening public awareness and producing more environment-friendly alternatives can be helpful to reduce the production of MPs from the source. Manage plastic waste reasonably is also a good strategy and the most important part of the management is recycling. The shortcomings of the current study and the direction of future research are also highlighted in the review.
Dip coating consists of withdrawing a substrate from a bath to coat it with a thin liquid layer. This process is well understood for homogeneous fluids, but heterogeneities, such as particles dispersed in liquid, lead to more complex situations. Indeed, particles introduce a new length scale, their size, in addition to the thickness of the coating film. Recent studies have shown that, at first order, the thickness of the coating film for monodisperse particles can be captured by an effective capillary number based on the viscosity of the suspension, providing that the film is thicker than the particle diameter. However, suspensions involved in most practical applications are polydisperse, characterized by a wide range of particle sizes, introducing additional length scales. In this study, we investigate the dip coating of suspensions having a bimodal size distribution of particles. We show that the effective viscosity approach is still valid in the regime where the coating film is thicker than the diameter of the largest particles, although bidisperse suspensions are less viscous than monodisperse suspensions of the same solid fraction. We also characterize the intermediate regime that consists of a heterogeneous coating layer and where the composition of the film is different from the composition of the bath. A model to predict the probability of entraining the particles in the liquid film depending on their sizes is proposed and captures our measurements. In this regime, corresponding to a specific range of withdrawal velocities, capillarity filters the large particles out of the film.
The withdrawal of a liquid or the translation of a liquid slug in a capillary tube leads to the deposition of a thin film on the inner wall. When particles or contaminants are present in the liquid, they deposit and contaminate the tube if the liquid film is sufficiently thick. In this article, we experimentally investigate the condition under which particles are deposited during the air invasion in a capillary tube initially filled with a dilute suspension. We show that the entrainment of particles in the film is controlled by the ratio of the particle and the tube radii and the capillary number associated with the front velocity. We also develop a model which suggests optimal operating conditions to avoid contamination during the withdrawal of a suspension from a tube. This deposition mechanism can also be leveraged in coating processes by controlling the deposition of particles on the inner walls of channels.
Plain Language Summary
Fluid invasion into porous media saturated with another more viscous, immiscible fluid exhibits various displacement patterns. The patterns are controlled by the competition between capillary and viscous forces and significantly affect oil recovery and CO2 trapping efficiency. Although invasion patterns have been studied intensively, the dynamic effect of viscous force on pattern transitions is neglected. The crossovers from compact displacement to capillary fingering are not well understood. To explore pattern transitions affected by wettability and flow rate, we perform theoretical analysis of flow behavior at the pore scale, and then establish a model to describe pattern transitions as functions of contact angle and capillary number. We rigorously consider the dynamic effect of viscous force and find that the critical contact angles for the crossover from compact displacement to capillary fingering decrease with capillary number. The model also well describes the crossover from capillary to viscous fingering. We evaluate the phase diagram using pore‐scale simulations, and microfluidic experiments performed in this and previous studies, justifying that the diagram can well capture the transitions under various wetting and flow‐rate conditions. This work extends the classic phase diagram and is also of practical significance in predicting displacement patterns in subsurface applications.
Printing inks typically consist of a functional component dispersed within a low-viscosity resin/solvent system where interparticle interactions would be expected to play a significant role in dispersion, especially for the high-aspect-ratio nanocarbons such as the graphite nanoplatelets (GNPs). Rheology has been suggested as a method for assessing the dispersion of carbon nanomaterials in a fluid. The effects of phase volume of ammonia plasma-functionalized GNPs on a near-Newtonian low-viscosity thermoplastic polyurethane (TPU) resin system have been studied using shear and quiescent oscillatory rheology. At low concentrations, the GNPs were well dispersed with a similar shear profile and viscoelastic behavior to the unfilled TPU resin, as viscous behavior prevailed indicating the absence of any long-range order within the fluid. Particle interactions increased rapidly as the phase volume tended toward maximum packing fraction, producing rapid increases in the relative viscosity, increased low shear rate shear thinning, and the elastic response becoming increasingly frequency independent. The nanoscale dimensions and high-aspect-ratio GNPs occupied a large volume within the flow, while small interparticle distances caused rapid increases in the particle–particle interactions to form flocculates that pack less effectively. Established rheological models were fitted to the experimental data to model the effect of high-aspect-ratio nanocarbon on the viscosity of a low-viscosity system. Using the intrinsic viscosity and the maximum packing fraction as fitting parameters, the Krieger–Dougherty (K–D) model provided the best fit with values. There was good agreement between the estimates of aspect ratio from the SEM images and the predictions of the aspect ratio from the rheological models. The fitting of the K–D model to measured viscosities at various phase volumes could be an effective method in characterizing the shape and dispersion of high-aspect-ratio nanocarbons.
An object withdrawn from a liquid bath is coated with a thin layer of liquid. Along with the liquid, impurities such as particles present in the bath can be transferred to the withdrawn substrate. Entrained particles locally modify the thickness of the film, hence altering the quality and properties of the coating. In this study, we show that it is possible to entrain the liquid alone and avoid contamination of the substrate, at sufficiently low withdrawal velocity in diluted suspensions. Using a model system consisting of a plate exiting a liquid bath, we observe that particles can remain trapped in the meniscus which exerts a resistive capillary force to the entrainment. We characterize different entrainment regimes as the withdrawal velocity increases: from a pure liquid film, to a liquid film containing clusters of particles, and eventually individual particles. This capillary filtration is an effective barrier against the contamination of substrates withdrawn from a polluted bath and finds application against biocontamination.
The coating of a plate withdrawn from a bath of a suspension of non-Brownian, monodisperse and neutrally buoyant spherical particles suspended in a Newtonian liquid has been studied. Using laser profilometry, particle tracking and local sample weighing we have quantified the thickness h and the particle content of the film for various particle diameters d and volume fractions (0.10 φ 0.50). Three coating regimes have been observed as the withdrawal velocity is increased: (i) no particle entrainment (h d), (ii) a monolayer of particles (h ∼ d), and (iii) a thick film (h d), where the suspension behaves as an effective viscous fluid following the Landau-Levich-Derjaguin law. We discuss the boundaries between these regimes, as well as the evolution of the liquid and solid content of the coating over the whole range of withdrawal capillary number and volume fractions.
Managed aquifer recharge is an effective strategy for urban stormwater management. Chemical ions are normally retained in stormwater and groundwater and may accelerate clogging during the recharge process. However, the effect of water chemistry on physical clogging has not previously been investigated. In this study, we investigated the hydrogeochemical mechanism of saturated porous media clogging in a series of column experiments. The column was packed with river sand and added suspensions of kaolinite particles. Calcium chloride and sodium chloride are used as representative ions to study chemical effects. We found that an increase in ionic strength resulted in retention of kaolinite solids in the column, with a breakthrough peak of C/C0 value of 1 to 0.2. The corresponding hydraulic conductivity decreased with increased solids clogging. Divalent cations were also found to have a greater influence on kaolinite particle clogging than monovalent cations. The enhanced hydrochemical‐related clogging was caused by kaolinite solids flocculating and increasing the deposition rate coefficient by 1‐2 times in high ionic strength conditions. Three clogging mechanisms of kaolinite solids are proposed: surface filtration, inner blocking, and attachment. This study further deepens the understanding of the mechanisms of solids clogging during aquifer recharge and demonstrates the significance of ionic strength on recharge clogging risk assessments.
