Microsphere Handling at the Microscale: New Tools and New Effects
Abstract and Figures
Analytical techniques are essential in detecting components within complex samples retrieved from various applications, e.g., rapidly detecting pathogens to prevent an outbreak. High Performance Liquid Chromatography (HPLC), is an analytical technique, in which a sample is pumped through a packed bed of microspheres (the column). Within this column, the components of the injected sample interact differently with the beads, hence leaving the column at different intervals.
Improving the efficiency of HPLC systems might benefit the clinical diagnostics of diseases, e.g., cancer. One approach to achieve this is by changing the packing of microspheres in the column from a random to an ordered state. We propose to accomplish this goal by a layer-by-layer assembly of these beads. This strategy is pursued by exploring the concept of a vacuum-driven force capturing a monolayer of precisely positioned beads on a micromachined device. As simple as it sounds, it suffices to say that it has been a challenge to assemble the particle monolayer. The main difficulties originated from the aggregation of these beads and their uncontrollable supply under dry conditions.
We have applied various techniques, such as rubbing, high voltage power supply systems, and shakers, to break the large cluster of microspheres, prior to offering them to the experimental setup. Furthermore, we have studied the interaction forces of silica or polystyrene beads on several surfaces to understand the mechanism of why they would stick on surfaces. It was observed that after rubbing, the microspheres unexpectedly had a preference to stick on a Teflon-like material. This result was explained by the tribocharging mechanism, which is the same mechanism responsible for the charging of a balloon while rubbing it on your hair. A key part of the project involved the design and fabrication of devices using micromachining technology. These devices were deployed in several domains of the project: to break the clusters, to control the supply of single particles with a filter, and to capture the particles with the vacuum force. Our studies revealed that droplets carrying the beads enhance the supplement as well as the quality of the obtained particle assembly. Moreover, funnel-like structures on which the microspheres are captured on the device, have proven to enhance the quality of the closely packed assemblies significantly.
![The capillary bridge formed over a distance D between a hydrophilic silica particle with radius R and another hydrophilic planar substrate. θ i is the contact angle on the respective surfaces, α is the filling angle of the liquid bridge with radii of curvature x and r. Adapted from Fukunishi et al.. [63]](https://www.researchgate.net/profile/Ignaas-Jimidar/publication/350610856/figure/fig4/AS:11431281091692002@1666604605582/The-capillary-bridge-formed-over-a-distance-D-between-a-hydrophilic-silica-particle-with_Q320.jpg)
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... blue circle in Fig. 1b-right image) [2]. To overcome and improve this limitation and previous efforts, profiled perforated devices, as shown in Fig. 1c, resembling a funnel-like structure, were micromachined [3]. As a result, the area between the pores decreases, ensuring that particles are guided adequately towards the pores. ...
Here, we propose a universal technique to firmly capture dispersed microparticles on any desired two-dimensional array of through-pores on a surface. The 10 μm silica or polystyrene particles are reversibly captured, making them accessible for direct manipulation and inspection or subsequent transfer to other surfaces. To obtain perfect arrays with a pitch of 1.25 μm, perforated devices with profiled surfaces were required. Additionally, the method has proven successful for both types of particles, either dispersed in water or ethanol. The assembly technique may serve as a platform for manufacturing hierarchical materials, e.g., ordered chromatography packings, or performing cellular assays.
Characterization of the morphology, identification of patterns and quantification of order encountered in colloidal assemblies is essential for several reasons. First of all, it is useful to compare different self-assembly methods and assess the influence of different process parameters on the final colloidal pattern. In addition, casting light on the structures formed by colloidal particles can help to get better insight into colloidal interactions and understand phase transitions. Finally, the growing interest in colloidal assemblies in materials science for practical applications going from optoelectronics to biosensing imposes a thorough characterization of the morphology of colloidal assemblies because of the intimate relationship between morphology and physical properties (e.g. optical and mechanical) of a material. Several image analysis techniques developed to investigate images (acquired via scanning electron microscopy, digital video microscopy and other imaging methods) provide variegated and complementary information on the colloidal structures under scrutiny. However, understanding how to use such image analysis tools to get information on the characteristics of the colloidal assemblies may represent a non-trivial task, because it requires the combination of approaches drawn from diverse disciplines such as image processing, computational geometry and computational topology and their application to a primarily physico-chemical process. Moreover, the lack of a systematic description of such analysis tools makes it difficult to select the ones more suitable for the features of the colloidal assembly under examination. In this review we provide a methodical and extensive description of real-space image analysis tools by explaining their principles and their application to the investigation of two-dimensional colloidal assemblies with different morphological characteristics.
