Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session W31: Rheology, Flow, Deformation, Fracture and Friction of Soft Matter |
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Sponsoring Units: DSOFT Chair: John Kolinski, Ecole Polytechnique Federale de Lausanne Room: 503 |
Friday, March 6, 2020 8:00AM - 8:12AM |
W31.00001: Hydrodynamic coupling to the electrical response of fluid suspensions of non-Brownian conducting particles Matthew Snell, Jeffrey Richards We report on the electrical response of conducting non-Brownian suspensions to applied steady shear. We show using dc-conductivity measurements in a rheo-dielectric device that a shear-rate dependent conductivity increases both with the shear intensity and the particle volume fraction. The conductivity increases instantaneously upon flow start-up and returns instantaneously and reversibly to the quiescent value upon flow cessation. For volume fractions exceeding 30 vol% microspheres, the ratio of the conductivity under flow to that in the quiescent state can exceed a factor of 105. We rationalize these results based on a simplistic scaling law that confirms the hydrodynamic forces couple to the interparticle electron transfer rate. These observations help to reconcile emerging experimental evidence for the role of particle mobility in determining electrical transport in colloidal fluids and suspensions and could prove a basis for new mechano-electric sensing modalities as well as improved electrochemical storage technologies. |
Friday, March 6, 2020 8:12AM - 8:24AM |
W31.00002: Rheological analysis of hydrocolloidal Basil seed mucilage in the context of seed physiology Bhuvsmita Bhargava, Jacob John, Susy Varughese Hydration of the seeds of the Ocimum basilicum develops a translucent mucilage around the seed that plays a vital role in its germination, adhesion and dispersal. This functionality stems from the structure of the mucilage, which comprises of adherent and non-adherent layers, that have varying concentrations of complex polysaccharides such as pectin, cellulose, and hemicellulose. Pectin crosslinks in the presence of Ca+2 ions to form hydrocolloidal gels. The interplay between the pectin and calcium concentrations and degree of esterification determines the nature and extent of crosslinking. However, the extent to which it contributes to the gel structure, its rheology and the resulting seed physiology is not completely understood. This work uses Large Amplitude Oscillatory Shear (LAOS) rheology as a tool to estimate the factors that affect crosslinking in the Basil seed mucilage. We observe that the mucilage behavior is gel-like, with a constant modulus plateau at low strains and shear thinning at higher strains. The non-linear signatures such as the loss modulus overshoot and strain stiffening observed for mucilage gels are compared with those of the pectin-Ca gels to estimate the extent of Ca+2 crosslinking and the degree of esterification of the pectin present in the mucilage. |
Friday, March 6, 2020 8:24AM - 8:36AM |
W31.00003: Rheology of glassy and jammed emulsions Cong Cao, jianshan liao, victor breedveld, Eric Weeks We study the rheology of monodisperse and polydisperse emulsions with various mean droplet sizes. Above a critical volume fraction φ, these systems exhibit solid-like behavior and possess a yield stress. Previous simulation work suggests that for small thermal particles, rheology will see a glass transition at φ≈0.58; for large athermal systems, rheology will see a jamming transition at φ≈0.63. However, for intermediate sized droplets at the crossover of thermal and athermal regimes, the glass and jamming transitions may both be observable in the same sample. We use a rheometer to shear different sizes of TPM emulsion droplets. 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. We successfully measure rheological properties near the critical volume fraction(s). We then further measure how their yield stress change with volume fraction. |
Friday, March 6, 2020 8:36AM - 8:48AM |
W31.00004: Athermal Shear-Driven Flow of Mixtures of Frictionless Rods and Disks in Two Dimensions Stephen Teitel, Theodore A Marschall We numerically simulate the athermal steady-state shearing of a mixture of frictionless circular disks and elongated spherocylinders (rods) in two dimensions, suspended in a host medium. We study behavior as the total packing fraction and the fraction of rods are varied, at slow shear strain rates. We find that the average angular velocity of the shear-driven rod rotations decreases, and the extent of the nematic ordering of the rods increases, as the fraction of the rods decreases. In the limit of a single rod in a sea of disks, the rod appears to cease rotating, except at very low packing fractions. We find that, as one shears from an initial random configuration, the system tends to form clusters of parallel contacting rods. We believe that such clusters form so as to reduce the pressure in the system, and that these clusters play an important role in the shear-driven rotation of the rods. |
Friday, March 6, 2020 8:48AM - 9:00AM |
W31.00005: Brownian motion in near-surface pressure driven flows with 3D-nanometric spatial resolution Alexandre Vilquin, Pierre Soulard, Vincent Bertin, Gabriel Guyard, David Lacoste, Elie Raphael, Frederic Restagno, Thomas Salez, Joshua McGraw In near-surface flows, interfaces play a major role by imposing (typically) no-slip boundary conditions, greatly reducing the fluid velocity compared to the central part of a channel. With total internal reflection fluorescence (TIRF), a flow is illuminated with an evanescent field decaying over a few hundred nanometers into the channel; this decay allowing a determination of nanoparticle altitudes. Combined with particle tracking, experimental determination of the velocity profile and local velocity distributions in three dimensions are possible. Here we present a detailed look at the statistics of near-surface particle motions in pressure-driven water for which diffusion is important compared to advection. The distribution of displacements in the invariant flow direction is Gaussian as for normal diffusion. Significant anomalies are however observed for both of the other spatial dimensions. Combining experiments and simulations, we disentangle contributions from so-called Taylor-Aris dispersion, nanoparticle polydispersity and the optical measurement system. This description of TIRF allows for the study of many Brownian motion problems, such as near-surface polymer solution dynamics or particle motion near soft boundaries. |