Macro-plastic pollution is found in terrestrial and marine environments and is degraded to micro-particles (MP) and nano-particles (NP) of plastic. These can enter the human food chain either by inhalation or by ingestion, particularly of shellfish and crustaceans. Absorption across the gastrointestinal tract is relatively low, especially for MPs, which appear to have little toxicity. However, NPs are more readily absorbed and may accumulate in the brain, liver and other tissues in aquatic species and other animals. Studies using nanoparticles of other materials suggest that toxicity could potentially affect the central nervous system and the reproductive system, although this would be unlikely unless exposure levels were very high and absorption was increased by physiological factors.
Pearling instabilities of slender viscoelastic threads have received much attention, but remain incompletely understood. We study the instabilities in polymer solutions subject to uniaxial elongational flow. Two distinctly different instabilites are observed: beads on a string and blistering. The beads-on-a-string structure arises from a capillary instability whereas the blistering instability has a different origin: it is due to a coupling between stress and polymer concentration. By varying the temperature to change the solution properties we elucidate the interplay between flow and phase separation.
When confined in a liquid-filled circular cylinder, a long air bubble moves slightly faster than the bulk liquid as a small fraction of the liquid leaks through a very thin annular gap between the bubble and the internal wall of the cylinder. At low velocities, the thickness of this lubricating film formed around the bubble is set only by the liquid properties and the translational speed of the bubble and thus can be tuned in a simple fashion. Here, we use this setting to filter, based on size, micron-size particles that are originally dispersed in a suspension. Furthermore, we apply this process for separation of particles from a polydisperse solution. The bubble interface is free of particles initially, and particles of different sizes can enter the liquid film region. Particle separation occurs when the thickness of the lubricating liquid film falls between the diameters of the two different particles. While large particles will be collected at the bubble surface, smaller particles can leak through the thin film and reach the fluid region behind the bubble. As a result, the film thickness can be fine-tuned by simply adjusting the speed of a translating confined bubble, so as to achieve separation of particles by size based on the relative particle diameter compared to the film thickness.
The contamination of the environment with microplastic, defined as particles smaller than 5 mm, has emerged as a global challenge because it may pose risks to biota and public health. Current research focuses predominantly on aquatic systems, whereas comparatively little is known regarding the sources, pathways, and possible accumulation of plastic particles in terrestrial ecosystems. We investigated the potential of organic fertilizers from biowaste fermentation and composting as an entry path for microplastic particles into the environment. Particles were classified by size and identified by attenuated total reflection-Fourier transform infrared spectroscopy. All fertilizer samples from plants converting biowaste contained plastic particles, but amounts differed significantly with substrate pretreatment, plant, and waste (for example, household versus commerce) type. In contrast, digestates from agricultural energy crop digesters tested for comparison contained only isolated particles, if any. Among the most abundant synthetic polymers observed were those used for common consumer products. Our results indicate that depending on pretreatment, organic fertilizers from biowaste fermentation and composting, as applied in agriculture and gardening worldwide, are a neglected source of microplastic in the environment.
Plastic debris is an environmentally persistent and complex contaminant of increasing concern. Understanding the sources, abundance and composition of microplastics present in the environment is a huge challenge due to the fact that hundreds of millions of tonnes of plastic material is manufactured for societal use annually, some of which is released to the environment. The majority of microplastics research to date has focussed on the marine environment. Although freshwater and terrestrial environments are recognised as origins and transport pathways of plastics to the oceans, there is still a comparative lack of knowledge about these environmental compartments. It is highly likely that microplastics will accumulate within continental environments, especially in areas of high anthropogenic influence such as agricultural or urban areas. This review critically evaluates the current literature on the presence, behaviour and fate of microplastics in freshwater and terrestrial environments and, where appropriate, also draws on relevant studies from other fields including nanotechnology, agriculture and waste management. Furthermore, we evaluate the relevant biological and chemical information from the substantial body of marine microplastic literature, determining the applicability and comparability of this data to freshwater and terrestrial systems. With the evidence presented, the authors have set out the current state of the knowledge, and identified the key gaps. These include the volume and composition of microplastics entering the environment, behaviour and fate of microplastics under a variety of environmental conditions and how characteristics of microplastics influence their toxicity. Given the technical challenges surrounding microplastics research, it is especially important that future studies develop standardised techniques to allow for comparability of data. The identification of these research needs will help inform the design of future studies, to determine both the extent and potential ecological impacts of microplastic pollution in freshwater and terrestrial environments.
When one liquid is introduced into another immiscible one, it ultimately fragments due to hydrodynamic instability. In contrast to neck pinch-off without external actuation, the viscous two-fluid system subjected to an oscillatory flow demonstrates higher efficiency in breaking fluid threads. However, the underlying dynamics of this process is less well understood. Here we show that the neck-thinning rate is accelerated by the amplitude of oscillation. By simply evaluating the momentum transfer from external actuation, we derive a dimensionless pre-factor to quantify the accelerated pinch-off. Our data ascribes the acceleration to the non-negligible inner fluid inertia, which neutralizes the inner phase viscous stress that retards the pinch-off. Moreover, we characterize an equivalent neck-thinning behavior between an actuated system and its unactuated counterpart with decreased viscosity ratio. Finally, we demonstrate that oscillation is capable of modulating satellite droplet formation by shifting the pinch-off location. Our study would be useful for manipulating fluids at microscale by external forcing.
We report here a review of particle-laden interfaces. We discuss the importance of the particle’s wettability, accounted for by the definition of a contact angle, on the attachment of particles to the fluid interface and how the contact angle is strongly affected by several physicochemical parameters. The different mechanisms of interfacial assembly are also addressed, being the adsorption and spreading the most widely used processes leading to the well-known adsorbed and spread layers, respectively. The different steps involved in the adsorption of the particles and the particle-surfactant mixtures from bulk to the interface are also discussed. We also include here the different equations of state provided so far to explain the interfacial behavior of the nanoparticles. Finally, we discuss the mechanical properties of the interfacial particle layers via dilatational and shear rheology. We emphasize along that section the importance of the shear rheology to know the intrinsic morphology of such particulate system and to understand how the flow-field-dependent evolution of the interfacial morphology might eventually affect some properties of materials such as foams and emulsions. We dedicated the last section to explaining the importance of the particulate interfacial systems in the stabilization of foams and emulsions.
