Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session C59: Rheology and Flow of Soft Matter IFocus

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Sponsoring Units: GSOFT GSNP Chair: LiChiun Cheng, Massachusetts Institute of Technology Room: BCEC 257B 
Monday, March 4, 2019 2:30PM  2:42PM 
C59.00001: Rheology of glassy and jammed emulsions Cong Cao, Eric Weeks We study the rheology of monodisperse emulsions with various mean droplet sizes. Above a critical volume fraction, these systems will exhibit solidlike behaviors and possess a yield stress. Previous simulation work suggests that for small thermal particles, rheology will see a glass transition; for large athermal systems, rheology will see a jamming transition. However, at the crossover of thermal and athermal regimes, the glass and jamming transitions may be well separated and observed at different volume fractions for a fixed mean droplet size. We conduct an experiment by shearing different sizes of emulsion droplets (500 nm5 μm diameter) with a rheometer. In this way, we measure rheological properties near the critical volume fraction. By varying the shear rate and particle size, our experiments cover a wide range of Péclet number (the ratio of shear and thermal motions), including the crossover regime. 
Monday, March 4, 2019 2:42PM  2:54PM 
C59.00002: Viscous fingering instabilities in carbon black gels Thibaut Divoux, Badis Marsit, Yacouba Kaloga, Irmgard Bischofberger Pattern formation in fluids occurs in numerous physical processes in which mechanical mixing, chemical reactions, evaporation and/or surface effects play a key role. When the pattern develops in a nonNewtonian fluid, the nonlinear rheology interferes with the patterning process, which often generates a richer dynamics than that commonly observed for a Newtonian fluid. Here we focus on the viscous fingering instability in a timedependent yield stress fluid. We study experimentally the flow of a carbon black gel sandwiched in a parallel plate geometry, for which the upper plate is being lifted up at constant velocity. We show the existence of a critical initial gap spacing and a critical lift velocity, above which the flow becomes unstable, leading to the growth of viscous finger originating from the SaffmanTaylor instability at the fluidair interface. The resulting pattern in the gel consists in a treelike branched structure, whose wavelength surprisingly follows the scaling established for Newtonian fluids. The signature of the fluid nonlinear rheology lays in the spatial extent of the pattern, which is governed by the yield strain of the gel. Finally, we show that varying the shear history of the gel, one can produce patterns with a wealth of new morphologies. 
Monday, March 4, 2019 2:54PM  3:06PM 
C59.00003: Microrheology of pHResponsive Nanoparticle Monolayers at Fluid Interfaces Shiyi Qin, Xin Yong The effect of shear on the structure and dynamics of polyelectrolytegrafted nanoparticles (PGNP) straddling at a water–oil interface was investigated through mesoscale simulations. Using electrostatic dissipative particle dynamics, longrange electrostatic forces are solved in a triclinic simulation box, which allows us to apply continuous shear deformation to the monolayer. The monolayers with different particle coverages were examined under shear and degree of ionization of PGNPs were set to be high. While undisturbed, the particles arrange themselves on a hexagonal lattice due to the longrange electrostatic interactions. As the monolayer with low particle concentration is subjected to the shear flow, the free voids allow particles to move in the shear direction. However, strong interparticle forces at high particle concentration result in the collective motion of domains to repair the adjacent defect. The inplane structure of monolayer is analyzed by structure factor and Voronoi diagram. The dynamics of local domains are observed through snapshots and averaged velocity contours. The rheology measurements of the monolayer were also performed under smallamplitude oscillatory shear and the results were correlated with the detailed structure evolution. 
Monday, March 4, 2019 3:06PM  3:18PM 
C59.00004: Microscopic Rearrangements in the Flow of Polydisperse Dense Emulsions Yonglun Jiang, Carlos S Orellana, Eric Weeks We study the flow of dense polydisperse quasitwodimensional emulsions. In particular, we are interested in highly polydisperse samples with the largest droplets as much as ten times the size of the smallest. The droplets are confined between two parallel glass plates and driven to flow by a syringe pump. We use video microscopy to examine how rearrangements differ for large and small droplets. In particular, we find the large droplets follow the mean flow, while the small droplets move more erratically. We try to quantify these results by looking at the nearest neighbor changes and nonaffine motions.We explore different means of defining affine motion, including definitions based on nearest neighbors and definitions based on regions of space. The former varies quite a bit depending on the size of droplets, whereas the latter is simpler but lacks a sensitivity to droplet size. 
