Session C59: Rheology and Flow of Soft Matter I

Rheology of glassy and jammed emulsions

We study the rheology of monodisperse emulsions with various mean droplet sizes. Above a critical volume fraction, these systems will exhibit solid-like 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 nm-5 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.
  

Viscous fingering instabilities in carbon black gels

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 non-Newtonian fluid, the non-linear 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 time-dependent 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 Saffman-Taylor instability at the fluid-air interface. The resulting pattern in the gel consists in a tree-like branched structure, whose wavelength surprisingly follows the scaling established for Newtonian fluids. The signature of the fluid non-linear 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.   

Microrheology of pH-Responsive Nanoparticle Monolayers at Fluid Interfaces

The effect of shear on the structure and dynamics of polyelectrolyte-grafted nanoparticles (PGNP) straddling at a water–oil interface was investigated through mesoscale simulations. Using electrostatic dissipative particle dynamics, long-range 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 long-range 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 inter-particle forces at high particle concentration result in the collective motion of domains to repair the adjacent defect. The in-plane 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 small-amplitude oscillatory shear and the results were correlated with the detailed structure evolution.
  

Microscopic Rearrangements in the Flow of Polydisperse Dense Emulsions

We study the flow of dense polydisperse quasi-two-dimensional 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.   

Rheological and Microstructural studies of semi-dense and dense suspensions in a Periodic Poiseuille Flow using Core-Modified Dissipative Particle Dynamics

Shear-thickening is a nonlinear rheological behaviour often observed in semi-dense and dense suspensions at increasing shear rates and is commonly associated with microstructural changes in the material. Therefore, understanding the rheology-microstructure correlation has a fundamental importance in the development of many industrial and technological processes. Core-Modified Dissipative Particle Dynamics was employed as a computational method to capture the physics involved in the flow of semi-dense 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 shear-thickening 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.   

Active Microrheology in Emulsion Glass

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 oil-in-water emulsion system. We apply a well-defined 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 semi-quantitative agreement.
  

Tuning friction and rheology at material-nanoparticle-liquid interfaces with an external electric field.

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 electro-tunable control of their flow in liquid. We report the use of electrophoretic forces to tune friction and rheology at material-nanoparticle-liquid 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 electro-tunable 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 glass-like time scales. The studies also reveal that nanoparticles manipulated by electric fields can act as "cantilever-free" atomic force probes capable of “tapping mode” exploration of interfacial properties and nanoscale interactions in geometries inaccessible to optical and micromechanical probes.   

Understanding the effect of confinement on the viscosity of bacterial suspensions

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.
  

Upstream vortex and elastic wave in the viscoelastic flow around a confined cylinder

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 three-dimensional 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.   

Glassy Dynamics in a Simulated Cell Monolayer with Division and Death

Recent experiments have indicated that quasi two-dimensional 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 particle-based 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 vertex-type model of motile tissue modified to include a balanced rate of cell division and death. The division and death rate competes with the motility-driven 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 motility-dominated and division/death-dominated rheology.   

Reversibility and diffusion in a meso-scale model for amorphous under cyclic shear.

We present results on a meso-scale 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 long-time diffusion. This behavior is consistent with what is seen in experiments on 2D amorphous particle rafts and confocal microscopy of emulsions and in athermal quasi-static particle-based simulations. We further show that, in the large amplitude regime, the steady state single-cycle plastic strain field is localized along line-like 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.   

Front Microrhelogy of the non-Newtonian behaviour of blood

We introduce a new framework to study the non-Newtonian behaviour of fluids at the microscale based on the analysis of front advancement. We apply this methodology to study the non-linear rheology of blood in microchannels. We carry out experiments in which the non-linear viscosity of blood samples is quantified at different haematocrits and ages. Under these conditions, blood exhibits a power-law 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 alpha-thalasemic blood samples with the bending rigidity of the erytrocyte membrane [3].

[1] C. Trejo-Soto et al. Soft Matter, 2017, 13, 3042
[2] G. R. Lazaro et al. Soft Matter, 2014, 10, 7195
[3] C. Trejo-Soto et al. Preprint, 2018   

Coarse-graining amorphous plasticity: shear-banding and rejuvenation

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.