The effect of fluid rheology on particle migration induced by fluid viscoelasticity in a square-shaped microchannel is reported. Three water polymer solutions of PolyEthylene Oxyde at different concentrations, corresponding to different elasticity and degree of shear thinning, are prepared and rheologically characterized. Experiments are carried out for a wide range of flow rates, and the particle distributions over the channel cross section are reconstructed by combining particle tracking measurements and numerical simulations of the fluid velocity profile. The particle distributions show that the migration direction strongly depends on the fluid rheology. Specifically, when particles explore the constant viscosity region of the suspending liquids, they are focused around the channel centerline. Such an effect is more and more pronounced as the flow rate increases. On the other hand, for particles suspended in a shear-thinning fluid, a different scenario appears: At low flow rates, i.e., in the constant viscosity region, particles still migrate toward the channel centerline, while at high flow rates, i.e., in the shear thinning region, the migration reverts direction and the particles are driven toward the corners of the channel cross section. Those experimental observations elucidate the relevant and competing role of elasticity and shear thinning, with obvious implications in designing microfluidic devices for particle manipulation. Finally, our results highlight the weak effect of inertia on particle migration as compared to viscoelastic effects, even for low elastic suspending liquids.
When nonwetting fluid displaces wetting fluid in a porous rock many rapid pore-scale displacement events occur. These events are often referred to as Haines jumps and any drainage process in porous media is an ensemble of such events. However, the relevance of Haines jumps for larger scale models is often questioned. A common counter argument is that the high fluid velocities caused by a Haines jump would average-out when a bulk representative volume is considered. In this work, we examine this counter argument in detail and investigate the transient dynamics that occur during a Haines jump. In order to obtain fluid-fluid displacement data in a porous geometry, we use a micromodel system equipped with a high speed camera and couple the results to a pore-scale modeling tool called the Direct HydroDynamic (DHD) simulator. We measure the duration of a Haines jump and the distance over which fluid velocities are influenced because this sets characteristic time and length scales for fluid-fluid displacement. The simulation results are validated against experimental data and then used to explore the influence of interfacial tension and nonwetting phase viscosity on the speed of a Haines jump. We find that the speed decreases with increasing nonwetting phase viscosity or decreasing interfacial tension; however, for the same capillary number the reduction in speed can differ by an order of magnitude or more depending on whether viscosity is increased or interfacial tension is reduced. Therefore, the results suggest that capillary number alone cannot explain pore-scale displacement. One reason for this is that the interfacial and viscous forces associated with fluid-fluid displacement act over different length scales, which are not accounted for in the pore-scale definition of capillary number. We also find by analyzing different pore morphologies that the characteristic time scale of a Haines jump is dependent on the spatial configuration of fluid prior to an event. Simulation results are then used to measure the velocity field surrounding a Haines jump and thus, measure the zone of influence, which extends over a distance greater than a single pore. Overall, we find that the time and length scales of a Haines jump are inversely proportional, which is important to consider when calculating the spatial and temporal averages of pore-scale parameters during fluid-fluid displacement.
The instability in a jet of a viscoelastic semidilute entangled polymer solution under high stretching is discussed. Initially the jet is stable due to solution elasticity stopping the fluctuation growth. The modeling shows that a jet stretching results in a radial gradient in the polymer distribution: the polymer is concentrated in the jet centre, whereas the solvent is remaining near the surface. This viscous liquid shell demonstrates Raleigh-type instability resulting in the formation of individual droplets on the oriented filament. All process stages were observed experimentally.
Graphical abstract
Newly developed high-speed, synchrotron-based X-ray computed microtomography enabled us to directly image pore-scale displacement events in porous rock in real time. Common approaches to modeling macroscopic fluid behavior are phenomenological, have many shortcomings, and lack consistent links to elementary pore-scale displacement processes, such as Haines jumps and snap-off. Unlike the common singular pore jump paradigm based on observations of restricted artificial capillaries, we found that Haines jumps typically cascade through 10-20 geometrically defined pores per event, accounting for 64% of the energy dissipation. Real-time imaging provided a more detailed fundamental understanding of the elementary processes in porous media, such as hysteresis, snap-off, and nonwetting phase entrapment, and it opens the way for a rigorous process for upscaling based on thermodynamic models.
Agricultural soil is a sink of microplastics (MPs) in the environment. MPs in topsoil can be transferred deeply or into surrounding water by rainfall. However, little is known about rainfall-induced migration pattern of different MPs in agricultural soil. In this study, soil leaching experiments of 21 d were performed on Nile red-stained size-different polyethylene terephthalate (PET) particles, and shape-different polyethylene (PE) MPs under simulated or natural rainfall. Results showed that simulated rainfall of 5–25 mm/d caused intensity-dependent migration of MPs in horizontal and vertical directions. Maximum migration depth of MP particles arrived up to 4–7 cm. Rise of soil slopes could significantly increase horizontal mobility of MPs. Comparatively, natural rainfall of similar intensity caused relatively high mobility of MPs. Moreover, under both simulative and natural rainfall, mobility of MPs presented size/shape-different characteristics. Comparatively, small-size MPs (especially <1 mm) showed relatively high mobility in horizontal or vertical direction, and had high-frequency presence in runoff water. Of four MPs' shapes, fiber and film had relatively high mobility in comparison to particles. These results indicate that rainfall can cause size/shape-dependent migration of MPs in agricultural soil. It suggests size/shape-different environment fate of MPs, and provides a reference for MP control.
Microplastic (MP) pollution has become a global concern given its wide occurrence and potential ecological risks. The retention/transport features of MPs in porous media govern the fate and risks of MPs in subsurface environments. Polystyrene (PS) microspheres are employed as representative MPs to explore the migration behaviors in water-saturated quartz sand columns. The hydrodynamic size mainly determines the deposition and size exclusion straining of MPs in porous media, and further the attachment efficiency. PS50 (PS with 50 nm diameter) shows a total migration rate greater than 85% in each of the studied conditions. In contrast, PS500 commonly exhibits slower migration velocities and higher attachment efficiencies than those of PS50 and PS100. The ionic strength, pH, and dissolved organic matter content of the solution show obvious effects on the retention/transport of PS MPs. The influences of solution chemical properties are consistent with the prediction of Derjaguin-Landau-Verwey-Overbeek (DLVO) theory. The results in this study clarify the size-dependent migration characteristics of MPs in porous media and provide a basis for risk assessment of MPs in terrestrial environments.
