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 H36: Microscale Flows: Particles |
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Chair: Michael Loewenberg, Yale University Room: 618 |
Monday, November 25, 2019 8:00AM - 8:13AM |
H36.00001: Fiber buckling in confined viscous flows Anke Lindner, Jean Cappello, Olivia du Roure, Camille Duprat, Mathias Bechert, François Gallaire Studying fluid structure interactions in confined flows is important to understand locomotion of micro-organisms in soils or medical conducts as well as the movement of long fibers in fractures, where they are used as in-situ probes for example in oil recovery. Here we look at the dynamics of a model system, constituted by flexible fibers freely transported in pressure-driven flows in a Hele-Shaw cell. Fiber height is comparable to channel height and in this very confined geometry the fiber dynamics are dominated by viscous friction with top and bottom walls. We focus on the dynamics of fibers oriented parallel to the flow direction and show that a buckling instability occurs under certain conditions. The fibers deform into a wavy shape resembling a wave-packet, with a well-defined dominant wavelength. Such an instability is triggered by a competition between viscous forces and elasticity and is observed only for long fibers, at least one order of magnitude larger than the observed wavelength. We characterize experimentally the instability and show that the wavelength of the deformation is proportional to an elasto-viscous length. We furthermore study the growth rate of the instability for different fiber geometries, flow strength, and mechanical properties of the fibers. [Preview Abstract] |
Monday, November 25, 2019 8:13AM - 8:26AM |
H36.00002: Colloidal particle migration within microchannels in combined Poiseuille and electrokinetic flows Shaurya Prakash, Varun Lochab Recent discoveries show that dilute colloidal particle suspensions with diameters \textless 1$\mu m$ are assembled to distinct colloidal bands within microchannels (100 -- 300$\mu m$ wide x 34$\mu m$ deep x 4cm long). Band formation requires opposing Poiseuille and electrokinetic flows. Band formation also requires a minimum applied potential threshold at a given shear rate. Band formation is a function of particle size and volume fraction, particle and channel wall zeta potential, electrolyte concentration, and the minimum electric field thresholds change non-monotonically for particle mixtures. Here, we discuss the broad parameter ranges to elucidate robustness of particle migration to and away from walls, formation of particle bands, and influence of fluid properties on particle migration and band formation. Interestingly, in co-flow with Poiseuille and electrokinetic flows, particles migrate away from the microchannel walls with aggregation near the bulk of the microchannel. Colloidal particle migration away from or towards the microchannel walls is likely due to the particle slip velocity with respect to the fluid. The particle migration is attributed to an electrophoretic lift-like force, analogous to the inertial lift forces in sedimentation flows. [Preview Abstract] |
Monday, November 25, 2019 8:26AM - 8:39AM |
H36.00003: Particle motion nearby rough surfaces Christina Kurzthaler, Amir Pahlavan, Lailai Zhu, Howard A. Stone Hydrodynamic interactions of particles with surrounding surfaces play a pivotal role in a variety of biological systems and microfluidic applications. Here, we study the motion of an externally driven particle through a viscous fluid along different boundaries that are characterized by periodic and rough surface shapes. We derive analytical expressions for the translational velocities of the particle using the Lorentz reciprocal theorem and find that the particle follows the surface shape, as anticipated by the time-reversal symmetry of Stokes flow. Moreover, our theoretical framework permits the statistical analysis of random rough surfaces, which do not affect particle motion on average but manifest themselves in the dispersion of particle trajectories. Our results are supported by numerical simulations using a boundary integral method and experimental observations. Overall, they should lay the foundation for our future understanding of microswimmer motion nearby random, heterogeneous boundaries. [Preview Abstract] |
Monday, November 25, 2019 8:39AM - 8:52AM |
