Session A28: Microscale Flows: General

Formation of a colloidal band via pH-dependent electrokinetics

Electroosmosis on non-uniformly charged surfaces often induces intriguing flow behaviors, which can be utilized in applications such as mixing processes and designing micromotors. Here, we demonstrate non-uniform electroosmosis induced by electrochemical reactions. Water electrolysis creates pH gradients near the electrodes that cause a spatiotemporal change in the wall zeta potential, leading to non-uniform electroosmosis. Such non-uniform electroosmotic flows induce multiple vortices, which promote continuous accumulation of particles that subsequently form a colloidal band. The band develops vertically into a “wall” of particles that spans from the bottom to the top surface of the chamber. Such a flow-driven colloidal band can be potentially used in colloidal self-assembly and separation processes irrespective of the particle surface properties. For instance, we demonstrate these vortices can promote rapid segregation of soft colloids such as oil droplets and fat globules.   

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Colloidal particles experience directed motion when placed in an imposed solute concentration gradient, a phenomenon known as diffusiophoresis. Recent advances in microfluidic techniques helped uncover many of the fundamental aspects of these out-of-equilibrium, electrokinetic flows, as well as propose a variety of potential applications for them, in particular in the fields of particle separation, concentration, or screening. [1] Microfluidic chips with continuously reactive side walls have been shown to constitute interesting platforms for water purification through diffusiophoretic exclusion, but the stationary nature of the reaction limits their versatility. [2] In this work, we realize reactive walls by embedding photocatalytic particles in a hydrogel matrix. [3] Thus, we can switch on or off the reaction through the shining of UV light, creating gradients of solutes at the walls at will, and controlling the diffusiophoretic motion of the particles nearly instantaneously. We take advantage of this new technique to generate temporally- and spatially-dependent particle concentration profiles in linear and circular channel geometries.

[1]        S. Shin, Phys. Fluids 32, 101302 (2020).

[2]        H. Lee, J. Kim, J. Yang, S. W. Seo, and S. J. Kim, Lab Chip 18, 1713 (2018).

[3]        H.-J. Koo and O. D. Velev, J. Mater. Chem. A 1, 11106 (2013).

    

Diffusiophoresis in a Taylor-dispersing solute

We consider coupled solute and particle dynamics in a long, narrow, two-dimensional channel flow, where the motion of each species is the result of advection, diffusion, and diffusiophoresis. We describe the evolution of the solute and particle concentration fields at early times, distinguishing this work from existing studies that consider one-dimensional or long-time behaviors. We use a lubrication approximation to describe the particle diffusiophoresis that is driven by a solute concentration field experiencing Taylor dispersion in a background Poiseuille flow. Theoretical predictions are based on the assumption of constant zeta potential. To validate these results, we then compare the theoretical predictions at a fixed zeta potential with numerical simulation results that consider variable zeta potential, which show good agreement. Finally, we comment on the assumption of constant zeta potential for such problems.   

Improving efficiency by enhancing mass transfer at CO reduction cathodes

Transforming CO₂ to commodity chemicals to close the carbon cycle is possible through first converting CO₂ to CO in a solid oxide cell, then reducing the CO to multicarbon products like ethylene. However, CO reduction electrolysis suffers from poor energy efficiency due to high full cell voltages, low reactant conversion rates, and parasitic reactions which decrease product selectivity.  These losses are most prominent at the cathode side, which comprises the carbon fiber gas diffusion layer and a thick, porous catalyst layer containing copper catalyst and Teflon nanoparticles to balance hydrophilicity and hydrophobicity. We have built a 2D computational model that fully couples the kinetics of multiple electrochemical reactions, ion transport, mass transfer in porous media by viscous and diffusive mechanisms, and an overall material balance. Changing key parameters such as the flow rate, pressure, temperature, and catalyst loading has allowed us to identify tradeoffs between figures of merit like faradaic efficiency and CO conversion. High energy efficiency can be achieved when a high partial pressure of CO and high CO conversion are simultaneously maintained. We will also present a sensitivity analysis and model validation against experimental results.   

Multi-scale dynamics of semi-dilute and concentrated suspensions of dielectric particles in an electric field.

The multi-scale responses of the semi-dilute and concentrated suspensions of dielectric particles in an electric field are studied via a large-scale Stokesian dynamics simulation. These suspensions also share similarities with electrorheological (ER) fluids. Here, we discuss the concentration-dependent mechanisms of structural formation by probing the temporal evolution of the particle kinetics and suspension dynamics at different length scales. Below the transition concentration of 30%, we observe the well-known chain formation and subsequent slow coarsening process. Around the transition concentration, the steady-state structures show minimum anisotropy but maximum micro-and mesoscopic heterogeneity compared to other concentrations. Above that concentration, however, the suspensions directly undergo the formation of mesoscopic structures. Lastly, we investigate the effect of confinement on the suspension. It is shown that the confinement alters the structures primarily at the mesoscopic level rather than the microscale, especially at the moderately concentrated regime where the screening effect is still not dominant.   