The addition of micro cavities is a promising passive method to enhance mass transfer performance in microchannels. Experimental and numerical methods are adopted to investigate the flow characteristics of velocity field, droplet shape and vorticity for the liquid-liquid two-phase flows in microchannels with micro cavities. The results indicate that the droplet expands in cavities for slug and transitional flow patterns with droplet length longer than the channel width, while the expansion is not obvious for droplet flow patterns. Dimensionless expansion width increases with decreasing Capillary number and with increases in droplet length. For long droplet with length more than 2.5 times of the channel width, the expansion width is nearly the same with increasing droplet length. Due to the sudden decrease and increase of cross-section areas in cavities, the droplet moving speed fluctuates in the cavities, leading to the pulsating flow in cavity-shaped microchannels. When the droplet passes through the cavity structure, the continuous phase near the wall enters the cavity and induces small vortexes in the cavities. The intensity of vortex is more pronounced with longer droplet due to the blocking effect of droplet expansion in cavities. The vorticity magnitude in cavities is always higher than the value in straight microchannel, indicating that micro cavities can enhance the fluid mixing in the microchannel.
Hypothesis
The stability of fluid–fluid interface is key to control the displacement efficiency in multiphase flow. The existence of particles can alter the interfacial dynamics and induce various morphological patterns. Moreover, the particle aggregations are expected to have a significant impact on the interface stability and patterns.
Experiments
Monodisperse polyethylene particles of different sizes are uniformly mixed in silicone oil to form the granular mixtures, which are injected into a transparent radial Hele-Shaw cell through different strategies to obtain the homogeneous and inhomogeneous (with particle aggregations) initial states. Subsequently, a systematic study of morphology and interface stability during the withdrawal of granular mixtures is performed.
Findings
For homogeneous mixtures, we observe earlier onset of fingering, more fingers and lower gas saturation at breakthrough than for pure fluid with equivalent viscosity. This effect can be attributed to the particle-induced perturbations. For inhomogeneous mixtures, particle clusters and bands significantly enhance the interface instability. Furthermore, we find that particle deposition due to liquid film entrainment occurs above a critical local flow velocity, and we elucidate the responsible mechanism through force balance analysis and the thin film theory. This work could be of practical significance in geoenergy and industrial applications.
Hypothesis
The Landau-Levich-Derjaguin (LLD) theory is widely applied to predict the film thickness in the dip-coating process. However, the theory was designed only for flat plates and thin fibers. Fifty years ago, White and Tallmadge attempted to generalize the LLD theory to thick rods using a numerical solution for a static meniscus and the LLD theory to forcedly match their numeric solution with the LLD asymptotics. The White-Talmadge solution has been criticized for not being rigorous yet widely used in engineering applications mostly owing to the lack of alternative solutions. A new set of experiments significantly expanding the range of White-Tallmadge conditions showed that their theory cannot explain the experimental results. We then hypothesized that the results of LLD theory can be improved by restoring the non-linear meniscus curvature in the equation. With this modification, the obtained equation should be able to describe static menisci on any cylindrical rods and the film profiles observed at non-zero rod velocity.
Experiment
To test the hypothesis, we distinguished capillary forces from viscous forces by running experiments with different rods and at different withdrawal velocities and video tracking the menisci profiles and measuring the weight of deposited films. The values of film thickness were then fitted with a mathematical model based on the modified LLD equation. We also fitted the meniscus profiles.
Findings
The results show that the derived equation allows one to reproduce the results of the LLD theory and go far beyond those to include rods of different radii. A new set of experimental data together with the White-Tallmadge experimental data are explained with the modified LLD theory. A set of simple formulas approximating numeric results have been derived. These formulas can be used in engineering applications for the prediction of the coating thickness.
This paper investigates a CFD-based analysis for gas-liquid and liquid-liquid Taylor flows through a circular axisymmetric microchannel with a sudden enlargement. A series of simulations are conducted by exploring the influence of different superficial velocity ratios, apparent viscosities, and channel expansion on the hydrodynamics of slug flow. A concentric junction introduces dispersed airflow into a continuous flow of water for gas-liquid flow, and the junction introduces dispersed water into a continuous flow of dodecane for liquid-liquid flow. The air-bubble and water-slug evolution processes, slug breakup, and slug expansion are investigated. In all cases, the lengths of air bubbles and water slugs increase with increasing superficial velocity ratio, particularly before the expansion. For gas-liquid flow, the apparent viscosity ratio causes a fluctuating interface over the uniform film region. However, the water slug length is shorter and the film region is slightly thicker in liquid-liquid compared to gas-liquid flow. The numerical analysis developed in this paper is in good agreement with the existing correlations and experimental data in the literature.
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Terrestrial soils are not only a large reservoir for Microplastics (MPs), but also a possible entrance to the subsurface environment, posing potential risks to the subterranean habitats and groundwater. In this study, we examined the vertical transport of MPs of four polymers, i.e., polyethylene (PE), polyethylene terephthalate (PET), polypropylene (PP) and polyamide (PA), in porous sand media driven by wet-dry cycling. The effects of polymer properties, MP size, sand particle size, wet-dry cycles, and dissolved organic matter (DOM) on their migration behavior were investigated. Surface hydrophobicity showed a strong positive correlation with MP mobility, with PA exhibiting the greatest movement potential, followed by PE, PET, and PP. The penetration depth of MP particles increased with decreasing MP particle size (dMP) and increasing sand diameter (dsand). MP particles migrated deeper in sand media when dMP/dsand < 0.11. Furthermore, frequent wet-dry cycles and the presence of DOM promoted the vertical migration of MPs in the sand. The results revealed multiple factors influencing the vertical migration of MPs in sand, which is instructive for understanding the ecological risk of MPs in potentially contaminated soil (e.g., farmland with long-term mulching) to the subsurface environment and potential negative impact to public health.
The intensive use and wide-ranging application of plastic- and plastic-derived products have resulted in alarming levels of plastic pollution in different environmental compartments worldwide. As a result of various biogeochemical mechanisms, this plastic litter is converted into small, ubiquitous and persistent fragments called microplastics (< 5 mm), which are of significant and increasing concern to the scientific community. Microplastics have spread across the globe and now exist in virtually all environmental compartments (the soil, atmosphere, and water). Although these compartments are often considered to be independent environments, in reality, they are very closely linked. Ample research has been done on microplastics, but there are still questions and knowledge gaps regarding the emission, occurrence, distribution, detection, environmental fate and transport of MPs in different environmental compartments. The current article is intended to provide a systematic overview of MP emissions, pollution conditions, sampling and analytical approaches, transport, fates and transformation mechanisms in different environmental compartments. It also identifies research gaps and future research directions and perspectives.
Fluid-driven granular transport is involved in many important geomorphological processes and industrial applications such as unconventional hydrocarbon recovery. Yet it remains challenging to fully understand the granular transport mechanisms in confined geometries. By performing simulations based on a coupled computational fluid dynamics and discrete element method (CFD-DEM) approach, we systematically investigate the particle transport patterns and mechanisms driven by fluid flow between two parallel plates. Depending on the local drag force, the particles can settle or suspend in the fluid, leading to fluid-driven particle transport by creeping or by suspension. In the case of settle particle layers, fluid velocity variation in the vertical direction contributes to the relative motion between particle layers. Fluid-induced fingering patterns are observed in the upper layer of settled particles during the sliding. It is shown that the average finger length increases linearly with time and is affected by the flow rate and particle volume fraction. Increasing the flow rate shift the particle migration from creeping and sliding to suspension, which generally improves the particle transport efficiency. The obtained understanding of particle transport patterns in the confined geometries may have practical implications for industrial scenarios such as proppant transport in hydraulic fractures and sand production in extraction systems.