Monday, March 4, 2019 3:18PM  3:30PM 
C59.00005: Rheological and Microstructural studies of semidense and dense suspensions in a Periodic Poiseuille Flow using CoreModified Dissipative Particle Dynamics Erika Barcelos, Shaghayegh Khani, Arman Boromand, Mônica Naccache, Joao Maia Shearthickening is a nonlinear rheological behaviour often observed in semidense and dense suspensions at increasing shear rates and is commonly associated with microstructural changes in the material. Therefore, understanding the rheologymicrostructure correlation has a fundamental importance in the development of many industrial and technological processes. CoreModified Dissipative Particle Dynamics was employed as a computational method to capture the physics involved in the flow of semidense and dense suspensions subjected to a periodic Poiseuille flow at two confinement ratios. The interplay between hydrodynamic and frictional interactions in promoting shear thickening was investigated as well as the microstructure evolution of those systems at increasing shear rates. Velocity profiles were found to increase with the Péclet number and the shearthickening response was stronger and took place sooner for dense suspensions at narrower gap sizes. The microstructure followed the rheological trend, clusters of particles being bigger in size for denser and more confined systems. 
Monday, March 4, 2019 3:30PM  3:42PM 
C59.00006: Active Microrheology in Emulsion Glass Nesrin Senbil, Markus Gruber, Chi Zhang, Matthias Fuchs, frank Scheffold Microscopic observations of probe particles under passive or force driven motion provides unique insights into the dynamics of colloidal dispersions. Here, we study experimentally the motion of polystyrene probe particles seeded in a micron scale oilinwater emulsion system. We apply a welldefined constant force on the probe particles via a gradient intensity laser line trap and determine the displacements and probability distributions at various forces in the fluid and glass. Over the range studied, our emulsion droplets acts like hard spheres displaying a jamming and glass transition at 64% and 59% packing fractions, respectively. Both PS particles and emulsion droplets are sterically stabilized and identical in size. The crossover from localized to delocalized behavior happens at a threshold force which highly depends on the composition and corresponding cage strength (in the glass) and cage relaxation (in the fluid). Our experiments reveal intermittent dynamics and bimodal van Hove distribution functions around a depinning transition at a threshold force. For smaller forces, linear response connects the mean displacement and the quiescent mean squared displacement. We compare our observations to Mode coupling theory and find qualitative and semiquantitative agreement. 
Monday, March 4, 2019 3:42PM  3:54PM 
C59.00007: Tuning friction and rheology at materialnanoparticleliquid interfaces with an external electric field. Biplav Acharya, Caitlin M Seed, Donald W Brenner, Alex I Smirnov, Jacqueline Krim Nanoparticles, both with and without polymeric surface coatings, dispersed in solutions are in common use as rheological and friction modifiers. The surface functionalization of the nanoparticles therefore provides a unique opportunity for active electrotunable control of their flow in liquid. We report the use of electrophoretic forces to tune friction and rheology at materialnanoparticleliquid interfaces with static or low frequency (0.6 – 50 mHz) electric fields. Negatively charged TiO2 or positively charged Al2O3 nanoparticles suspended in water were repositioned relative to a planar platinum surface of a quartz crystal microbalance, which was then used to monitor friction levels. Active electrotunable control of friction was achieved, and investigated as a function of electric field frequency. Kinetic effects corresponding to nanoparticle repositioning at the solid interface were discovered to occur at glasslike time scales. The studies also reveal that nanoparticles manipulated by electric fields can act as "cantileverfree" atomic force probes capable of “tapping mode” exploration of interfacial properties and nanoscale interactions in geometries inaccessible to optical and micromechanical probes. 
Monday, March 4, 2019 3:54PM  4:06PM 
C59.00008: Understanding the effect of confinement on the viscosity of bacterial suspensions Zhengyang Liu, Kechun Zhang, Xiang Cheng Bacterial suspensions, a premier example of active fluids, show intriguing rheology different from their counterpart colloidal suspensions. When the confinement length scale is comparable or smaller than the intrinsic length scale of bacterial suspensions, we expect to see a qualitative change of their flow behaviors. Here, by using a microfluidic channel viscometer, we investigate the viscosity of E. coli suspensions under different degrees of confinement. For low concentration suspensions, we observe strong shear thickening at low flow rates. More importantly, a strong confinement effect is found when the confinement length is comparable to the running length of single bacterium. The viscosity of bacterial suspensions decreases by a factor of 2.5, when the confinement decreases from 50 down to 25 microns. In contrast, for high concentration suspensions, we observe strong shear thinning at low shear rates. The confinement leads to an increase rather than decrease of viscosity when the confinement length is comparable to the size of collective swarming of bacterial suspensions. Our study reveals the dynamics of bacterial suspensions under confinement and provides new insights into the transport of active fluids in confined geometries. 
