Bulletin of the American Physical Society
72nd Annual Meeting of the APS Division of Fluid Dynamics
Volume 64, Number 13
Saturday–Tuesday, November 23–26, 2019; Seattle, Washington
Session G36: Microscale Flows: Electrokinetics |
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Chair: Jae Sung Park, University of Nebraska Room: 618 |
Sunday, November 24, 2019 3:48PM - 4:01PM |
G36.00001: Time Periodic Electroosmotic Flow in Cylindrical Microchannel with Heterogeneous Surface Charge Aminul Khan, Prashanta Dutta Mixing of species is very slow in microfluidic device due to creeping flow. Time periodic electroosmotic flow (EOF) has been used for faster mixing in microdevice by rapid stretching and folding of fluid streams. Furthermore, surface inhomogeneities are also explored to expedite the mixing process. Although several analytical models exist for each individual case, there is no analytical solution for time-periodic EOF in a heterogeneously charged microchannel. In this work, a general analytical model has been developed for time-periodic EOF through cylindrical microchannel by solving Navier-Stokes equation with slip velocity conditions at the channel wall. Results show that the axial variation of surface charge yields diverse flow patterns containing counter-rotating vortices. The extent and strength of vortices are characterized by channel size, charge distribution and the period of electric field. As the electric field frequency or channel diameter increases, vortices are shifted towards the channel surface and the perturbed flow region confined near the channel wall. Also, the number of vorticities depends on the periodicity of the surface charge. Our analytical model can be used for effective micromixer design by manipulating the surface charge pattern. [Preview Abstract] |
Sunday, November 24, 2019 4:01PM - 4:14PM |
G36.00002: Particle trapping in merging flow junctions by fluid-solute-colloid-boundary interactions Sangwoo Shin, Jesse Ault, Amy Shen Merging of different streams in channel junctions represents a common mixing process that occurs in systems ranging from soda fountains and bathtub faucets to chemical plants and microfluidic devices. Here, we report a sudden trapping of colloidal particles in a merging flow junction when the merging streams have a salinity contrast. We show experimentally and numerically that the particle trapping is a consequence of complex interactions between diffusioosmosis, diffusiophoresis, and the freestream flow. A delicate balance of these transport processes results in a stable vortex near the junction that traps the particles in various modes depending on the flow conditions. We use 3-D particle visualization and numerical simulations to provide a rigorous understanding of the observed particle trapping phenomenon. The trapping mechanism we identify is unique from the well-known inertial trapping that is enabled by the vortex breakdown as the current particle trapping can occur even at Reynolds number below unity. Our study demonstrates a good example of how nonlinear, coupled fluid-solute-colloid-boundary dynamics can result in peculiar particle behavior in simple flow systems. [Preview Abstract] |
Sunday, November 24, 2019 4:14PM - 4:27PM |
G36.00003: Non-intuitive behavior in concentrated suspension of ideally polarizable particles in an electric field Siamak Mirfendereski, Jae Sung Park Large-scale numerical simulations are used to analyze the dynamics of ideally polarizable particles in concentrated suspensions under the effects of nonlinear electrokinetic phenomena. Particles are assumed to carry no net charge and considered to undergo the combination of dielectrophoresis and induced-charge electrophoresis termed dipolophoresis. The suspension dynamics seems to be hindered up to semi-dilute regimes by the increase in the magnitude of excluded volume interactions. Interestingly, a non-intuitive suspension behavior is observed in concentrated regimes, where the hydrodynamic diffusivity starts to increase with volume fraction and reach a local maximum before decreasing as approaching random close packing. This behavior is rationalized through an examination of the velocity fluctuations, suspension microstructure, and number-density fluctuations. We conclude that the non-intuitive behavior is attributed to a consequence of particle contacts, depending on the dominant mechanism of particle paring. While contacts are expected to occur along the field direction in dilute or semi-dilute regimes, very strong and massive contacts along the direction perpendicular to the applied field arise, promoting the non-intuitive behavior observed in concentrated regimes. [Preview Abstract] |
Sunday, November 24, 2019 4:27PM - 4:40PM |
G36.00004: Diffusiophoresis in Multivalent Electrolytes Jessica L. Wilson, Suin Shim, Ankur Gupta, Howard A. Stone Diffusiophoresis is the spontaneous movement of colloidal particles in a concentration gradient of ions. As a small-scale phenomenon that harnesses energy from concentration gradients, diffusiophoresis may prove useful for passively manipulating particles in lab-on-a-chip type applications. Though naturally occurring ions are often multivalent, experimental studies on diffusiophoresis have been mostly limited to monovalent electrolytes. In this work, we investigate the motion of negatively charged polystyrene particles in one-dimensional salt gradients for a variety of multivalent electrolytes. Our results indicate that the valence combination of cation and anions significantly impacts the diffusiophoretic mobility of the particles. In addition, the ion valence also modifies the ambipolar diffusivity, which in turn influences the motion of the particles by changing the timescale at which the concentration gradients evolve. We also develop a 1D model and obtain a good agreement between our experimental and modeling results. Our results are applicable to systems where the chemical concentration gradient is made up of multivalent ions, and motivate future research to manipulate particles by exploiting ion valence. [Preview Abstract] |