Plastics are widely used in many fields due to their stable physical and chemical properties, and their global production and usage increase significantly every year, which leads to the accumulation of microplastics in the entire ecosystem. Numerous studies have shown that microplastics (MPs) have harmful effects on living organisms. This review aims to provide a comprehensive conclusion of the current knowledge of the impacts of MPs on the stability of the gut microenvironment, especially on the gut barrier. Studies showed that exposure to MPs could cause oxidative damage and inflammation in the gut, as well as the destruction of the gut epithelium, reduction of the mucus layer, microbial disorders, and immune cell toxicity. Although there are few reports directly related to humans, we hoped that this review could bring together more and more evidence that exposure to MPs results in disturbances of the intestinal microenvironment. Therefore, it is necessary to investigate their threats to human health further.
Environmental pollutants like microplastics are posing health concerns on aquatic animals and the ecosystem. Microplastic toxicity studies using Caenorhabditis elegans (C. elegans) as a model are evolving but methodologically hindered from obtaining statistically strong data sets, detecting toxicity effects based on microplastics uptake, and correlating physiological and behavioural effects at an individual-worm level. In this paper, we report a novel microfluidic electric egg-laying assay for phenotypical assessment of multiple worms in parallel. The effects of glucose and polystyrene microplastics at two concentrations on the worms' electric egg-laying, length, diameter, and length contraction during exposure to electric signal were studied. The device contained eight parallel worm-dwelling microchannels called electric traps, with equivalent electrical fields, in which the worms were electrically stimulated for egg deposition and fluorescently imaged for assessment of neuronal and microplastic uptake expression. A new bidirectional stimulation technique was developed, and the device design was optimized to achieve a testing efficiency of 91.25%. Exposure of worms to 100 mM glucose resulted in a significant reduction in their egg-laying and size. The effects of 1 μm polystyrene microparticles at concentrations of 100 and 1000 mg/L on the electric egg-laying behaviour, size, and neurodegeneration of N2 and NW1229 (expressing GFP pan-neuronally) worms were also studied. Of the two concentrations, 1000 mg/L caused severe egg-laying deficiency and growth retardation as well as neurodegeneration. Additionally, using single-worm level phenotyping, we noticed intra-population variability in microplastics uptake and correlation with the above physiological and behavioural phenotypes, which was hidden in the population-averaged results. Taken together, these results suggest the appropriateness of our microfluidic assay for toxicological studies and for assessing the phenotypical heterogeneity in response to microplastics.
Understanding of microplastics transport mechanism is highly important for soil contamination and remediation. The transport behaviors of microplastics in soils are complex and influenced by various factors including soil and particle properties, hydrodynamic conditions, and biota activities. Via a microfluidic experiments we study liquid film entrainment and microplastics transport and retention during two-phase displacement in microchannels with one end connected to the air and the other connected to the liquid with suspended particles. We discover three transport patterns of microplastic particles, ranging from no deposition to particle entrapment and to particle layering within liquid films, depending on the suspension withdrawal rates and the particle volume fraction in the suspension. The general behavior of particle motion is effectively captured by the film thickness evolution which is shown to be dependent on a modified capillary number Ca0 taking into account the effects of flow velocity, particle volume fraction, and channel shape. We also provide a theoretical prediction of the critical capillary number Ca0* for particle entrapment, consistent with the experimental results. In addition, the probability of microplastics being dragged into the trailing liquid film near the gas invading front is found to be proportional to both particle volume fraction and the capillary number. This work elucidates the microplastics transport mechanism during unsaturated flow, and therefore is of theoretical and practical importance to understand the contaminant migration in many natural and engineered systems spanning from groundwater sources to water treatment facilities.
The cotransport behaviors of colloidal polystyrene microplastic particles (PSMPs) and tetracycline (TC) (20 mg/L) were investigated in saturated porous media in KCl and CaCl2 solutions of various ionic strengths (1, 10, 50, 100 mM). Furthermore, the effects of TC concentration (0, 1, 5, 10, 20 mg/L) on the cotransport behaviors of PSMPs and TC in 100 mM KCl solution were assessed. The cotransport behaviors were analyzed by comparing the individual transport behaviors of PSMPs or TC. When cotransported, the presence of TC (20 mg/L) slightly inhibited PSMPs mobility in K⁺ solutions (the C/C0 decreased in the range of 0–5.9%), but facilitated it in Ca²⁺ solutions (the C/C0 increased in the range of 6.7–42.6%). In KCl solutions, although the presence of TC (PSMPs) did not significantly affect the transport behaviors of PSMPs (TC), the attachment efficiencies of both PSMPs and TC showed a non-linear and non-monotonic change with increase in ionic strength. However, in CaCl2 solutions, the effects of TC (PSMPs) on the transport behaviors of PSMPs (TC) were remarkable and a non-linear non-monotonic change was observed. The adsorption of TC on PSMPs might play a critical role during the cotransport. Thus, the balance between the transport-inhibiting (e.g., the reduction in electrostatic repulsive force) and transport-facilitating effects (e.g., the effects on hydrophilicity/hydrophobicity of PSMPs due to TC adsorption) may be responsible for the observed changes. Overall, the results demonstrated that the cotransport behaviors of PSMPs and TC were more complicated than their individual transport behaviors in porous media, which might vary considerably with environmental conditions. This work could greatly improve our understanding of complex cotransport behaviors and environmental risk of PSMPs.
Microplastics (MPs) are ubiquitous in the environment and more abundant in the marine environment. Consequently, increasing focus has been put on MPs in oceans and seas, while little importance has been attached to their presence in freshwaters and soils. Therefore, this paper aimed to provide a comprehensive review of the occurrence, analysis and ecotoxicology of MPs. The abundance and distribution of MPs in several typical freshwater systems of China were summarized. It suggested that the surface water of Poyang Lake contained the highest concentration of 34 items/L MPs among all the 8 freshwater systems, and the content of MPs in sediments were higher than that of the surface water. Net-based zooplankton sampling methods are the most frequently utilized sampling methods for MPs, and density separation, elutriation and digestion are three major pretreatment methods. Fourier transform infrared spectroscopy, Raman spectroscopy and pyrolysis-gas chromatography coupled to mass spectrometry are often used to identify the polymer types of MPs. Besides, MPs might damage the digestive tract of various organisms and negatively inhibit their growth, feeding and reproduction. The ways of human exposure to MPs are by ingestion, inhalation and dermal exposure, digestive and respiratory system might be adversely influenced. However, potential health risks of MPs to humans are remained insufficiently researched. Overall, by showing the presence of MPs in freshwaters and soils as well as possible ecotoxicological effects on the environment and humans, this paper provided a framework for future research in this field.
