Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session A53: Rheology and Flow of Soft Materials |
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Sponsoring Units: GSOFT GSNP DPOLY Chair: Patrick Charbonneau, Duke University Room: LACC 513 |
Monday, March 5, 2018 8:00AM - 8:12AM |
A53.00001: Flow induced crystallization of penetrable particles Alberto Scacchi, Joseph Brader For a system of Brownian particles interacting via a soft exponential potential we investigate the interaction between equilibrium crystallization and spatially varying shear flow. For thermodynamic state points within the liquid part of the phase diagram, but close to the crystallization phase boundary, we observe that imposing a Poiseuille flow can induce nonequilibrium crystalline ordering in regions of low shear gradient. The physical mechanism responsible for this phenomenon is shear– induced particle migration, which causes particles to drift preferentially towards the center of the flow channel, thus increasing the local density in the channel center. The method employed is classical dynamical density functional theory. |
Monday, March 5, 2018 8:12AM - 8:24AM |
A53.00002: Insight into Shear Thickening Suspensions using Boundary Stress Microscopy Vikram Rathee, Daniel Blair, Jeffrey Urbach The bulk rheological response of shear thickening (ST) suspensions is well documented but the microscopic origin of ST remains poorly understood. Using direct measurement of spatially resolved surface stresses in ST using boundary stress microscopy (BSM), we recently reported the existence of clearly defined dynamic regions of substantially increased stress that appear intermittently at stresses above a critical value. Here we present measurements using a smaller system and lower magnification so that the whole suspension is visualized during shear and show that the localized high stress events observed in steady shear have a finite lifetime. In large amplitude oscillatory measurements, we do not observe any localized high stress events if the peak shear rate is less than a critical shear rate, but the events appear when the peak shear rate is large enough. At very high values, the heterogeneous events become well established. |
Monday, March 5, 2018 8:24AM - 8:36AM |
A53.00003: Rough colloids in shear flow: transient and steady state rheology Alan Jacob, Lilian Hsiao Although particle roughness is encountered in many technological and engineering applications, its role in the rheology of concentrated colloidal suspensions is not well understood. Here, we present experiments and simulations that demonstrate that surface roughness is key to shear thickening and jamming in colloidal suspensions. We suspend smooth and rough colloids in a solvent that is refractive index and density matched. The surface profiles of the colloids are characterized using atomic force microscopy. We recently developed state diagrams that characterize the transition from continuous to discontinuous shear thickening in flowing suspensions when the roughness is increased above a critical value [Hsiao et al., PRL 158001, (2017)]. A physical rationale behind these observations is presented by considering the competition of lubrication and friction in suspensions of rough colloids. Futhermore, we investigate the creep and strain recovery of these suspensions. At volume fractions below the hard sphere glass transition, rough colloids impart rheological properties that are reminiscent of colloidal glasses and granular materials. |
Monday, March 5, 2018 8:36AM - 8:48AM |
A53.00004: Short Time-scale Colloidal Motion in Complex Materials Andrew Hammond, Eric Corwin How do the macroscopic properties of materials derive from their microscopic interactions? For simple fluids the Navier-Stokes equations allow us to connect the micro-scale and the macro-scale. By contrast, drawing the connection between the local structure of a complex fluid and the global response is extremely difficult. We demonstrate a technique to examine a tracer particle at very short time and length scales as a probe on the properties of the carrier fluid, be it simple newtonian fluid, a more complex Maxwell fluid, or even a colloidal glass. By examining the thermal motion of a freely moving colloid we can uncover both the local structure of the material and the global state properties. We report on the verification of the Clercx-Schram law for dense fluids, a violation of the expected behavior of Maxwell fluids, and new results in the micro-scale behavior of colloidal glasses. |
Monday, March 5, 2018 8:48AM - 9:00AM |
A53.00005: Chain-length dependent rheology and relaxation dynamics in glass-forming oligomers and polymers Matthew Reynolds, Daniel Baker, Chinmay Das, Peter Hine, Robin Masurel, Peter Olmsted, Johan Mattsson Understanding how a glass is formed on a microscopic level remains an outstanding problem in condensed matter physics; both the approach towards the glassy state and the molecular behavior within the glass remain to be fully understood. Moreover, how chain connectivity in oligomers and polymers affect the relevant molecular behavior is not presently clear. The work presented here addresses these questions through a systematic experimental study ranging from small molecules to oligomers and polymers, including molecular systems of different characteristics (flexibility, fragility, etc). The effects of molecular chain-length on the rheology and chain, structural and secondary relaxations are explored using a range of experimental techniques combined with computer simulations. Results attained from rheology, broadband dielectric spectroscopy, calorimetry, field cycling NMR, and computer simulations will be discussed, outlining their role in determining the material response for molecules of different chain-length and chemistries. The experimental and computational work will be presented in the context of established models and behaviour previously reported in the literature. |
