Bulletin of the American Physical Society
71st Annual Meeting of the APS Division of Fluid Dynamics
Volume 63, Number 13
Sunday–Tuesday, November 18–20, 2018; Atlanta, Georgia
Session M24: Microscale Flows: Phoretic Effects 
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Chair: Federico Toschi, Eindhoven University of Technology Room: Georgia World Congress Center B312 
Tuesday, November 20, 2018 8:00AM  8:13AM 
M24.00001: CO_{2} driven diffusiophoresis and water cleaning: similarity solutions for predicting the exclusion zone in a channel flow Suin Shim, Mrudhula Baskaran, Howard A. Stone We investigate diffusiophoretic separation of negatively charged particles in a rectangular channel flow, driven by CO_{2} dissolution from one wall. Since the negatively charged particles create an exclusion zone near the boundary where CO_{2} is introduced, we model the problem by applying a shear flow approximation in a 2D configuration. From the form of the equations we define a similarity variable to transform the reactiondiffusion equations and particle distribution equations to ODEs. The definition of the similarity variable suggests a characteristic length scale for the particle exclusion zone. We consider heightaveraged and different orientation of the flow behaviors in rectangular channels to rationalize and connect our experimental observations with the model. Our observations and the theoretical model provide design parameters such as flow rate, channel dimensions and CO_{2} pressure for the inflow water cleaning systems. 
Tuesday, November 20, 2018 8:13AM  8:26AM 
M24.00002: Boosting CO_{2}driven diffusiophoresis of colloidal suspensions by permeation Orest Shardt, Sangwoo Shin When carbon dioxide dissolves into an aqueous colloidal suspension, the resulting transient pH gradients drive diffusiophoresis of the suspended particles. We investigate the effects of CO_{2} permeation on the extent of particle motion in a model system consisting of a suspensionfilled pore in a gaspermeable polymer. Pure CO_{2} is applied to both ends of the pore and the outer surface of the polymer is either sealed (preventing steady leakage of CO_{2}) or open to the atmosphere, in which case a steady CO_{2} (and pH) distribution is established. For both cases, we compute the transient CO_{2} distributions (in the water and polymer) and the diffusiophoretic migration of suspended particles. For the second (open) case, we also determine the steady particle distribution. Loss of CO_{2} into the surrounding material increases the magnitude and duration of the pH gradient and therefore boosts the migration of the particles. We evaluate the effects of the thickness of the polymer around the pore and the permeability of the polymer to CO_{2} (the product of the solubility and diffusivity of CO_{2} in the polymer). An optimal diffusivity maximizes the displacement of the particles. The numerical predictions are consistent with experiments in poly(dimethylsiloxane) blocks with varying thicknesses. 
Tuesday, November 20, 2018 8:26AM  8:39AM 
M24.00003: The Effect of Electrolyte on Diffusiophoresis in OneDimensional Salt Gradients Jessica L. Wilson, Suin Shim, Ankur Gupta, Howard A Stone We study the diffusiophoretic motion of negatively charged particles in one dimensional salt gradients using a deadend pore geometry and various salt solutions. We estimate the diffusiophoretic velocities for particles for a range of background electrolytes such as multivalent and asymmetric (nonz:z) electrolytes. Theoretical investigations of asymmetric electrolyte diffusiophoresis support our findings that the choice of electrolyte significantly influences the movement of particles, and in particular the chemiphoretic component of the diffusiophoretic velocity. By combining theoretical and experimental analysis, we present the relative importance of electrophoretic and chemiphoretic contributions in electrolyte solutions. 
Tuesday, November 20, 2018 8:39AM  8:52AM 
M24.00004: Colloidal accumulation in flow junctions induced by fluid flow and dissolved solutes Sangwoo Shin, Jesse T Ault, Patrick B Warren, Howard A Stone The flow of solutions containing solutes and colloidal particles in porous media is widely found in systems including underground aquifers, hydraulic fractures, estuarine or coastal habitats, water filtration systems, etc. In such systems, solute gradients occur when there is a local change in the solute concentration. While the effects of solute gradients have been found to be important for many applications, we observe an unexpected colloidal behavior in porous media driven by the combination of solute gradients and the fluid flow. When two flows with different solute concentrations are in contact near a junction, a sharp solute gradient is formed at the interface, which may allow strong diffusiophoresis of the particles directed against the flow. Consequently, the particles accumulate near the pore entrance, rapidly approaching the packing limit. These colloidal dynamics have important implications for the clogging of a porous medium, where particles that are orders of magnitude smaller than the pore width can accumulate and block the pores within a short period of time. 