The COVID-19 pandemic has had growing environmental consequences related to plastic use and follow-up waste, but more urgent health issues have far overshadowed the potential impacts. This paper gives a prospective outlook on how the disruption caused by COVID-19 can act as a catalyst for short-term and long-term changes in plastic waste management practices throughout the world. The impact of the pandemic and epidemic following through the life cycles of various plastic products, particularly those needed for personal protection and healthcare, is assessed. The energy and environmental footprints of these product systems have increased rapidly in response to the surge in the number of COVID-19 cases worldwide, while critical hazardous waste management issues are emerging due to the need to ensure destruction of residual pathogens in household and medical waste. The concept of Plastic Waste Footprint (PWF) is proposed to capture the environmental footprint of a plastic product throughout its entire life cycle. Emerging challenges in waste management during and after the pandemic are discussed from the perspective of novel research and environmental policies. The sudden shift in waste composition and quantity highlights the need for a dynamically reponsive waste management system. Six future research directions are suggested to mitigate the potential impacts of the pandemic on waste management systems.
We present experiments and theory for viscous fingering of a suspension of non-colloidal particles undergoing radial flow in a Hele-Shaw cell. As the suspension displaces air, shear-induced migration causes particles to move faster than the average suspension velocity and to accumulate on the suspension–air interface. The resultant particle accumulation generates a pattern in which low-concentration, low-viscosity suspension displaces high-concentration, high-viscosity suspension and is unstable due to the classic Saffman–Taylor instability mechanism. While the destabilising mechanism is well-understood, what remains unknown is the stabilising mechanism that suppresses fine fingers characteristic of miscible fingering. In this work, we demonstrate how the stable suspension–air interface interacts with the unstable miscible interface to set the critical wavelength. We present a linear stability analysis for the time-dependent radial flow and show that the wavenumber predicted by the analysis is in good agreement with parametric experiments investigating the effect of suspension concentration and gap thickness of the Hele-Shaw cell.
Microplastics (MPs) are an emerging concern and potential risk to marine and terrestrial environments. Surface soils are reported to act as a sink. However, MP vertical mobility in the subsurface remains uncertain due to a lack of scientific data. This study focused on MP penetration in sand soil column experiments. Here we report the mobility of five different MPs, which consisted of polyethylene (PE) and polypropylene (PP) particles of various sizes and densities. We observed that the smallest sized PE MPs (21 μm) had the greatest movement potential. Moreover, it was found that when these MPs were subjected to greater numbers of wet-dry cycles, the penetration depth significantly increased, with an apparent linear relationship between depth and wet-dry cycle number (r2 = 0.817). In comparison, increasing the volume of infiltration liquid or the surface MP concentration had only negligible or weak effects on migration depth (r2 = 0.169 and 0.312, respectively). Based on the observed wet-dry cycle trend, we forecast 100-year penetration depths using weather data for 347 cities across China. The average penetration depth was calculated as 5.24 m (95% CI = 2.78-7.70 m), with Beijing Municipality and Hebei, Henan and Hubei provinces being the most vulnerable to MP vertical dispersion. Our results suggest that soils may not only represent a sink for MPs, but also a feasible entryway to subsurface receptors, such as subterranean fauna or aquifers. Finally, research gaps are identified and suggested research directions are put forward to garner a better understanding MP vertical migration in soil.
Viscous fingering is observed experimentally when a bidisperse suspension displaces air inside a Hele-Shaw cell, despite the stabilising viscosity ratio between the invading (suspension) and defending (air) phases. Careful experiments are carried out to characterise this instability by either systematically varying the large-particle concentrations \unicode[STIX]{x1D719}_{l0} at constant total concentrations \unicode[STIX]{x1D719}_{0} , or changing \unicode[STIX]{x1D719}_{0} with fixed \unicode[STIX]{x1D719}_{l0} . Leading to the instability, we observe that larger particles consistently enrich the fluid–fluid interface at a faster rate than small particles. This size-dependent enrichment of the interface leads to an earlier onset of the fingering instability for bidisperse suspensions, compared to their monodisperse counterpart of all small particles. In particular, even the small presence of large particles is shown to effectively lower the total particle concentration needed for fingering, compared to the all-small-particle case. We hypothesise that the key mechanism behind this enhanced viscous fingering is the size-dependent nature of shear-induced migration of particles far upstream from the interface. A reduced equilibrium model is derived based on the modified suspension balance model to verify this hypothesis, in reasonable agreement with experiments.
Withdrawing a plate from a suspension leads to the entrainment of a coating layer of fluid and particles on the solid surface. In this article, we study the Landau-Levich problem in the case of a suspension of non-Brownian particles at moderate volume fraction 10% < ϕ < 41%. We observe different regimes depending on the withdrawal velocity U, the volume fraction of the suspension ϕ, and the diameter of the particles 2a. Our results exhibit three coating regimes. (i) At small enough capillary number Ca, no particles are entrained, and only a liquid film coats the plate. (ii) At large capillary number, we observe that the thickness of the entrained film of suspension is captured by the Landau-Levich law using the effective viscosity of the suspension η(ϕ). (iii) At intermediate capillary numbers, the situation becomes more complicated with a heterogeneous coating on the substrate. We rationalize our experimental findings by providing the domain of existence of these three regimes as a function of the fluid and particles properties.
Suspensions are composed of mixtures of particles and fluid and are omnipresent in natural phenomena and in industrial processes. The present paper addresses the rheology of concentrated suspensions of non-colloidal particles. While hydrodynamic interactions or lubrication forces between the particles are important in the dilute regime, they become of lesser significance when the concentration is increased, and direct particle contacts become dominant in the rheological response of concentrated suspensions, particularly those close to the maximum volume fraction where the suspension ceases to flow. The rheology of these dense suspensions can be approached via a diversity of approaches that the paper introduces successively. The mixture of particles and fluid can be seen as a fluid with effective rheological properties but also as a two-phase system wherein the fluid and particles can experience relative motion. Rheometry can be undertaken at an imposed volume fraction but also at imposed values of particle normal stress, which is particularly suited to yield examination of the rheology close to the jamming transition. The response of suspensions to unsteady or transient flows provides access to different features of the suspension rheology. Finally, beyond the problem of suspension of rigid, non-colloidal spheres in a Newtonian fluid, there are a great variety of complex mixtures of particles and fluid that remain relatively unexplored.