H36.00004: Hydrodynamic interactions between permeable particles Rodrigo Reboucas, Michael Loewenberg An analysis is presented for pairwise hydrodynamic interactions between permeable spherical particles in the limit of weak permeability, $K=k/a^{2} \ll 1$, where $k$ is the permeability, and $a$ is the reduced radius of the particles. Except for the near-contact motion of the particles, hydrodynamic interactions can be approximated by smooth spheres, the permeability having a weak perturbative effect under these conditions. However, non-zero particle permeability qualitatively affects the near-contact axisymmetric motion, eliminating the classical lubrication singularity for smooth spherical particles. An integrodifferential lubrication equation incorporating Darcy’s law for flow inside the particles describes the near-contact motion of permeable spheres under weak permeability conditions. Collision efficiencies and hydrodynamic diffusivities are calculated and it is shown that permeable spherical particles are hydrodynamically equivalent to rough spheres with roughness $\delta/a \approx K^{2/5}$. [Preview Abstract] |
Monday, November 25, 2019 8:52AM - 9:05AM |
H36.00005: Spreading of diffusiophoretic colloids under transient solute gradients: Super-diffusion, trapping and shuttling Henry Chu, Stephen Garoff, Robert Tilton, Aditya Khair Diffusiophoresis (DP) refers to the deterministic drift of one species induced by a concentration gradient of another species. Recent microfluidic experiments have focused on DP of micron-scale colloids in gradients of small ionic solutes. A solute concentration gradient results in a DP colloid velocity \textbf{\textit{u}}$_{\mathrm{DP}} \quad = \quad M\nabla $log$S$, where $M$ and $S$ are the DP mobility and solute concentration, respectively. The mobility $M$ can be positive or negative, corresponding to DP driving colloids up (solute-attracting) or down (solute-repelling) the solute gradient, respectively. Here, we calculate the advective-diffusive spreading of DP particles under transient solute gradients, highlighting novel transport phenomena for microscale sorting, deposition, and delivery of colloids. We show that evolution of an initial point source of colloids depends critically on the ratio of the DP mobility to solute diffusivity, with behavior ranging from spatial trapping for ``solute-attracting'' colloids, to long-time super-diffusion for ``solute-repelling'' colloids. Finally, a solute undergoing advective translation is shown to rapidly shuttle the colloids. [Preview Abstract] |
Monday, November 25, 2019 9:05AM - 9:18AM |
H36.00006: Inertial bifurcation of the equilibrium position of a neutrally-buoyant circular cylinder in shear flow between parallel walls Andrew Fox, James Schneider, Aditya Khair The dynamics of a neutrally-buoyant, rigid circular cylinder in shear flow between planar, parallel walls are quantified at various particle Reynolds numbers $Re_p$ and confinement ratios $\kappa$ via lattice Boltzmann simulations. An inertial lift force acting transverse to the ambient shear flow has a single zero crossing at the center of the channel below a critical $Re_p$, corresponding to a single stable transverse equilibrium position. Above the critical $Re_p$, the equilibrium position bifurcates, with a unstable equilibrium position at the center and two additional stable equilibria equidistant off-center. Trajectories of a force- and torque-free particle confirm the equilibrium position bifurcation, showing the cylinder reaches the center equilibrium position below the critical $Re_p$ and the off-center equilibria above; the stable equilibrium position is independent of the initial cylinder position, with the lone exception of the aforementioned unstable equilibrium. The critical $Re_p$ dependent on the confinement ratio, and thus particle size, and occurs below the transition to unsteady flow. [Preview Abstract] |
Monday, November 25, 2019 9:18AM - 9:31AM |
H36.00007: inFocus: Fast Inertial Lift Velocity Calculation In Arbitrary Geometry Samuel Christensen, Raymond Chu, Chris Anderson, Marcus Roper Inertial microfluidic devices use inertial lift forces to arrange particles into required positions and formations. However, fully realized three dimensional simulations of inertial focusing require long computation times, making predictive device design very difficult. Here we use a combination of asymptotic methods and computational fluid dynamics to develop a fast, open-source fluid flow solver in Matlab that calculates particle dynamics in channels with arbitrary geometry. We use the new solver to dissect the contributions of shear and wall curvature to particle focusing with the goal of developing mechanistic understanding of how, where, and with what speed particles focus under inertial forces. [Preview Abstract] |
Monday, November 25, 2019 9:31AM - 9:44AM |