Near-wall effects on suspended particle electrokinetics

Colloidal polystyrene particles in a very dilute (< 0.5 vol%) suspension become attracted towards, and accumulate in, the high shear regions near the wall, when the particles are subject to pressure and voltage gradients along the same direction.  This behavior is qualitatively similar to inertial migration, although the particles lead the flow in this case due to electrophoresis. Estimates based on tracking tracer particles that are about 1% of the radius a = 250 nm particles imply that near-wall particle concentrations grow exponentially by a factor of 100-200 as the particles are attracted to the wall, increasing the near-wall concentration to at least 25 vol%.  The time scales for accumulation suggest that the attractive force is comparable to that predicted by recent models of phoretic or weakly inertial lift.  Measurements of streamwise near-wall particle velocities, even when the average interparticle spacing exceeds 20a, differ significantly from the sum of the flow and particle electrophoretic velocities.  If the flow velocity is equal to what would be predicted by theory, these results suggest that wall effects suppress particle electrophoresis by a factor of 2-3, in disagreement with models that predict that wall effects enhance electrophoresis.   

Microscale hydrodynamic cloaking and shielding via electro-osmosis

We demonstrate theoretically and experimentally that injection of momentum in a region surrounding an object in microscale flow can yield both 'cloaking' conditions, where the flow field outside the cloaking region is unaffected by the object, and 'shielding' conditions, where the hydrodynamic forces on the object are eliminated. The experimental setup is based on a cylindrical obstacle in a Hele-Shaw cell exposed to pressure-driven flow. Momentum injection is performed using field-effect electro-osmosis in a region around the obstacle. We present a theoretical framework employing a shape-perturbation approach and analytical solutions for a range of geometrical shapes of the obstacle. Good agreement between experiments and theory is found. We also demonstrate the ability to dynamically switch between the cloaking and shielding states, which corresponds to a control principle with real-time adaptivity.   

Visualization and analysis of 3-dimensional AC electroosmotic flow in an elongated droplet

When an alternating voltage is applied between two parallel electrodes, the charge induced on the electrode surface causes a slip velocity in the direction perpendicular to the electrodes, regardless of the polarity of voltage. This flow has been extensively studied as an alternating current electroosmotic flow(AC EOF), and its typical form is paired vortices on the electrodes. Due to its handful control in microchannel, AC EOF has been employed as the core mechanism of trapping and aligning particles in the electrolyte, or generating a manipulated flow as a micropump using asymmetric electrodes. However, two-dimensional approximation is often invalid for AC EOF within droplets where 3-dimensional constriction is dominant. In this study, we visualized three-dimensional motions of AC EOF in an electrolyte existing in the form of elongated droplets on symmetric planar electrodes and conducted a 3D numerical simulation with liquid-gas interface. In this system, we demonstrated the generation of flow in the direction parallel to the electrodes, which cannot be predicted by two-dimensional assumption and analyzed the cause of the flow.   

Manipulating and characterizing individual bio-particles in nanochannels

Manipulation of matter at the nanoscale and at the single entity level is of utmost importance in a large variety of fields ranging from basic sciences to biology and medicine. It is particularly appealing in order to develop an understanding of complex entities such as viruses, characterize them and assert their biological/chemical function when interacting with other entities in biological fluids. Modern and successful approaches mostly exploit configurations where a strong gradient of field provides a stable attracting potential in space, generated by tightly focused propagating fields or in the near field of antennas. However, due to the volume dependence of their polarizability, sub-100 nm nano-objects require strong and potentially harmful field intensities.

In this work, we transport individual nano-objects in biological level ionic solutions to select locations in on-chip nanochannels using electrokinetic nanovalves. In-between two such nanovalves the nano-object is confined and we study its judiciously restricted thermal motion. The confined particle dynamics is analyzed and provides important properties at the individual nano-object level such as the diffusion coefficient, hydrodynamic diameter, trap stiffness and electrical conductivity of the individual particles. We show the versatility of our system by assessing the properties of polystyrene nanospheres, conjugated polymer nanoparticles and adenoviruses.

The collaborative effect of the applied alternating current (AC) electric field between nanoelectrodes and a designed and fabricated nanochannel topography locally amplifies the near-field gradient, forming together a harmonic trap between a nanovalves system with no-moving-parts effective for various dielectric nanoparticles. This work is an important advancement in manipulating and characterizing individual nano-objects in biologically relevant ionic solutions, including novel medical nanomaterials, or specific viruses.