Optical tweezers have emerged as a powerful tool for the non-invasive trapping and manipulation of colloidal particles and biological cells1,2. However, the diffraction limit precludes the low-power trapping of nanometre-scale objects. Substantially increasing the laser power can provide enough trapping potential depth to trap nanoscale objects. Unfortunately, the substantial optical intensity required causes photo-toxicity and thermal stress in the trapped biological specimens³. Low-power near-field nano-optical tweezers comprising plasmonic nanoantennas and photonic crystal cavities have been explored for stable nanoscale object trapping4–13. However, the demonstrated approaches still require that the object is trapped at the high-light-intensity region. We report a new kind of optically controlled nanotweezers, called opto-thermo-electrohydrodynamic tweezers, that enable the trapping and dynamic manipulation of nanometre-scale objects at locations that are several micrometres away from the high-intensity laser focus. At the trapping locations, the nanoscale objects experience both negligible photothermal heating and light intensity. Opto-thermo-electrohydrodynamic tweezers employ a finite array of plasmonic nanoholes illuminated with light and an applied a.c. electric field to create the spatially varying electrohydrodynamic potential that can rapidly trap sub-10 nm biomolecules at femtomolar concentrations on demand. This non-invasive optical nanotweezing approach is expected to open new opportunities in nanoscience and life science by offering an unprecedented level of control of nano-sized objects, including photo-sensitive biological molecules.
The patterning of structural colored materials has a significant impact on various applications such as flexible displays, anti‐counterfeiting patches, colorimetric sensors, etc. Herein, a sessile microdroplet‐based writing board is presented to pattern magnetochromatic hydrogels with abundant structural colors and improved optical performance. It is demonstrated that predesigned hydrophilic patterns on a hydrophobic writing board can capture a mixture of polymer and Fe3O4@SiO2 magnetic nanoparticles inks with a spatial resolution of ≈100 pin per 1 cm2 while retaining magnetic field responsibility to the lower limit of 84 Gs. The inks are self‐partitioned into microdroplet arrays, which would in situ transform into structural colored hydrogels within a short time via thiol‐Michael addition. In contrast to conventional evaporation induced assembly of colloidal photonic crystals in sessile droplets, the resulting structural colored hydrogel microarrays show not only good stability and optical adjustability but tunable morphologies. In addition, the introduction of the microfluidic mixing and ink dispensing system greatly shortens the time interval from the polymer mixing to sessile droplet generation, circumvents the challenge of short operation time for the self‐crosslinking ink components, and enables the direct handwriting of high quality structural colored patterns.
This research study focuses on wall fouling and electrostatic charging in gas–solid fluidized beds. Experiments were conducted with glass bead particles with different mass median diameters in an acrylic column at different humidity levels. The coverage ratio of particles on a wall was measured with two different methods. To obtain the local coverage ratio, the number of particles was counted with digital image processing. To achieve the required average coverage ratio, all the particles which adhered to the wall were weighed. The electrostatic charges of these particles and in the dense phase of the fluidized beds were measured individually with the use of a Faraday cup combined with a vacuum device. The surface potential of the wall was also measured with an electrostatic potential meter. The coverage ratio of the particles was high at low humidity because of the electrostatic attraction that was affected considerably by the surface potential of the wall compared with the surface charge density of the particles. The relationship between the wall fouling and electrostatic charging could be explained based on (a) the charge that the wall provided to the particles according to the triboelectric charging and (b) the number of particles that acquired the charge.
