Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session F03: Microinertia Effects in Particulate FlowsFocus

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Sponsoring Units: GSOFT Chair: Antony Beris, Univ of Delaware Room: LACC 150C 
Tuesday, March 6, 2018 11:15AM  11:51AM 
F03.00001: Inertial effects on the stress generation in active matter Invited Speaker: John Brady Suspensions of selfpropelled bodies generate a unique mechanical stress owing to their motility that impacts their largescale collective behavior. For microswimmers suspended in a fluid with negligible particle inertia, we have shown that the virial swim stress is a useful quantity to understand the rheology and nonequilibrium behaviors of active soft matter systems. For larger selfpropelled organisms such as fish, it is unclear how particle inertia impacts their stress generation and collectivemovement. Here we analyze the effects of finite particle inertia on the mechanical pressure (or stress) generated by a suspension of selfpropelled bodies. We find that swimmers of all scales generate a unique swim stress and Reynolds stress that impact their collective motion. We discover that particle inertia plays a similar role as confinement in overdamped active Brownian systems, where the reduced run length of the swimmers decreases the swim stress and affects the phase behavior. Although the swim and Reynolds stresses vary individually with the magnitude of particle inertia, the sum of the two contributions is independent of particle inertia. This points to an important concept when computing stresses in computer simulations of nonequilibrium systems: The Reynolds and the virial stresses must both be calculated to obtain the overall stress generated by a system. 
Tuesday, March 6, 2018 11:51AM  12:03PM 
F03.00002: The rotation of a nonspherical particle in simple shear flow Tomas Rosen, Minh DoQuang, Cyrus Aidun, Fredrik Lundell A single particle suspended in a flow with parallel streamlines is typically subject to a simple shear flow, which induces a rotational motion of the particle. The angular dynamics of an ellipsoidal particle in a simple shear flow was analytically derived by Jeffery [1] and shows that the axisymmetric ellipsoid (i.e. spheroid) is rotating in closed degenerate orbits. However, the derivation of these solutions assumes inertia of both fluid and particle to be negligible, i.e. the particle Reynolds number Re_{p} equals zero. Here, we will present results of a neutrally buoyant particle in simple shear flow and show how the degeneracy of the Jefferyorbits is broken due to inertia. Through direct numerical simulations and linear stability analysis, we are able to predict transitions between stable rotational states for spheroids ranging from thin discs to moderately slender fibers [2]. Furthermore, we find that fluid inertia is always dominating over particle inertia for neutrally buoyant particles at low Re_{p}. At moderate Re_{p}, particle inertia overcomes viscous damping, which increases the dimensionality of the dynamical system and can lead to chaotic rotational states. 
Tuesday, March 6, 2018 12:03PM  12:15PM 
F03.00003: Hydrodynamic Synchronization of Externally Driven Colloids along a Straight Path Takashi Taniguchi, Kosuke Teshigawara, John Molina, Ryoichi Yamamoto, Norihiro Oyama The collective dynamics of externally driven N_{p}colloidal systems (1 ≤ N_{p} ≤ 4) in a confined viscous fluid have been investigated using threedimensional direct numerical simulations with fully resolved hydrodynamics. The dynamical modes of collective particle motion are studied by changing the particle Reynolds number as determined by the strength of the external driving force and the confining wall distance. For a system with N_{p} = 3, we found that at a critical Reynolds number, a dynamical mode transition occurs from the doubletsinglet mode to the triplet mode, which has not been reported experimentally. The dynamical mode transition was analyzed in detail from the following two viewpoints: (1) spectrum analysis of the time evolution of a tagged particle velocity and (2) the relative acceleration of the doublet cluster with respect to the singlet particle. For a system with N_{p} = 4, we found similar dynamical mode transitions from the doubletsingletsinglet mode to the tripletsinglet mode and further to the quartet mode. 
Tuesday, March 6, 2018 12:15PM  12:27PM 
F03.00004: Macroscopic modeling of microinertia effects in particulate flows Paul Mwasame, Antony Beris, Norman Wagner A new approach to introduce microinertia into macroscopic models of particulate flows, based on an internal conformation tensor structural variable, is developed using the NonEquilibrium Thermodynamics bracket formalism (Beris and Edwards, Thermodynamics of Flowing Systems, Oxford U. Press, 1994). The proposed approach is first applied to a dilute emulsion with the structural tensor parameter physically identified with the deformed ellipsoidal geometry of the dispersed phase. In addition to a new kinetic term in the Hamiltonian, microinertia effects also introduce a new nonaffine term that couples the conformation and the vorticity tensors in the evolution equation for the conformation tensor. This derivation and resultant constitutive equation provide a new pathway to rigorously incorporate microinertia into general, conformation tensorbased, macroscopic models for multiphase systems. The model is consistent with previously developed in the literature asymptotic theories in the limit of small capillary, Ca, as well as small particle Reynolds, Re, numbers. These asymptotic solutions are also used to uniquely determine all the model parameters. Additional, more recent, applications to concentrated suspensions are also going to be outlined. 