Friday, March 6, 2020 9:00AM - 9:12AM |
W31.00006: Effect of Roughness and Elasticity on Interactions between Charged Colloidal Spheres Joseph Monti, Patricia M McGuiggan, Mark Robbins The effects of realistic roughness and elasticity on the interactions between charged silica spheres are studied as a function of surface potential, screening length, interfacial energy, and roughness. The repulsive force Frep that must be overcome to bring charged spheres into contact is relatively insensitive to elasticity unless spheres are hundreds of times softer than silica. Frep is also insensitive to roughness and interfacial energy. In contrast, roughness has a large effect on the binding energy of spheres and the force Fsep to separate them. Both are lowered by 1 to 2 orders of magnitude by the measured surface roughness of less than 1 nm on 1 μm silica spheres. The reason is that interactions between rigid spheres are dominated by the highest surface peaks rather than the entire spherical surface. Elasticity can increase the pull-off force of rough spheres by a factor of 2 or more because additional surface area can be brought into contact. The implications of these results for shear-thickening transitions are discussed. |
Friday, March 6, 2020 9:12AM - 9:24AM |
W31.00007: Near-surface dynamics of semidilute polymer solutions: diffusion, nonlinear rheology, and the hydrodynamic boundary condition Gabriel Guyard, Joshua McGraw, Alexandre Vilquin, Frederic Restagno Near-surface dynamics of polymer solutions challenge both experimental and theoretical efforts — especially in the case of semi-dilute solutions for which the chains overlap — yet evanescent wave microscopy allows for a three-dimensional characterization of such interfacial flows. Here we report nanoscale-resolved particle motions in microfluidic channels for pressure-driven flows of semidilute polymer solutions. The results using polymer-free water are in good agreement with Stokes-flow hydrodynamic and diffusive theory. Experiments using hydrogenated polyacrylamide at different volume fractions close to and above the overlap concentration are done in the same chips as for the water experiments. In contrast to Newtonian fluid behaviour, the shear-rate/pressure drop relation is non-linear for the polymer solution flows, suggesting nanometrically-resolved, shear-thinning effects, accompanied with a non-trivial hydrodynamic boundary condition. The diffusive motion of the tracer particles is also distinguished from that of the water experiments, and such motions are detailed here. These results set the basis for a study of near-wall hydrodynamic flow and diffusion in complex fluids, notably including semidilute polymer solutions. |
Friday, March 6, 2020 9:24AM - 9:36AM |
W31.00008: Emergence of surface roughness from plastic deformation of amorphous materials Richard Leute, Till Junge, Lars Pastewka Many man made and natural surfaces show intrinsic surface roughness which controls material properties such as adhesion or friction. Over several length scales and in different materials, ranging from rocks to gold crystals, roughness shows a self-affine scaling behaviour. We investigate the emergence of self-affine surface roughness in amorphous solids through plastic deformation. Our simulations are performed using a fast Fourier transform (FFT)-based continuum mechanics solver. The typical plastic deformation avalanche dynamics of an amorphous material is simulated by a fully tensorial stochastic plasticity model. The crucial role of avalanches for self-affine surface roughness is shown by comparing the stochastic plasticity model with a finite strain J2-plasticity formulation with linear isotropic hardening. We present avalanche statistics and analyse the spatiotemporal distribution of the plastic events. The non-affine deformations in the bulk of the material is related to the self-affine surface roughness of the free surface. |
Friday, March 6, 2020 9:36AM - 9:48AM |
W31.00009: Determining the Structural Characteristics of Deforming Objects from Their Scattering Signature GUAN-RONG HUANG, Yangyang Wang, Yuya Shinohara, Changwoo Do, Takeshi Egami, Wei-Ren Chen There has been much interest in understanding the structural characteristics of deforming materials on different length scales. In the context of scattering, the domain of interest can be splitted into a microscopic region within which the local configurational translation and rotation can be treated in detail, and a larger regime where the description of global shape is appropriate. From a perspective of geometric interpretation at the micro/macroscopic level, a central issue is how to quantitatively determine the structural characteristics from the anisotropic scattering intensity in a model-free manner. |
Friday, March 6, 2020 9:48AM - 10:00AM |
