[Show abstract][Hide abstract] ABSTRACT: Driven by its importance in nature and technology, droplet impact on solid surfaces has been studied for decades. To date, research on control of droplet impact outcome has focused on optimizing pre-impact parameters, e.g., droplet size and velocity. Here we follow a different, post-impact, surface engineering approach yielding controlled vectoring and morphing of droplets during and after impact. Surfaces with patterned domains of extreme wettability (high or low) are fabricated and implemented for controlling the impact process during and even after rebound -a previously neglected aspect of impact studies on non-wetting surfaces. For non-rebound cases, droplets can be morphed from spheres to complex shapes -without unwanted loss of liquid. The procedure relies on competition between surface tension and fluid inertial forces, and harnesses the naturally occurring contact-line pinning mechanisms at sharp wettability changes to create viable dry regions in the spread liquid volume. Utilizing the same forces central to morphing, we demonstrate the ability to rebound orthogonally-impacting droplets with an additional non-orthogonal velocity component. We theoretically analyze this capability and derive a We(-.25) dependence of the lateral restitution coefficient. This study offers wettability-engineered surfaces as a new approach to manipulate impacting droplet microvolumes, with ramifications for surface microfluidics and fluid-assisted templating applications.
[Show abstract][Hide abstract] ABSTRACT: Marine oil spills seriously endanger sea ecosystems, coastal environments, and result in a loss of energy resources. Environmental and economic demands emphasize the need for new methods of effectively separating oil-water mixtures, while collecting oil content at the same time. A new surface-tension-driven, gravity-assisted, one-step, oil-water separation method is presented for sustained filtration and simultaneous collection of oil from a floating spill. A bench-top prototype oil collection device uses selective wettability (superhydrophobic and superoleophilic) stainless steel mesh that attracts the floating oil, simultaneously separating it from water and collecting it in a container, requiring no pre-separation pumping or pouring. The collection efficiencies for oils with wide ranging kinematic viscosities (0.32 - 70.4 cSt at 40(o)C) are above 94%, including motor oil and heavy mineral oil. The prototype device also showed high stability and functionality over repeated use, and can be easily scaled for efficient clean-up of large oil spills on sea water. In addition, a brief consolidation of separation requirements for oil-water mixtures of various oil densities is presented to demonstrate the versatility of the material system developed herein.
[Show abstract][Hide abstract] ABSTRACT: Dropwise condensation (DWC) heat transfer depends strongly on the maximum diameter (Dmax) of condensate droplets departing from the condenser surface. This study presents a facile technique implemented to gain control of Dmax in DWC within vapor/air atmospheres. We demonstrate how this approach can enhance the corresponding heat transfer rate by harnessing the capillary forces in removal of the condensate from the surface. We examine various hydrophilic-superhydrophilic patterns, which, respectively sustain and combine DWC and filmwise condensation on the substrate. The material system uses laser-patterned masking and chemical etching to achieve the desired wettability contrast, and does not employ any hydrophobizing agent. By applying alternating straight parallel strips of hydrophilic (contact angle ~ 78(o)) mirror-finish aluminum and superhydrophilic regions (etched aluminum) on the condensing surface, we show that the average maximum droplet size on the less wettable domains is nearly 42% of the width of the corresponding strips. An overall improvement in condensate collection rate, up to 19%, (as compared to the control case of DWC on mirror-finish aluminum) was achieved by using an interdigitated superhydrophilic track pattern (on the mirror-finish hydrophilic surface) inspired by the vein network of plant leaves. The bioinspired interdigitated pattern is found to outperform the straight hydrophilic-superhydrophilic pattern design, particularly under higher humidity conditions in the presence of non-condensable gases (NCG), a condition that is more challenging for maintaining sustained DWC.
[Show abstract][Hide abstract] ABSTRACT: Rough surfaces immersed under water remain practically dry if the
liquid-solid contact is on roughness peaks, while the roughness valleys are
filled with gas. Mechanisms that prevent water from invading the valleys are
well studied. However, to remain practically dry under water, additional
mechanisms need consideration. This is because trapped gas (e.g. air) in the
roughness valleys can dissolve into the water pool, leading to invasion.
Additionally, water vapor can also occupy the roughness valleys of immersed
surfaces. If water vapor condenses, that too leads to invasion. These effects
have not been investigated, and are critically important to maintain surfaces
dry under water. In this work, we identify the critical roughness scale below
which it is possible to sustain the vapor phase of water and/or trapped gases
in roughness valleys - thus keeping the immersed surface dry. Theoretical
predictions are consistent with molecular dynamics simulations and experiments.