Tuesday, March 6, 2018 12:27PM  12:39PM 
F03.00005: Shearinduced gradient diffusivity of emulsions at finite inertia Abhilash Reddy Malipeddi, Kausik Sarkar The gradient diffusivity due to shearinduced diffusion in moderately concentrated emulsions is calculated for the first time using a direct numerical simulation. A concentrated layer of viscous drops, when subjected to shear, showed its thickness to increase with onethird exponent of time. The diffusivity is calculated using this relation as a function of capillary number. The results in Stokes limit are in good agreement with experimental observations and theoretical predictions in the literature. Note that in contrast to rough rigid sphere suspensions, the effort to compute gradient diffusivity in the dilute limit for deformable particles (drops, vesicles, cells etc.) by integrating over all pairwise trajectories leads to divergent integrals. The dependence of diffusivity on capillary number and inertia is explained by analyzing the effect on the shape of the drops and their pairwise interactions. 
Tuesday, March 6, 2018 12:39PM  12:51PM 
F03.00006: Effects of microinertia on average normal stress differences of a concentrated emulsion Kausik Sarkar, Priyesh Srivastava, Abhilash Reddy Malipeddi A viscous emulsion of drops typically experiences a positive first normal stress difference and a negative second normal stress difference in shear. However, our group has shown that small amount of particle inertia can change their signs (Li & Sarkar 2005, J. Rheol., 49, 1377). The result stems from the inertia induced increase in drop inclination angle. We have performed direct numerical simulation to investigate the phenomenon both in dilute limit and moderate concentration (527% volume fraction). The computed rheological properties (effective shear viscosity and first and second normal stress differences) in the Stokes limit match well with previous theoretical (Choi–Schowalter in the dilute limit) and simulated results (for concentrated systems) using the boundary element method. The critical Reynolds number for sign reversal increases with concentration and can be explained by relating it to dropdrop interactions and specifically to the contact pairdistribution function. The functional dependence of excess stresses with Reynolds number, capillary number, viscosity mismatch and volume fraction will be described. 
Tuesday, March 6, 2018 12:51PM  1:03PM 
F03.00007: Integration of inertial microchannels and droplet generators for controlled encapsulation of single cells Hamed Haddadi, Dino Di Carlo In this presentation, we describe a method for controlled encapsulation of single cells in microdroplets by combining straight inertial microchannels and flowfocusing droplet generators. Under fluid mechanical inertia, particles migrate toward inertial equilibrium positions inside the microchannel and form ordered trains of particles (cells). First we use microfluidic experiments and discreteparticle latticeBoltzmann simulations to describe the pair trajectory attractors which lead to formation of trains with preferred spacing between particles. Then we illustrate that train formation can be utilized to modulate the entry frequency of cells into a flow focusing droplet generator. Considering the inertia of the cellladen fluid, we control the wallconfinement in the orifice to segment the suspending fluid into uniform size droplets without forming stable or undulating jets. We show the capability of inertial microfluidic systems in controlled encapsulation of single cells in droplets for various cancer cell lines and human bonemarrow stem cells suspended in Newtonian Phosphate Buffered Saline (PBS) and weakly viscoelastic pregel fluids. 
Tuesday, March 6, 2018 1:03PM  1:15PM 
F03.00008: The Solid to Fluid Transition of a Granular Flow in a Powder Rheometer HanHsin Lin, Melany Hunt We study the transition from solidlike to fluidlike behavior of a granular material in a powder rheometer using sand and glass beads. The yielding stress and the volume fraction change before reaching the steady state; the free surface also changes its shape. A burping sound can be heard as the shear stress increases due to the change in the free surface and the pressure increasing. Based on the minimum stress ratio, the flow transits through the inertial region to the quasistatic region. A continuum model to determine the inertial region and quasistatic region inside the bulk is given. From the model, we specify the boundary between inertial zone and quasistatic zone. By this model, we estimate how the strain rate corresponds to the dilatant behavior locally. 