W31.00010: Maximum strain energy density drives path selection in dynamic cracks Lital Rozen-Levy, John Kolinski, Gil Cohen, Jay Fineberg Our fundamental understanding of dynamic ‘simple’ cracks in brittle solids is excellent, yet the criteria for path selection of moving cracks remain unknown. We evaluate the criteria for path selection in dynamic fracture in two ways: first, we deflect dynamic cracks using sparsely implanted defects, and second, we drive them to undergo an intrinsic oscillatory instability in defect-free media. Our experiments cover a wide range of crack velocities, from 10-95% of their limiting velocity within the brittle material. Our high-speed imaging data are used to obtain measurements of the strain fields very near the crack tip - and these measurements reveal that path selection for such rapid and strongly perturbed cracks is determined by the direction of maximal strain energy density rather than by the principle of local symmetry. These results suggest a novel mechanism for material toughening in brittle solids, whereby a crack is steered, deflected and even stopped by embedded inclusions. |
Friday, March 6, 2020 10:00AM - 10:12AM |
W31.00011: A scaling law to determine cracking lengths in shrinkable granular packings Han-Jae Jeremy Cho, Sujit Datta Hydrated granular packings can shrink and crack into discrete clusters of grains when dried. Cracking in paint and mud are familiar examples of this phenomenon. Despite its ubiquity, however, an accurate prediction of cracking length scales remains elusive. Here, we uncover the previously overlooked role of individual grain shrinkage in determining crack patterning. We perform experiments with shrinkable hydrogel particles and discrete-element simulations. By incorporating grain shrinkage into classical Griffith crack theory, we obtain a scaling law that quantifies how cluster size depends on the interplay between grain shrinkage, stiffness, and size—in agreement with experiments and simulations. The cracking predictions are applicable to a diverse array of shrinkable granular materials. |
Friday, March 6, 2020 10:12AM - 10:24AM |
W31.00012: The Prince’s Dilemma: How Local Heterogeneities Affect the Onset of Frictional Slip Mary Agajanian, Sam Dillavou, Vincent Stin, Amir Sagy, Emily Brodsky, Shmuel Rubinstein Peas in eider-down beds and asperities at frictional interfaces trouble fairy-tale princesses and nucleate slip events, respectively. We study the onset of such slip events at a two-dimensional frictional interface using soft, transparent elastomer slabs separated by a layer of sand. This system slows rupture propagation to time scales easy to capture experimentally and allows us to directly visualize the full displacement field at the interface. As the top plate is sheared across the interface, stress aggregation forces contact points at the frictional interface to slide and initiates slip events. We systematically introduce localized pockets of stress (peas) to the interface geometry and characterize the effects of these peas on the dynamics of slip nucleation and growth. By tracking the displacement field near the interface, we observe a plethora of these nucleation behaviors, including partial ruptures and local and traveling nucleation. |
Friday, March 6, 2020 10:24AM - 10:36AM |
W31.00013: Role of Mesoscopic Friction and Morphology in Large Deformation of Carbon Nanotube Network: a Distinct Element Method Study Yuezhou Wang, Grigorii Drozdov, Traian Dumitrica Carbon nanotubes (CNTs) are game-changing materials in various disciplines, but the limited capability for simulating CNT in bulk represents a significant obstacle for realizing their promising applications. We address this challenge by developing a new simulation methodology titled the mesoscopic Distinct Element Method (mDEM). With parameters input from atomistic simulations, mDEM bridges across the atomistic and mesoscopic length scales, providing an efficient tool to predict the large deformation mechanism of CNT network. Validated through experiments, mDEM is utilized in hypothesis-driven research to demonstrate the internal structural evolution of CNT network stretched until fracture. At small and moderate deformations, zipping relaxations along the applied strain direction dictates the microstructural evolution. At larger deformations, the occurrence of energetic elasticity promotes yarn densification, by relaxing CNT waviness and eliminating squashed pores. Besides the strain-induced alignment process, phononic and polymeric friction promote CNT alignment by enabling load transfer. Our parametric studies not only offer guidance in manufacturing industry to achieve high quality yarn, but also potentially offer an effective way to model fibrous materials in general. |
Friday, March 6, 2020 10:36AM - 10:48AM |
W31.00014: Tracking Tabletop Earthquake Nucleation Sam Dillavou, Vincent Stin, Mary Agajanian, Amir Sagy, Emily Brodsky, Shmuel Rubinstein Where, when, and how does an earthquake start? Stress concentrations are thought to play an important role in nucleating earthquakes, but their roles are far from trivial. How stressed areas (known as asperities) interact to control the location and timing of quake nucleation is an inherently two-dimensional elasto-dynamic crack problem where the effects of heterogeneity are far from obvious. We experimentally probe the dynamics of nucleation using a tabletop model fault. We reduce the speed of the dynamic ruptures by using soft transparent elastomers and filling the interface with a thin layer of sand. Heterogeneous stress concentrations are imposed by compressing the interface with a rigid plate and adjustable beads to form localized asperities. The clear material allows us to directly image the propagating slip, which would not be possible in natural rock. We find that nucleation location and style is controlled by the stress concentrations. Weaker concentrations result in more migratory nucleation and more global slip events. Furthermore the location of these regions dictates their interactions; widely spaced concentrations are less likely to slip simultaneously, resulting in fewer global events and slower stress release in the case of a global event. |
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