Monday, March 4, 2019 4:06PM  4:18PM 
C59.00009: Upstream vortex and elastic wave in the viscoelastic flow around a confined cylinder Paul Salipante, Boyang Qin, Steven Hudson, Paulo E. Arratia The flow of a viscoelastic fluid past a cylinder is a classic benchmark problem that is not completely understood. Using novel holographic particle velocimetry to measure 3D flow fields, we report two main discoveries of the elastic instability upstream of a single cylinder in confined microchannel flow. After the onset of corner vortices upstream of the cylinder, we find that the vortex becomes unsteady and switches between two distinct flow states, leading to symmetry breaking perpendicular to the cylinder axis that is highly threedimensional in nature. Second, we show that the disturbance of the elastic instability propagates far upstream via an elastic wave and is weakly correlated with that in the cylinder wake. The wave speed and the extent of the instability increase with Weissenberg number, indicating an absolute instability in viscoelastic fluids. 
Monday, March 4, 2019 4:18PM  4:30PM 
C59.00010: Glassy Dynamics in a Simulated Cell Monolayer with Division and Death Michael Czajkowski, Daniel Sussman, M. Cristina Marchetti, M. Lisa Manning Recent experiments have indicated that quasi twodimensional epithelial tissues undergo a transition to a rigid state which shares many characteristics with traditional particulate glass. At the same time, numerical work has suggested that the presence of cell division and death in dense particlebased models for tissues will always destroy signatures of glassy dynamics such as caging. Can then glassy behavior be recovered in real tissues where cells commonly divide and die? We address this question with a vertextype model of motile tissue modified to include a balanced rate of cell division and death. The division and death rate competes with the motilitydriven rate of cell rearrangement to control the tissue dynamics. We show that glassy dynamics is recovered for slow division and death rates. Further quantifying the displacements coming from a single division or death event, we are able to accurately predict the crossover between motilitydominated and division/deathdominated rheology. 
Monday, March 4, 2019 4:30PM  4:42PM 
C59.00011: Reversibility and diffusion in a mesoscale model for amorphous under cyclic shear. Kareem Abdelshafy, Botond Tyukodi, Damien Vandembroucq, Craig Maloney We present results on a mesoscale model of amorphous plasticity subject to cyclic shear. We show that, after a transient, depending on the amplitude of cycling, the steady state behavior falls into one of three cases, in order of increasing strain amplitude: i) pure elastic behavior with cessation of all plastic activity, ii) reversible periodic plasticity with the period being an integer number of strain cycles (not necessarily a single cycle), and iii) irreversible plasticity with longtime diffusion. This behavior is consistent with what is seen in experiments on 2D amorphous particle rafts and confocal microscopy of emulsions and in athermal quasistatic particlebased simulations. We further show that, in the large amplitude regime, the steady state singlecycle plastic strain field is localized along linelike structures similar to the ones seen during avalanches during steady plastic shear, but there is little persistence in the localization from one cycle to the next. 
Monday, March 4, 2019 4:42PM  4:54PM 
C59.00012: Front Microrhelogy of the nonNewtonian behaviour of blood Aurora HernandezMachado We introduce a new framework to study the nonNewtonian behaviour of fluids at the microscale based on the analysis of front advancement. We apply this methodology to study the nonlinear rheology of blood in microchannels. We carry out experiments in which the nonlinear viscosity of blood samples is quantified at different haematocrits and ages. Under these conditions, blood exhibits a powerlaw dependence on the shear rate. In order to analyse our experimental data, we put forward a scaling theory which allows us to define an adhesion scaling number. By applying this scaling theory to samples of different ages, we are able to quantify how the characteristic adhesion energy varies as time progresses [1]. We also analyze numerically the rheology of dilute red blood cell suspensions in pressure driven flows at low Reynolds number in terms of the elasticity of the cells. We identify the relevant aspects of cell elasticity that contribute to the rheological response of blood [2]. We have related the viscosity of healthy, anemic and alphathalasemic blood samples with the bending rigidity of the erytrocyte membrane [3]. 
Monday, March 4, 2019 4:54PM  5:06PM 
C59.00013: Coarsegraining amorphous plasticity: shearbanding and rejuvenation Botond Tyukodi, Armand Barbot, Reinaldo Garcia Garcia, Matthias Lerbinger, Sylvain Patinet, Damien Vandembroucq The plastic behavior of glasses and disordered solids displays a rich and complex phenomenology, from scale free avalanches to localization of the strain field in shear bands. The recent years have seen an increasing effort of modeling of these phenomena at different length and time scales. In complement of numerous numerical simulations at atomistic scale of model or more realistic glasses, a new class of lattice models has emerged at mesoscopic scale that rely on the coupling between a local threshold dynamics and elastic interactions. The latter "Eshelby" interaction is associated to the internal stress induced by local rearrangeùment taking place in the surrounding elastic matrix and is characterized by a quadrupolar symmetry. While these lattice models reproduce most of the phenomenology observed in amorphous plasticity, a quantitative link remains to be done with atomistic sipmulations. Here we use a technique recently developed to characterize local yield stress in atomistic simulations to propose and study the quantitative connection between simulations operationg at atomistic and mesocopic scales. 
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