Tuesday, November 20, 2018 8:52AM  9:05AM 
M24.00005: Diffusiophoresis in a Weakly Viscoelastic Fluid Shabab Saad, Giovanniantonio Natale 
Tuesday, November 20, 2018 9:05AM  9:18AM 
M24.00006: Abstract Withdrawn Motivated by carbon capture and sequestration processes, we experimentally investigate the dynamics of salt precipitation as a gas injection into an initially fullysaturated microfluidic network with brine. Salt gradually precipitates as the porebrine slowly dries out. Based on the growth rates of the salt nucleation, the development of salt nucleation can be categorized into three stages: (i) initial slow, (ii) rapid growth, and (iii) final stages, which correspond to three different drying rates of the residual brine. Higher drying rate of brine leads to a rapid nucleation growth. In addition, the porosity of porestructure plays an important role that controls the correlation between brine drying and salt nucleation rates. Microfluidic networks with a low porosity show a higher drying rate and, hence, a greater nucleation growth rate. Two primary salt crystal structures—bulk crystal and polycrystalline aggregate—and their crystallization dynamics were analyzed. The capillary, wettability, and porosity effects on salt precipitation dynamics will be discussed. 
Tuesday, November 20, 2018 9:18AM  9:31AM 
M24.00007: Onchip microscale thermophoresis using MEMSfabricated fluidic channel Tetsuro Tsuji, Satoyuki Kawano A temperature difference in microscale spatial distance in fluid can produce a strong temperature gradient, which induces thermophoresis of dispersed micro/nano particles. Such a particle transport, microscale thermophoresis, is expected to develop a new particle manipulation technique used in micro and nanofluidic devices, since it has different physical characteristics from common methods such as electrophoresis. In the present study, we fabricate a microfluidic device with a micro heater using microelectromechanical systems (MEMS) technologies and demonstrate that the microscale thermophoresis effectively controls the flow of microparticles. In particular, we find that polystyrene and silica microparticles have opposite responses to the temperature gradient, realizing the selective particle transport in the microfluidic system. 
Tuesday, November 20, 2018 9:31AM  9:44AM 
M24.00008: Hydrodynamic fluctuations in quasitwo dimensional diffusion Florencio Balboa Usabiaga, Raúl P Peláez, Sergio Panzuela, Qiyu Xiao, Rafael DelgadoBuscalioni, Aleksandar Donev A common feature of many biological systems is the diffusion of particles in a twodimensional interface immersed in a threedimensional fluid. Think for example of proteins diffusing in a lipid membrane. It is known that in these systems the interplay between hydrodynamic interactions and Brownian fluctuations leads to anomalous diffusion, in which the collective diffusion coefficient diverges like the inverse of the wavenumber. Here, we use particle simulations and fluctuating hydrodynamics to study the diffusion of colloids in a simplified system, ideal 
Tuesday, November 20, 2018 9:44AM  9:57AM 
M24.00009: Brownian diffusion of wetted particle at fluctuating interface Xiao Xue, Mauro Sbragaglia, Luca Biferale, Federico Toschi We study the effects of thermally induced capillary waves on the diffusivity of a wetted particle at a fluctuating interface. Recent studies have shown that the wetted particle feels an unexpected viscous drag at the fluctuating interface which is larger than the one in the bulk. Our numerical investigations can address the problem by using a fluctuating multicomponent lattice Boltzmann (LB) model for nonideal multicomponent fluids, including nonequilibrium stochastic fluxes mimicking the effects of molecular forces at the nanoscales coupled with wetted finitesize particle. We will present results on the Brownian diffusion for the wetted colloids both in bulk and at the fluctuating interface. Beyond the practical importance of our findings for kinetically driven assembly of particles, our study also explores a novel application of LB with colloidfluid interactions in the realm of nanofluidic phenomena. 
(Author Not Attending)

M24.00010: Emergent Structures in Colloidal Suspensions Sedimenting Down an Incline Aleksandar Donev, Blaise Delmotte, JosephRiley Cruise, Michio Tanaka, Michelle M Driscoll, Paul Chaikin The presence of a nearby noslip boundary strongly affects the structures emerging in sedimenting colloidal suspensions. We combine experiments, theory and large scale numerical simulations to study the dynamics of colloidal suspensions sedimenting down an incline. By varying the inclination angle we observe different regimes of sedimentation. At low angles, when sedimentation is mostly directed toward the floor, the suspensions forms a monolayer with a dense traveling front. This traveling front can be described using a simple one dimensional nonlocal PDE. The front then transitions into fingerlike structures whose width depends on the particle size and height from the floor. As the inclination angle increases, more and more particles are lifted away from the floor due to hydrodynamic interactions. The particle height distribution becomes bimodal: a second layer of more rapid particles forms, the fingers then move faster and exhibit a larger wavelength. After a characteristic time, the fingers sediment back on the incline, only to be lifted again by the flows generated by the particles at the rear. This cycle leads to unusual and rich longtime dynamics. 
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