H36.00008: High Frequency Inertial Particle Focusing Giridar Vishwanathan, Dianzhuo Wang, Gabriel Juarez Inertial Focusing in micro-channels is a simple and reliable means of sorting, separating and controlling particle position, usually accomplished by producing steady flow in a long micro-channel. Recently, oscillatory flows have been shown to enable focusing of sub-micron particles, in much shorter channel lengths and at decreased pressure gradients even for frequencies $<20$ Hz. Considering the substantial improvement of focusing efficiency even at relatively low oscillation frequencies, we present our experimental observations on the focusing of particles in the high frequency ($20-1000$ Hz) range. The role of the channel Womersley number on the focusing performance is critically examined. [Preview Abstract] |
Monday, November 25, 2019 9:44AM - 9:57AM |
H36.00009: Solute-Driven Colloidal Particle Manipulation in Continuous Flows Past Microstructured channels. Guido Bolognesi, Goran T. Vladisavljevic, Francois Nadal, Cecile Cottin-Bizonne, Christophe Pirat, Naval Singh Recent advancements in the chemical and biological analysis have led to the integration of colloidal particle manipulation capabilities into microfluidic devices. Various active techniques have been employed for particle operations. The ability to manipulate particles by diffusiophoresis -- a phoretic phenomenon leading to particle motion along a solute concentration gradient without the use of an external field -- has gained an increasing attention. The aim of this study is to explore diffusiophoresis to enable particle filtration, trapping, and accumulation within a microfluidic environment under continuous flow settings. A microchannel, made of an optical adhesive and fitted with a micro-structured wall, was fabricated by photo-/soft-lithography. The charged fluorescent colloidal particles were accumulated within the channel microstructures by pumping electrolyte solutions into the device junction to generate salt concentration gradients. The spatial distribution of particles was characterized via confocal microscopy. This novel approach of particle handling in lab-on-a-chip device by solute driven transport can unlock potential applications in point of care industry, drug delivery and biosensing. [Preview Abstract] |
Monday, November 25, 2019 9:57AM - 10:10AM |
H36.00010: Numerical Study of a particle migration in a liquid-liquid stratified flow in a microchannel. T. Krishnaveni, T. Renganathan, S. Pushpavanam Inertial focusing is a passive separation technique in an axial flow where particles migrate laterally to equilibrium positions in the presence of finite inertia. These equilibrium positions mainly depend on the two counteracting forces namely the wall lift force and the shear gradient force. The equilibrium positions can be altered by changing the velocity profile of the fluid. ~In this work, we model the inertial focusing of a particle in the laminar, liquid-liquid stratified flow in a microchannel. This is used for the separation and recovery of cells from one fluid to the other without using any membrane.~~The particle is sent through one fluid and due to the shear gradient force and interfacial force, the particle may focus into the other fluid or stuck at the interface. A flow between infinite parallel plates is considered to study the particle migration. An immersed boundary method coupled with the level set method is used to study the hydrodynamics and the particle dynamics.~It is observed that the particle focus in the low viscous fluid beyond a critical flowrate ratio of two liquids. The effect of other parameters like phase holdup, Reynolds number, Weber number, particle size and wettability is analyzed on the particle migration. [Preview Abstract] |
Monday, November 25, 2019 10:10AM - 10:23AM |
H36.00011: Spontaneous Lift of Particles Through Inhomogeneous Slip Surfaces in Lubricated Contacts Aidan Rinehart, Ugis Lacis, Thomas Salez, Shervin Bagheri We reveal how inhomogeneous surfaces can accomplish wear reduction and increased mobility for particles traveling near walls. We consider a model problem of a cylinder near a wall where the surface (cylinder or wall) has non-homogeneous slip properties. We demonstrate that through variations in surface slip length the lubrication pressure symmetry is broken. Using lubrication theory we provide the analytical solutions to the hydrodynamic force and torques acting on the cylinder. Using numerical simulations, we also report various particle trajectories arising from inhomogeneous slip surfaces, including migration, oscillation, and self-propulsion. We find a linear scaling between wall normal migration and slip length, $\Delta \sim l$, for wall parallel motion over a slip to no-slip wall transition. These findings are relevant especially for micro-fluidic and biological systems where particles typically reside in close proximity to walls. [Preview Abstract] |
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