Soils are essential components of terrestrial ecosystems that experience strong pollution pressure. Microplastic contamination of soils is being increasingly documented, with potential consequences for soil biodiversity and function. Notwithstanding, data on effects of such contaminants on fundamental properties potentially impacting soil biota are lacking. The present study explores the potential of microplastics to disturb vital relationships between soil and water, as well as its consequences for soil structure and microbial function. During a 5-weeks garden experiment we exposed a loamy sand soil to environmentally relevant nominal concentrations (up to 2 %) of four common microplastic types (polyacrylic fibers, polyamide beads, polyester fibers, and polyethylene fragments). Then, we measured bulk density, water holding capacity, hydraulic conductivity, soil aggregation, and microbial activity. Microplastics affected the bulk density, water holding capacity, and the functional relationship between the microbial activity and water stable aggregates. The effects are underestimated if idiosyncrasies of particle type and concentrations are neglected, suggesting that purely qualitative environmental microplastic data might be of limited value for the assessment of effects in soil. If extended to other soils and plastic types, the processes unravelled here suggest that microplastics are relevant long-term anthropogenic stressors and drivers of global change in terrestrial ecosystems.
A series of one-dimensional column experiments were conducted to investigate the transport and retention of micron-sized plastic spheres (MPs) with diameters of 0.1-2.0 μm in seawater-saturated sand. In seawater with salinity of 35 PSU (practical salinity units), the mass percentages recovered from the effluent (Meff) of the larger MPs increased from 13.6% to 41.3%, as MP size decreased from 2.0 μm to 0.8 μm. This occurred because of the gradual reduction of physical straining effect of MPs in the pores between sands. The smaller MPs (0.6, 0.4, and 0.1 μm) showed the stronger inhibition of MPs mobility, with Meff values of 11.5%, 11.9%, and 9.8%, respectively. This was due to the lower energy barriers (from 108 kBT to 16 kBT) between the smaller MPs and the sand surface, when compared with the larger MPs (from 296 kBT to 161 kBT). In particular, the aggregation of MPs (0.6 or 0.4 μm) triggered a progressive decrease in MP concentration in the effluent. Retention experiments showed that the vertical migration distance of most MP colloids was 0-4 cm at the inlet of column. For 0.6 or 0.4 μm MPs, the particles were concentrated over a 0-2 cm vertical distance. Moreover, the salinity (35-3.5 PSU) did not affect the transport of the larger MPs (2.0-0.8 μm). However, as seawater salinity decreased from 35 PSU to 17.5 or 3.5 PSU, the aggregation of the smaller MPs (0.6-0.1 μm) was dramatically inhibited or completely prevented. Meanwhile, ripening of the sand surface by the MPs (0.6 and 0.4 μm) no longer occurred. By contrast, all MPs in deionized water (0 PSU) achieved complete column breakthroughs because of the strong repulsive energy barrier (from 218 kBT to 4192 kBT) between the MPs and the sand surface. Consequently, we find that the transport and retention of MPs in sandy marine environment strongly relies on both the MP size and the salinity levels.
Plastic litter is widely acknowledged as a global environmental threat and poor management and disposal lead to increasing levels in the environment. Of recent concern is the degradation of plastics from macro- to micro- and even to nanosized particles smaller than 100 nm in size. At the nanoscale, plastics are difficult to detect and can be transported in air, soil and water compartments. While the impact of plastic debris on marine and fresh waters and organisms has been studied, the loads, transformations, transport, and fate of plastics in terrestrial and subsurface environments are largely overlooked. In this review, we first present estimated loads of plastics in different environmental compartments. We also provide a critical review of the current knowledge vis-à-vis nanoplastic (NP) and microplastic (MP) aggregation, deposition, and contaminant co-transport in the environment. Important factors that affect aggregation and deposition in natural subsurface environments are identified and critically analyzed. Factors affecting contaminant sorption onto plastic debris are discussed, and we show how polyethylene generally exhibits a greater sorption capacity than other plastic types. Finally, we highlight key knowledge gaps that need to be addressed to improve our ability to predict the risks associated with these ubiquitous contaminants in the environment by understanding their mobility, aggregation behavior and their potential to enhance the transport of other pollutants.
As an air bubble translates in a microchannel, a thin film of liquid is formed on the bounding walls. In a microchannel with a rectangular cross section, the liquid in the film leaks towards the low-pressure corners of the geometry, which leads to the appearance of local minima in the film thickness in the cross-sectional plane. In such a configuration, theory suggests that the minimum film thickness scales with Ca and Ca(4/3) depending on the distance from the nose of the bubble, where Ca = μUb/γ is the flow capillary number based on the bubble velocity Ub, liquid viscosity μ and surface tension γ. We show that the film of a partially wetting liquid dewets on the channel wall at the sites of the local minima in the film thickness as it acquires thicknesses smaller than 100 nm. Our experiments show that the distance Lw between the nose of the bubble and the initial dewetting location is a function of Ca and surface wettability. For channels of different wettability, Lw always scales proportional to Caα, where 1.7 < α < 2 for the range of 10⁻⁵ < Ca < 10⁻². Moreover, Lw increases up to 10 times by enhancing the wettability of the surface at a given Ca. Our present measurements of Lw provide a design constraint on the lengths of bubbles to maintain a liquid wet channel without dry patches on the wall.
Slickwater fracturing is a popular stimulation treatment in the unconventional oil and gas industry. It creates thin and long fractures that connect to pre-existing natural fractures and generate complex fracture networks. A large fraction of the fractured area is not usually propped due to the high density of typical proppants (sand) and low viscosity of the fracturing fluid. The goal of this work is to understand and optimize proppant transport in complex fracture networks. In this paper, proppant transport in fracture intersections is studied experimentally (using laboratory size slots) and numerically (using a multiphase dense discrete phase model). The orientation of natural fractures, proppant size and shear rate have been varied and the injected proppant volume is kept constant. Both experiments and simulations show three zones: bottom immobile sand bed zone, middle flowing slurry zone, and top clear fluid zone. The sand injected early forms the bottom of the sand bed; the sand injected later moves downstream and forms the top part of the bed. The entrance eroded region increases as the shear rate (or equivalently the water injection rate) increases. The sand bed length increases as the shear rate increases. The equilibrium sand bed height decreases as the shear rate increases and the sand size decreases. Proppant placement in the bypass slot increases as the shear rate increases and the bypass angle decreases. The numerical model using a dense discrete phase model (DDPM) captures the key features of the sand bed formation and transport.
A 1972–76 survey of over 300 beaches showed that small plastic pellets and granules, of the kinds commonly being recorded on shores around the North Atlantic, are also widely distributed on the New Zealand foreshore. Most of the pellets are virgin polyolefins, the imported feedstock of the local plastics industry. The pellets find their way into the environment through accidental spillage during transport and handling; they are not litter or waste in the usual sense. The quantity of these virgin plastic pellets on New Zealand beaches today possibly exceeds 1000 t and has a value in excess of NZ$1 000 000. Virgin polystyrene pellets are rare: virgin polyvinyl chloride pellets were never seen.Numbers of pellets are greatest near Auckland, Wellington, and Christchurch, which are the important source areas. However, pellets are also found on beaches remote from these cities, and some may have come from eastern Australia. Because they degrade slowly, plastics can be a significant contributor to coastal pollution, but the environmental hazards of their accumulation are uncertain.