[Show abstract][Hide abstract] ABSTRACT: Among numerous challenges to meet the rising global energy demand in a
sustainable manner, improving phase change heat transfer has been at the
forefront of engineering research for decades. The high heat transfer rates
associated with phase change heat transfer are essential to energy and industry
applications; but phase change is also inherently associated with poor
thermodynamic efficiencies at low heat flux, and violent instabilities at high
heat flux. Engineers have tried since the 1930's to fabricate solid surfaces
that improve phase change heat transfer. The development of micro and
nanotechnologies has made feasible the high-resolution control of surface
texture and chemistry over length scales ranging from molecular levels to
centimeters. This paper reviews the fabrication techniques available for
metallic and silicon-based surfaces, considering sintered and polymeric
coatings. The influence of such surfaces in multiphase processes of high
practical interest, e.g., boiling, condensation, freezing, and the associated
physical phenomena are reviewed. The case is made that while engineers are in
principle able to manufacture surfaces with optimum nucleation or thermofluid
transport characteristics, more theoretical and experimental efforts are needed
to guide the design and cost-effective fabrication of surfaces that not only
satisfy the existing technological needs, but also catalyze new discoveries.
MRS Energy & Sustainability--A Review Journal 09/2014; 1.
[Show abstract][Hide abstract] ABSTRACT: Many important applications in fluid management could benefit from unidirectional transport through porous media via a simple, large-area, low-cost coating treatment; in essence, a fluid diode, demonstrated herein for water using common cellulosic paper substrates. In electronics, the diode is an electrical component with asymmetric current transfer characteristics. A light (< 2g/m2) superhydrophobic conformal coating applied onto one side of a porous substrate is shown to create liquid transport function analogous to the electronic diode, facilitating fluid movement in one direction under negligible penetration pressures, but opposing it in reverse up to greater pressures. The phenomenon is driven by capillary action, and can be observed using any similarly thin fluid barrier applied on only one side (i.e., wettability contrast) of an absorbent porous matrix. Diodic action and liquid transport rates are shown to be highly tunable, determined by fiber diameter and spacing, in combination with coating deposition amount. As an example, the device is used to separate an oil/water mixture relying upon the surface tension differences of the mixture constituents, and may be implemented in multicomponent fluid filtration/separation technologies.
[Show abstract][Hide abstract] ABSTRACT: Electromagnetic interference (EMI) shielding reduces coupling between signals, crosstalk among electrical components, noise in cables and communication systems, etc. With the increasing speed of terahertz (THz) electronic circuits and systems, THz EMI shielding is becoming increasingly more important. We review recent and pioneering studies on shielding property–structure characterization and applications of carbon nanocomposite materials in the THz frequency domain. The theory of EMI shielding by nanocomposite materials is summarized first. A description of relevant fabrication methods follows, along with structural characterization details. THz probing and characterization of carbon nanofillers and their composites as EMI shielding and attenuation materials is presented next. Finally, the application of these materials in quasi-optical THz components, including polarizers and potentially mesh filters, as well as related manufacturing techniques are reviewed and discussed. Specific examples are presented in some detail to introduce the reader to this exciting and rapidly evolving technological area.
[Show abstract][Hide abstract] ABSTRACT: We demonstrate the fabrication of free-standing polymeric nanocomposite films, which present magnetic and electrically conductive anisotropic properties. Magnetically functionalized carbon nanofibers are dispersed in a polymeric solution and, upon casting under a weak external magnetic field, are easily oriented and permanently assembled in a head-to-tail orientation in the polymer film during solvent evaporation. Magnetic and conductive property studies reveal that the resulting films have a high degree of anisotropy in both cases, thus allowing their use in functional complex devices. As a proof of concept, we demonstrate the potential application of these films as flexible THz polarizers. The detailed study shows that very high attenuation values per unit film thickness and fiber mass concentration are achieved, paving thus the way for cost-effective fabrication of substrate-free systems that have advantage over conventional devices realized so far.