It is well-known that polymer brushes can degraft in aqueous liquids. Here, we show that brushes can deteriorate in humid air too. We observe that the detachment rate of the brushes increases with increasing relative humidity and hydrophilicity of the brushes. We relate this to the increase in water absorption upon increasing these parameters. Our results imply that protective measures - that are at present being developed for application of brushes in liquid - will also be key in enabling the long-term storage and utilization of hydrophilic brushes in air.
The rapid progress in flexible electronic devices has attracted immense interest in many applications, such as health monitoring devices, sensory skins, and implantable apparatus. Here, inspired by the adhesion features of mussels and the color shift mechanism of chameleons, a novel stretchable, adhesive, and conductive structural color film is presented for visually flexible electronics. The film is generated by adding a conductive carbon nanotubes polydopamine (PDA) filler into an elastic polyurethane (PU) inverse opal scaffold. Owing to the brilliant flexibility and inverse opal structure of the PU layer, the film shows stable stretchability and brilliant structural color. Besides, the catechol groups on PDA impart the film with high tissue adhesiveness and self‐healing capability. Notably, because of its responsiveness, the resultant film is endowed with color‐changing ability that responds to motions, which can function as dual‐signal soft human‐motion sensors for real‐time color‐sensing and electrical signal monitoring. These features make the bio‐inspired hydrogel‐based electronics highly potential in the flexible electronics field.
Triboelectric charging of small particles is a common problem in industrial processes because it can lead to various undesired consequences such as electric discharges or surface adhesion. Despite the many studies on the phenomenon, there remains significant gaps in understanding regarding the charge transfer mechanisms responsible for triboelectric charging, impeding the ability to make predictions regarding a contact between two dielectric materials. An experimental setup measuring the charge generated during a single impact between a dielectric particle and a dielectric target under well controlled conditions is used to investigate the triboelectric charging of a glass bead against polymer targets. Mechanical parameters such as the normal and tangential velocities and their influence over the charge generated at impact are studied statistically against a Polytetrafluoroethylene (PTFE) and a Polyurethane (PU) target, and two different behaviours are exhibited. Additional tests were performed on beads of different diameter in order to enrich the observations previously made on mechanical properties.
Particle (monolayer) assembly is essential to various scientific and industrial applications, such as the fabrication of photonic crystals, optical sensors, and surface coatings. Several methods, including rubbing, have been developed for this purpose. Here, we report on the serendipitous observation that microparticles preferentially partition onto the fluorocarbon-coated parts of patterned silicon and borosilicate glass wafers when rubbed with polydimethylsiloxane slabs. To explore the extent of this effect, we varied the geometry of the pattern, the substrate material, the ambient humidity and the material and size of the particles. Partitioning coefficients amounted up to a factor 12 on silicon wafers, and even ran in the 100’s on borosilicate glass wafers at zero humidity. Using Kelvin Probe Force Microscopy, the observations can be explained by tribo-electrification, inducing a strong electrostatic attraction between the particles and the fluorocarbon zones, while the interaction with the non-coated zones is insignificant or even weakly repulsive.
Processes based on electrostatic projection are used extensively in industry, e.g., for mineral separations, electrophotography, or manufacturing of coated abrasives, such as sandpaper. Despite decades of engineering practice, there are still unanswered questions. In this paper, we present a comprehensive experimental study of the projection process of more than 1500 individual spherical alumina particles with a nominal size of 500μm, captured by high-speed video imaging and digital image analysis. Based on flight trajectories of approximately 1100 projected particles, we determine the acquired charge and dynamics as a function of the relative humidity (RH) and the electric field intensity and compare the results with classical theories. For RH levels of 50% and above, more than 85% of distributed particles are projected, even when the electric field intensity is at its minimum level. This suggests that, beyond a critical value of the electric field intensity, the RH plays a more critical role in the projection process. We also observe that the charging time is reduced dramatically for RH levels of 50% and above, possibly due to the buildup of thin water films around the distributed particles, which can facilitate charge transfer. In contrast, projected particles at the 30% RH level exhibit excessive amounts of electric charge, between 2 and 4 times than that of the saturation value, which might be attributed to triboelectric charging effects. Finally, the physics of electrostatic projection is compared and contrasted with those of induced-charge electrokinetic phenomena, which share similar field-square scaling, as the applied electric field acts on its own induced charge to cause particle motion.