Monday, March 5, 2018 9:00AM - 9:12AM |
A53.00006: Theory for the rheology of dense non-Brownian suspensions: divergence of viscosities
and μ-J rheology Hisao Hayakawa, Koshiro Suzuki A systematic microscopic theory for the rheology of dense non-Brownian suspensions characterized |
Monday, March 5, 2018 9:12AM - 9:24AM |
A53.00007: Relation between Flows in Complex Geometries and the Dynamics of Entangled Polymer Chains: a Multiscale Simulation Study Takeshi Sato, Takashi Taniguchi Macroscopic flows of entangled polymer melts are determined by the dynamics of a vast number of constituent polymer chains. In other words, macroscopic flows are tightly connected with the microscopic polymer chain dynamics. Therefore, from a simulation perspective, a combined macro-micro description is required to properly analyze polymeric flows. In contrast to conventional approaches that employ constitutive equations, Murashima and Taniguchi developed a MultiScale Simulation (MSS) method for entangled polymer melts (Murashima et al., 2011). In the MSS method, a microscopic model that describes entangled polymer dynamics is combined with a Lagrangian fluid particle method to track the flow history of the strain rate. As a development of our MSS method, flow simulations in a contraction-expansion channel are performed. Using our MSS method, we investigated the relation between the macroscopic flow and the microscopic dynamics of the entangled polymer chains. In particular, we found that the number of entanglements strongly decreases around the middle of the polymer chains. This kind of information can be helpful when designing polymer melts with specific properties. |
Monday, March 5, 2018 9:24AM - 9:36AM |
A53.00008: Normal stress differences in attractive colloid/polymer mixtures Nayoung Park, Jacinta Conrad Hard-sphere particles in flowing suspensions can move across streamlines to locally increase concentration above that of the bulk through shear-induced migration, which is theoretically proposed to arise from the particle normal stresses. How polymers in solution alter mechanisms affecting migration, including flocculation and normal stresses, is poorly understood. We develop a model system well-suited for normal stress measurements and microscopic imaging on colloid/polymer mixtures. Colloidal (trifluoroethyl methacrylate)-co-(tert-butyl methacrylate) particles were nearly refractive-index and density-matched in glycerol/water mixtures, and poly(acrylamide) was added to induce attractive depletion interactions. We measure shear-rate-dependent viscosity and first normal stress differences for mixtures at constant particle concentration and image suspensions before, and after shear using confocal microscopy. These measurements allow us to test various physical pictures for mechanisms leading to nonzero normal stress differences in colloid/polymer mixtures, providing insight into physical processing underlying migration in complex mixtures. |
Monday, March 5, 2018 9:36AM - 9:48AM |
A53.00009: Emergence and persistence of flow inhomogeneities in the yielding and fluidization of dense soft solids Vishwas Vasisht, Gabrielle Roberts, Emanuela Del Gado The response to shear of the dense soft solids features a stress overshoot and a persistent shear banding before reaching a homogeneously flowing state. In 3D large scale simulations we analyze the time required for the onset of homogeneous flow, the normal stresses and structural signatures at different shear rates and in different flow geometries, finding that the stress overshoot, the shear band formation and its persistence are controlled by the presence of overconstrained microscopic domains in the initially solid samples. Being able to identify such domains in our model by prevalently icosahedrally packed regions, we show that they allow for stress accumulation during the stress overshoot and that their structural reorganization controls the emergence and the persistence of the shear banding. |
Monday, March 5, 2018 9:48AM - 10:00AM |
A53.00010: Shear-induced heterogeneity in flocculating micro/nanofibrillated cellulose microstructures Emily Facchine, Orlando Rojas, Saad Khan Aqueous suspensions of micro/nanofibrillated cellulose are known to exhibit gel-like behavior at low concentrations. The physical gel properties are a result of extensive hydrogen bonding, fibril entanglement, and flocculation. In this work, we used dynamic oscillatory rheology and steady shear rheology to assess the microstructural behavior. Three phenomena were studied at length: (1) flow instability in the form of a deviation from typical shear-thinning viscosity at intermediate shear rates (1-10 s-1), (2) reduced structural recovery after breakdown under intermediate values of oscillatory shear (5-8 Pa), (3) yield stress of the system as measured by two different rheological methods. These dynamic behaviors provide insight to the fundamental structure of the suspension, which behaves in the manner of a highly flocculated system. Furthermore, the properties of MNFC were compared with a system of (2,2,6,6-Tetramethylpiperidin-1-yl)oxyl (TEMPO)-oxidized cellulose nanofibrils (CNF), which differ from MNFC in their smaller cross sections and negative surface charges. These properties result in increased stability of the TEMPO-CNF suspension. Between these two related systems, yield stress, floc dynamics, and critical concentration of gel formation were compared and analyzed. |