[Show abstract][Hide abstract] ABSTRACT: Surface tension driven transport of liquids on open substrates offers an enabling tool for open micro total analysis systems that are becoming increasingly popular for low-cost biomedical diagnostic devices. The present study uses a facile wettability patterning method to produce open microfluidic tracks that - due to their shape, surface texture and chemistry - are capable of transporting a wide range of liquid volumes (~1-500 μL) on-chip, overcoming viscous and other opposing forces (e.g., gravity) at the pertinent length scales. Small volumes are handled as individual droplets, while larger volumes require repeated droplet transport. The concept is developed and demonstrated with coatings based on TiO2 filler particles, which, when present in adequate (~80 wt.%) quantities within a hydrophobic fluoroacrylic polymer matrix, form composites that are intrinsically superhydrophobic. Such composite coatings become superhydrophilic upon exposure to UV light (390 nm). A commercial laser printer-based photo-masking approach is used on the coating for spatially selective wettability conversion from superhydrophobic to superhydrophilic. Carefully designed wedge-patterned surface tension confined tracks on the open-air devices move liquid on them without power input, even when acting against gravity. Simple designs of wettability patterning are used on versatile substrates (e.g., metals, polymers, paper) to demonstrate complex droplet handling tasks, e.g., merging, splitting and metered dispensing, some of which occur in 3-D geometries. Fluid transport rates of up to 350 μL s(-1) are attained. Applicability of the design on metal substrates allows these devices to be used also for other microscale engineering applications, e.g., water management in fuel cells.
[Show abstract][Hide abstract] ABSTRACT: We present a facile approach for the fabrication of low-cost surface biomicrofluidic devices on superhydrophobic paper created by drop-casting a fluoroacrylic copolymer onto microtextured paper. Wettability patterning is performed with a common household printer, which produces regions of varying wettability by simply controlling the intensity of ink deposited over prespecified domains. The procedure produces surfaces that are capable of selective droplet sliding and adhesion, when inclined. Using this methodology, we demonstrate the ability to tune the sliding angles of 10 μL water droplets in the range from 13° to 40° by printing lines of constant ink intensity and varied width from 0.1 mm to 2 mm. We also formulate a simple model to predict the onset of droplet sliding on printed lines of known width and wettability. Experiments demonstrate open-air surface microfluidic devices that are capable of pumpless transport, mixing and rapid droplet sampling (~0.6 μL at 50 Hz). Lastly, post treatment of printed areas with pH indicator solutions exemplifies the utility of these substrates in point-of-care diagnostics, which are needed at geographical locations where access to sophisticated testing equipment is limited or non-existent.
[Show abstract][Hide abstract] ABSTRACT: The intense commercial demand for efficient fluid management (e.g., water barriers) has resulted in a recent proliferation of methods intended to impart liquid repellency to various substrates. Many such methods involve wet-based processing of hydrophobic polymers and thus rely heavily on organic solvents whose properties pose environmental challenges when scaled up from the laboratory bench. The current study presents a one-step, environmentally safe (>97 wt % water), room-temperature, low-cost, polymer-based technique that imparts superhydrophobicity to commercially relevant porous substrates. The method features aqueous dispersions of a commercially available fluoroacrylic copolymer and hydrophilic bentonite nanoclay and uses spray casting to apply coatings—which are subsequently dried in open air—to form thin conformal films. Wettability measurements demonstrate that the coating formulation imparts considerable resistance to water penetration in polymeric nonwoven and cellulosic substrates. In addition to the benign environmental impact of the aqueous formulation, all ingredients are commercially available, thus opening many technological opportunities in this area.
[Show abstract][Hide abstract] ABSTRACT: Low-cost, large-area, superhydrophobic coating treatments are of high value to technological applications requiring efficient liquid repellency. While many applications are envisioned, only few are realizable in practice due to either the high cost or low durability of such treatments. Recently, spray deposition of polymer-particle dispersions has been demonstrated as an excellent means for producing low-cost, large-area, durable, superhydrophobic composite coatings/films; however, such dispersions generally contain harsh or volatile solvents, which are required for solution processing of polymers as well as for dispersing hydrophobic nanoparticles, thus inhibiting scalability due to the increased cost in chemical handling and environmental safety concerns. Moreover, such coatings usually contain fluoropolymers due to their inherent low surface energy -a requirement for superhydrophobicity- but concerns over their bio-persistence has provided an impetus for eliminating these chemicals. For spray coating, the former problem can be overcome by replacing organic solvents with water, but this situation seems paradoxical: Producing a highly water-repellent coating from an aqueous dispersion. We report a water-based, non-fluorinated dispersion for the formation of superhydrophobic composite coatings applied by spray on a variety of substrates. We stabilize hydrophobic components (i.e., polymer, nanoparticles) in water, by utilizing chemicals containing acid functional groups (i.e., acrylic acid, carboxylic acid) that can become ionized in aqueous environments under proper pH control (pH > 7). The polymer utilized in this study is a co-polymer of olefin and acrylic acid, while the particle filler is exfoliated graphite nanoplatelet (xGnP®), which contains acid functional groups at its periphery. Once spray deposited and dried, the components become insoluble in water, thus promoting liquid repellency. Such coatings can find a wide range of applications due to their benign processing nature as well as the variety of substrates on which they can be deposited.