Sunday, November 24, 2019 4:40PM - 4:53PM |
G36.00005: Electrophoresis of Colloids via Asymmetric Rectified Electric Fields Timothy Hui, Joshua Lau, S.M.H. Hashemi Amrei, Gregory Miller, William Ristenpart Charged particles suspended in a dilute electrolyte solution exhibit complex aggregation and levitation behaviors in response to applied oscillatory fields. In particular, an experimentally observed bifurcation in the particle height over an electrode was recently shown to be qualitatively consistent with a force balance between gravity and an electrophoretic force due to the Asymmetric Rectified Electric Fields (AREF) that occur in electrolytes with unequal mobilities. Here, we elaborate on the dynamics of particle electrophoresis in response to AREFs. Using a combination of optical and confocal fluorescence microscopy, we measure the colloidal particle levitation and aggregation dynamics as a function of the applied field properties and electrolyte composition. We show that the dynamics are broadly consistent with numerical calculations of the AREF driving force, and we discuss the implications for precise control and electrophoretic trapping of particles using oscillatory fields. [Preview Abstract] |
Sunday, November 24, 2019 4:53PM - 5:06PM |
G36.00006: Nonlinear concentration waves in current monitoring method for measurement of electroosmotic flow Mohammad Babar, Kaushlendra Dubey, Supreet Singh Bahga Current monitoring method for measurement of electroosmotic flow (EOF) in a microchannel involves displacement of a binary electrolyte initially filled in the channel by a similar electrolyte but having a different conductivity, under the effect of electric field. A fixed voltage is applied across the channel ends and the temporal change in current is measured. Because a conductivity gradient in a binary electrolyte migrates only due to EOF and not due to electromigration, the time taken by one electrolyte to displace another gives an estimate of EOF. This displacement time is usually independent of whether a high-conductivity electrolyte displaces a low-conductivity electrolyte or vice versa. However, few studies have reported that, for low-conductivity electrolytes, the displacement time depends on whether the displacing electrolyte has higher or lower conductivity than the displaced electrolyte. In this study, we show that this directional dependence of displacement time is a result nonlinear concentration waves, such as shock and rarefaction waves, that form when surface conduction is comparable with bulk conduction. We present analytical expressions for current-time relationship in this regime and validate them with experimental observations. [Preview Abstract] |
(Author Not Attending)
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G36.00007: Temperature-gradient induced enhanced load bearing capacity of lubricated systems Siddhartha Mukherjee, Sunando DasGupta, Suman Chakraborty Lubricated systems, which are pretty common in several engineering and biological settings, perform on the basis of their load bearing performances. In recent years, a good volume of research has been directed to strategize the performance enhancement of such devices by coupling interfacial phenomenon like electrokinetics with the substrate deformability. However, the deformation characteristics and subsequent load carrying capacity of such systems in presence of thermal gradient have not been explored. Here, we have unveiled that by exploiting a unique coupling of electrokinetics, externally applied thermal gradient, hydrodynamics and substrate flexibility, one can modulate the load bearing capacity of a planar slider bearing considerably. Our studies reveal that the alteration in the electrical potential distribution upon application of temperature gradient and its influence on the resulting hydrodynamics, coupled with substrate compliance, may give rise to significant enhancement in load capacity. We envisage that this analysis may construct a new window in the context of improved design of lubricated systems where the interplay between thermal, electrokinetic and hydrodynamic aspects can be coupled together. [Preview Abstract] |
Sunday, November 24, 2019 5:19PM - 5:32PM |
G36.00008: Electric Field Induced Instabilities in Viscosity Stratified Miscible Microflows: Transition from Linear Instabilities to Coherent Vortices Satarupa Dutta, Partho Sarathi Gooh Pattader, Dipankar Bandyopadhyay We experimentally demonstrate three distinctive regimes of instabilities in a pressure-driven flow of a pair of viscosity stratified dielectric miscible fluids, inside a microchannel, upon application of electric field -- a linear-onset regime, a time-periodic non-linear regime with the generation of vortices qualitatively similar to the von K\'{a}rm\'{a}n vortex street, and eventually a coherent regime which leads to the mixing of the two fluids. A linear stability analysis reveals the occurrence of five finite wavenumber modes of instabilities. The electro-hydrodynamic stresses originating upon application of electric field stimulate a pair of shorter-wavelength electric field modes E-I and E-II beyond a critical value of electric Rayleigh number. For higher viscosity difference between the fluids, the relative longer wavelength viscous mode (V-mode) appears. Beyond a critical Schmidt number, a diffusive mode (D-mode) appears, which is qualitatively similar to the interfacial instabilities of the immiscible fluids. The K-mode of instability appears due to contrast in ionic mobility values. The reported phenomenon can be harnessed for microscale mixing, pumping, heat and mass transfer, and reaction engineering. [Preview Abstract] |
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