Tuesday, March 6, 2018 1:15PM  1:27PM 
F03.00009: Microscopic fluctuations in a sheared liquid studied using a complex plasma ChunShang Wong, John Goree, Zach Haralson In a liquid undergoing a laminar shear flow, there are microscopic fluctuations in viscous heating and in entropy production. These fluctuations include brief violations of the second law of thermodynamics, which have been predicted to obey the fluctuation theorem. To demonstrate this theorem experimentally, we overcome several technical challenges including measuring viscous heating on molecular scales. We do this using an analog system, with imaging to track the motion of electrically charged polymer microspheres suspended in a weaklyionized gas, i.e. a dusty plasma, also known as a complex plasma. This method has much in common with 2D charged colloid experiments. Our microspheres are in a single 2D layer that never buckles. Their motion is underdamped, so that it is possible drive a shear flow in the collection of microspheres using laser manipulation, without any flow of the background gas. Our particle tracking data allow us to calculate fluctuations in the microscopic shear stress and a time series for the microscopic entropy production rate. With these observables, we are able to experimentally confirm the fluctuation theorem of Evans et al. PRL 1993. This talk is based on Wong et al. Nat. Phys. (2017). DOI:10.1038/nphys4253. 
Tuesday, March 6, 2018 1:27PM  1:39PM 
F03.00010: Enhancing Controlled Colloidal Migration Through Engineering SolutoInertial Synergy Anirudha Banerjee, Ian Williams, Todd Squires The interactions and forces that are relevant at the colloidal scale are well understood. However, our ability to control colloidal behavior is limited by the fact that equilibrium interactions are typically restricted to the micron scale, and often much less. We demonstrate various strategies to circumvent this limitation, using millimeter scale, nonequilibrium suspension interactions enabled by solutoinertial (SI) "beacons". These beacons establish and maintain longlived, nonequilibrium solute fluxes in solution that drive colloidal particles to migrate via diffusiophoresis (DP). We explore the consequences of the SI phenomenon and demonstrate strategies that employ combinations of suitably engineered beacons that work harmoniously to enhance DP migration of particles. The synergy between multiple beacons increases the interaction range, imparts directionality, enhances migration velocity and prolongs equilibration. The versatility of the SI phenomena highlighted here suggests new possibilities for sorting and separating colloidal mixtures, targeted particle delivery, and enhancing the rate of suspension flocculation, especially in dilute suspensions where even commercial flocculants would take much longer to form colloidal aggregates. 
Tuesday, March 6, 2018 1:39PM  1:51PM 
F03.00011: A random Tetris model for analyzing the flow mechanism in a hopper discharging hard discs through an adjustable obstacle GuoJie Gao We propose a random Tetris model to study harddisc particles passing through a hopper including an obstacle placed on its centerline. The distance between the obstacle and the exit of the hopper is tunable. Governed by a Gaussian displacement function, each particle can freely move perpendicular to the obstacleexit direction and towards the exit. Our model rejects a particle’s movement if it creates overlaps between the particle and others, the obstacle, or boundaries of the hopper. Unlike the results in the studies using molecular dynamics simulations, where including an obstacle can increase or decrease the hopper flow rate, our model reveals a monotonically increasing flow rate as we place the obstacle further from the exit of the hopper. We also show that if the variance of the Gaussian displacement function of a particle increases as the particle keeps updating its position successfully, the flow rate always increases as long as the obstacle is close to the exit within a critical range. This can be explained by a freefly zone forming between the obstacle and the exit. However, if we place the obstacle outside the critical range, the freefly zone presumably could turn into a jammingprone zone which causes the flow rate to decrease. 
Tuesday, March 6, 2018 1:51PM  2:03PM 
F03.00012: Traction Force Rheology: a new technique to understand the mechanics of colloidal solids Zsolt Terdik, David Weitz, Frans Spaepen We present recent results on the structural and mechanical response of colloidal solids in response to externally imposed shear strain. We introduce a new technique, traction force rheology, that combines confocal and traction force microscopy. Using this technique, we simultaneously visualize the structural changes occurring within a colloidal solid during elastic and plastic deformation, and directly measure the mechanical response of the colloidal solid. We present stressstrain curves, measured insitu, combined with full visualization of the mechanisms occurring within the colloidal solid at every point along the stressstrain curve. For colloidal polycrystals, these mechanisms include dislocation dynamics, grain boundary sliding, and grain refinement; for colloidal glass we visualize shear transformations. Experimental details, challenges, and current results will be discussed. 
Tuesday, March 6, 2018 2:03PM  2:15PM 
F03.00013: Colloidal crystals on a wire: global curvature constraints at finite temperature William Wilkin, Nabila Tanjeem, Vinothan Manoharan, Christopher Rycroft A two dimensional crystal on the surface of a cylindrical wire is frustrated by a global commensurability constraint, and as a result typically incorporates an extended one dimensional chiral lineslip defect in its ground state. Using both experiment and simulation we investigate the kinetics of crystal growth and evolution at finite temperature, and find that single crystal regions have a characteristic length determined by minimal stability against thermal fluctuations and random stress transmitted across grain boundaries. In addition, we observe a new class of chiral defects at finite temperature that roughen the line slip and modify the effective interactions between line slip defects and grain boundaries. 
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