[Show abstract][Hide abstract] ABSTRACT: Carbon Nanotube (CNT) membranes hold the promise of extraordinary fast water transport for applications such as energy efficient filtration and molecular level drug delivery. However, experiments and computations have reported flow rate enhancements over continuum hydrodynamics, that contradict each other by orders of magnitude. We perform large scale Molecular Dynamics simulations emulating, for the first time, the micrometer thick CNTs membranes used in experiments. We find transport enhancement rates that are length dependent, due to entrance and exit losses, but asymptote to two orders of magnitude over the continuum predictions. These rates are far below those reported experimentally. The results suggest that the reported superfast water transport rates cannot be attributed to interactions of water with pristine CNTs alone.
[Show abstract][Hide abstract] ABSTRACT: We report an approach for cost-effective manufacturing of THz quasi-optical polarizers by inkjet printing of polymer-carbon nanowhisker (CNW) dispersions. The electromagnetic interference properties of coatings with fixed CNW/polymer composition and varying thickness are quantified by a frequency domain THz spectroscopy system in the range 570–630 GHz. A shielding effectiveness of ∼40 dB is attained for 70 μm-thick coatings. A prototype THz polarizer printed on Mylar film displayed transmission and absorbance that varied with polarization orientation. The degree of polarization for film thickness of ∼1 μm was 0.35. This performance can be improved by refining grid dimensions, increasing coating thickness and adopting multi-layer polarizer structures.
[Show abstract][Hide abstract] ABSTRACT: Surface tension confined (STC) open tracks for pumpless transport of low-surface tension liquids (e.g., acetone, ethanol, hexadecane) on microfluidic chips are fabricated using a large-area, wet-processing technique. Wettable, paraffin-wax, submillimeter-wide tracks are applied by a fountain-pen procedure on superoleophobic, fluoroacrylic-carbon nanofiber (CNF) composite coatings. The fabricated anisotropic wetting patterns confine the low-surface-tension liquids onto the flow tracks, driving them with meniscus velocities up to 3.1 cm s(-1). Scaling arguments and Washburn's equation provide estimates of the liquid velocities measured in the STC tracks. These tracks are also shown to act as rails for directional sliding control of mm-sized water droplets. The present facile top-down patterned wettability approach can be extended to deposit micrometer-wide tracks, which bear promise for pumpless handling of low-surface tension liquids (e.g., aqueous solutions containing alcohols or surfactants) in lab-on-a-chip type applications or in low power, high-throughput bio-microfluidics for health care applications.
[Show abstract][Hide abstract] ABSTRACT: Surfaces patterned with alternating (binary) superhydrophobic-superhydrophilic regions can be found naturally, offering a bio-inspired template for efficient fluid collection and management technologies. We describe a simple wet-processing, thermal treatment method to produce such patterns, starting with inherently superhydrophobic polysilsesquioxane-silica composite coatings prepared by spray casting nanoparticle dispersions. Such coatings become superhydrophilic after localized thermal treatment by means of laser irradiation or open-air flame exposure. When laser processed, the films are patternable down to ∼100 μm scales. The dispersions consist of hydrophobic fumed silica (HFS) and methylsilsesquioxane resin, which are dispersed in isopropanol and deposited onto various substrates (glass, quartz, aluminum, copper, and stainless steel). The coatings are characterized by advancing, receding, and sessile contact angle measurements before and after thermal treatment to delineate the effects of HFS filler concentration and thermal treatment on coating wettability. SEM, XPS and TGA measurements reveal the effects of thermal treatment on surface chemistry and texture. The thermally induced wettability shift from superhydrophobic to superhydrophilic is interpreted with the Cassie-Baxter wetting theory. Several micropatterned wettability surfaces demonstrate potential in pool boiling heat transfer enhancement, capillarity-driven liquid transport in open surface-tension-confined channels (e.g., lab-on-a-chip), and select surface coating applications relying on wettability gradients. Advantages of the present approach include the inherent stability and inertness of the organosilane-based coatings, which can be applied on many types of surfaces (glass, metals, etc.) with ease. The present method is also scalable to large areas, thus being attractive for industrial coating applications.