The total stress of a concentrated suspension of noncolloidal spheres in a Newtonian fluid was characterized by independent measurements in viscometric flows. Using a suspension balance formulation, the normal stress in the vorticity direction (Σ33) for a suspension undergoing simple shear was extracted from Acrivos et al.’s [Int. J. Multiphase Flow19, 797 (1993)] resuspension data in a Couette device. Employing a new correlation for the relative viscosityμr which obeys the Einstein relation in the dilute limit while diverging at random close packing, it was found that Σ33/τ (where τ is the magnitude of the shear stress) was a strong function of the solid volume fraction φ, scaling as φ3e2.34φ. The relative viscosity,measured in a parallel plate viscometer, was in good agreement with the proposed correlation, while the normal stress differences N1 and N2 for concentrated suspensions(φ=0.30–0.55) were characterized using parallel plate and cone-and-plate geometries, as well as laser profilometry measurements of the suspension surface deflection in a rotating rod geometry. The normal stresses were proportional to the shear stress τ, and with β≡N1/τ and δ≡N2/τ, the parameter combinations resulting from the three experimental geometries, β−δ, β, and δ+12 β, were all seen to increase with φ according to the derived scaling φ3e2.34φ. Furthermore, the best-fit N1 and N2 values consistent with the set of experiments were both negative, with |N2| > |N1| at any given concentration and shear rate. Taken together, the results obtained allow a complete determination of the total stress of a sheared suspension and in particular enabled us to compute the shear-induced particle-phase pressure Π, as defined in Jeffrey et al. [Phys. Fluids A 5, 2317 (1993)].
An analysis of forces which are expected to arise in colloidal systems under shear has been used to model the flow instability which leads to discontinuous and dilatant viscosity behavior in concentrated suspensions of polymeric resins. Observation of this phenomenon was reported in the first paper of this series. The analysis is carried out on the presumption that the dominant forces leading to the flow instability are van der Waals-London attraction, electric double layer repulsion and the shear stress acting on groups of particles. Conditions for the onset of the instability are cast in terms of a torque balance and an energy balance, and a set of dimensionless numbers are obtained to characterize the phenomenon. Experimental data obtained from tests on a number of concentrated suspensions are given in support of the theory.
This paper presents an extension of the theory of retention of fine particles in formations. The mechanism of interest is straining, which depends only on the shape and distribution of constrictions in pore space. To quantify the geometry of constrictions, we used computer-generated dense random packs of spheres as model porous media. A characteristic feature of dense packings is a pair of neighboring grains that do not quite touch. We refer to the void space between pairs of such spheres as gaps. Gaps are smaller than pore throats, which are the void space between three spheres. Geometric analysis of throats and gaps was combined with a new methodology to compute flow rates in gaps. The results were input to an existent straining theory to study the dependence of straining rate on fines size. Only the flow of a single fluid phase is considered.
Numerous experiments show evidence of straining of particles smaller than the smallest pore throat in the column, under conditions that exclude the possibility of filtration. The simplest test of a theory is whether it can account for these anomalous observations. Existing theories that consider pore throats as the constrictions cannot explain these observations, whereas including gaps in the set of pore space constrictions correctly predicts the observations.
A more stringent test of straining theory is to predict the dependence of particle straining rate in a column flow experiment upon the size of the particles. Classical pore-throat-based theories make the physically reasonable assumption that once a particle enters a small constriction, it is guaranteed to be trapped there. Thus the probability of trapping is proportional to the flow rate through the constriction. These theories significantly overestimate the influence of particle size for small particles. We argue that the probability of particle retention in a gap need not be 100% even if the particle enters the gap. The particle may rebound from the grain(s) rather than become wedged in the gap. If this happens, the particle may eventually be swept around the gap. We therefore extend the theory by postulating retention probability to be proportional to flow rate through the gap and inversely proportional to the momentum or to the kinetic energy of the carrier fluid. The extensions bracket the scaling behavior reported in experiments.
Our analysis of particle straining in gaps explained a long-standing set of observations for which previous theories could not account. These results suggest that a more detailed study of particle/grain collisions is needed to explain fully the straining rates observed in experiments.
Introduction
Fines are small particles of clay, quartz or similar materials that are present in most naturally occurring porous media. Kaolinite and illite are the most common migrating clays. Fines generally have a size of the order 1 µm and a net surface charge. The migration of fine particles includes their release from the porous media, their motion with the flow of permeate, and finally their capture at some pore sites or their passage out of the porous medium. The movement of these fine particles within the reservoir formation is due to drag forces during oil and gas production. This phenomenon often occurs in an unconsolidated formation, or when an incompatible completion fluid releases fine particles. Geilikman et al.1 presented a model to quantify the impairment in well productivity caused by fines migration during the bean-up operations. A change in the chemical composition of the fluid can mobilize fine particles attached to pore surfaces. A review of the mechanisms for fines migration can be found in the paper by Hibbeler et al.2
Formation damage in sandstone oil reservoirs is often caused by the dispersion of fine clay particles when the salinity of the interstitial water is reduced or the ionic composition is changed. This phenomenon is known as water sensitivity of sandstones and it can cause a strong reduction in permeability resulting in drastic decline in oil production.3 The problem can be particularly acute in high-rate wells in poorly consolidated formations. The experimental work of Nguyen et al.4 presented a method to control the fines migration to maintain permeability in both consolidated and unconsolidated formations. Fines can also exit the porous medium causing erosion in fluid handling facilities. The production of fines increases the porosity of the formation and can promote structural failure especially near the wellbore. The simultaneous flow of two fluids often contributes to fines migration, but single phase flow is sufficient in many situations and thus is the focus of this study.
Colloid transport in subsurface has received considerable attention recently because mobile colloids can facilitate the transport of heavy metals in soils to contaminate groundwater. Many studies on colloid mobility in the subsurface consider soils as well-defined porous media. Though similar in many aspects, soils are different from well-defined porous media. The authors emphasize the impacts of soil properties on soil-colloid deposition, release, and association with heavy metals to provide an overview of colloidal dynamics in natural soils. The electrical double layer and Derjaguin-Landau-Verwey-Overbeek (DVLO) theories are summarized in Section II as theoretical bases for further discussions of colloid dynamics in soils, and their interactions with heavy metals. After discussions of theory developments and experimental results on the characteristics of soil colloids in Section III and soil porous media in Section IV, the authors compares the deposition of colloidal particles in well-defined porous media with that in natural soils in Section V. In Section VI, processes that affect colloid release in soils are summarized and theories of ion transfer processes in soils during colloid release are reviewed and discussed. Finally, the authors give a brief overview of the adsorption and precipitation of heavy metals to soil colloidal particles and their influences on colloid surface charge development in Section VII. The authors conclude with remarks on the importance of colloid deposition and release in soils and their association with heavy metals.