[Show abstract][Hide abstract] ABSTRACT: A solution-based, large-area coating procedure is developed to produce conductive polymer composite films consisting of hollow-core carbon nanofibers (CNFs) and a fluoroacrylic co-polymer available as a water-based dispersion. CNFs (100 nm dia., length ∼130 μm) were dispersed by sonication in a formic acid/acetone co-solvent system, which enabled colloidal stability and direct blending of the CNFs and aqueous fluoroacrylic dispersions in the absence of surfactants. The dispersions were sprayed on smooth and microtextured surfaces, thus forming conformal coatings after drying. Nanostructured composite films of different degrees of oil and water repellency were fabricated by varying the concentration of CNFs. The effect of substrate texture and CNF content on oil/water repellency was studied. Water and oil static contact angles (CAs) ranged from 98° to 164° and from 61° to 164°, respectively. Some coatings with the highest water/oil CAs displayed self-cleaning behavior (droplet roll-off angles <10°). Inherent conductivity of the composite films ranged from 63 to 940 S/m at CNF concentrations from 10 to 60 wt.%, respectively. Replacement of the long CNFs with shorter solid-core carbon nanowhiskers (150 nm dia., length 6–8 μm) produced stable fluoropolymer–nanowhisker dispersions, which were ink-jetted to generate hydrophobic, conductive, printed line patterns with a feature size ∼100 μm.
[Show abstract][Hide abstract] ABSTRACT: We report a highly efficient technique to form novel fluoropolymer blend dispersions containing poly(vinylidene fluoride) (PVDF) and a fluorinated acrylic copolymer using a cosolvent system comprising N-methyl-2-pyrrolidone (NMP), acetone, and water under pH control. We also show that certain surface-functionalized, high-aspect-ratio nanostructured materials, such as organoclay and carbon nanowhiskers (CNWs), can be easily dispersed in these fluoropolymer blends to fabricate durable and functional superhydrophobic composite coatings upon spray casting. Both clay and CNW superhydrophobic coatings also repel lower surface tension liquids, such as water–alcohol mixtures (40 mN/m). Repellency is characterized using droplet sessile contact angle and contact angle hysteresis. Both clay and CNW-based composite coatings display self-cleaning properties (low contact angle hysteresis) for both water and water–alcohol mixtures. Additionally, electrical conductivity measurement of CNW composite coatings demonstrates the ability to fabricate multifunctional superhydrophobic composites using these fluoropolymer dispersions. The nanoparticle concentration required in these composite coatings for water and water–alcohol repellency is compared with a previously reported PVDF-based coating system. Wettability is interpreted within the framework of the Wenzel and Cassie–Baxter wetting theories.
[Show abstract][Hide abstract] ABSTRACT: Elastomeric superhydrophobic nanostructured composite coatings scalable to large areas are prepared by spray casting particle-polymer dispersions. The dispersions consist of nanostructured carbon black particles along with submicrometer-sized poly(tetrafluoroethylene) particles dispersed in nitrile rubber solution in acetone, with the goal to attain superhydrophobicity with minimal content of particle fillers. The coatings are applied on various flexible substrates, which are subsequently stretched uniaxially. Upon drying, water droplet roll-off (sliding) angle measurements are performed to quantify the self-cleaning ability of coated substrates being stretched uniaxially to 30% strain (coated silicone rubber) and 70% strain (coated polyester fabric). The coatings conform to the stretching of the substrates, while maintaining the self-cleaning property throughout this range. Self-cleaning is also maintained for cyclical stretching of coated substrates for strains 0–30% (coated silicone rubber) and 0–70% (coated polyester fabric) demonstrating the coatings’ functional recovery. Droplet roll-off angles below 10° reveal good self-cleaning ability for these coatings.
Composites Part A Applied Science and Manufacturing 08/2011; 42(8):979-985. · 3.01 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We report design and synthesis of polymer-based large-area superhydrophobic carbon nanofiber (CNF) composite coatings for tunable electromagnetic interference shielding and attenuation in the terahertz (THz) frequency regime. Such coatings with different CNF/polymer weight ratios are characterized by a frequency domain THz spectroscopy system. A maximum THz shielding effectiveness of ∼ 32 dB was measured in the examined frequency range of 570–630 GHz. Coating attenuation level varied with CNF loading. Two-dimensional distributions of power attenuation at 600 GHz showed good spatial uniformity. The present composite coatings, in addition to their self-cleaning property, have high potential for advanced technology high-frequency applications.