Monday, March 5, 2018 10:00AM - 10:12AM |
A53.00011: Rate-dependent diffusion in a mesoscopic model of amorphous plasticity: the role of viscous drag and disorder Kareem Khirallah, Botond Tyukodi, Craig Maloney We study the distribution of energy dissipation rates, stress drops and displacements in a mesoscopic, hybrid lattice-particle model of amorphous plasticity. In the model, the plane is tiled up with plaquettes defined by their vertices, which follow an overdamped dynamics. Plasticity is introduced via a continuous strain energy function with subsequent minima. In the lack of disorder, the strain energy function is periodic, whereas in the presence of disorder the distances between the minima are distributed. We show that in the lack of disorder, after finite strain, the system is locked into a homogeneous stress state with system-wide, synchronous bursts. As disorder is gradually introduced, this synchronous state is suppressed and the jerky plastic flow appears. For the latter case, we observe the occurence of slip lines and a corresponding diffusive behavior and we study the rate dependence of the displacement distributions. Finally, we investigate the role of the particular form of the drag by considering either a viscous or a Stokes drag and compare our results to particle simulations. |
Monday, March 5, 2018 10:12AM - 10:24AM |
A53.00012: Rate-Dependence of the Flow Stress in Amorphous Media: Effects of Disorder and Sample Preparation Botond Tyukodi, Kareem Khirallah, Craig Maloney We present results on the rate dependence of the flow stress in a realistic mesoscopic model of amorphous plasticity. In our model, the material is divided into adjacent, non-overlapping tiles and each tile is associated a continuous strain energy function, whereas transition between subsequent local minima corresponds to plastic deformation. Distances between subsequent minima are distributed in order to account for structural disorder. In contrast to previous studies, we find that a Herschel-Bulkley kind of rate dependence can only be recovered if the strain energy function is smooth. By tuning the sharpness of the potential at the barriers, we show that the rate dependence disappears as the potential becomes sharper. We show that the rate dependence is a consequence of the competition between the time scale of the barrier crossings and the time scale of the driving and these two have to be commensurate for rate dependence to take place. |
Monday, March 5, 2018 10:24AM - 10:36AM |
A53.00013: Effect of Polymer Molecular Weight on the Viscosity Reduction in Graphene Oxide Suspension Yul Hui Shim, Kyung Eun Lee, Sang Ouk Kim, So Youn Kim Graphene oxide (GO) has been of particular interest in material science, because it has potential in many applications based on the excellent mechanical and electrical property of the GO. It also exhibits an unique colloidal property based on the high aspect ratio of GO and a noteworthy discovery was that GO can exhibit liquid crystal property in aqueous solution. However, relatively less attention has been paid to GO as two-dimensional colloids. In this study, we examine the detailed microstructure and rheological property of GO dispersions, focusing on their colloidal property. While adding poly(ethylene glycol) (PEG) into GO suspensions can change the state of dispersion and reduce the viscosity of suspension, increasing molecular weight of PEG results in an unexpected viscosity drop. Extensive small angle x-ray scattering and rheology studies were employed to investigate the molecular weight dependent GO dispersions. |
(Author Not Attending)
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A53.00014: Stress Correlations in Shear Thickening Suspensions Abhinendra Singh, Omer Sedes, Bulbul Chakraborty, Lou Kondic, Morton Denn, Jeffrey Morris In concentrated suspensions of neutrally buoyant particles, the apparent viscosity is often found to undergo an abrupt increase making a transition from a low-viscosity to a high-viscosity state, termed as discontinuous shear thickening (DST). The observed behavior has recently been linked to a transition from “lubricated” rheology, where close interactions between suspended particles take place through a thin liquid film, to a “frictional” rheology, where particles make unlubricated frictional contacts. Particle simulations that led to this concept have been successful in quantitatively reproducing the non-Newtonian behavior of thickening suspensions [1, 2]. We find that the system shows features of classical phase transition with the critical point being the volume fraction at which the derivative of the shear rate with respect to the shear stress becomes zero [3]. This point is associated with a pairing of solid fraction and friction coefficient, and respectively. The temporal and spatial correlations of the shear (and normal) stress at the critical point and their variation with the system size will be presented. |
Monday, March 5, 2018 10:48AM - 11:00AM |
A53.00015: Correlated Time-Variation of Bulk Microstructure and Rheology in Asphalt Binders Adam Ramm, Sakib Nazmus, Amit Bhasin, Michael Downer We use near-infrared dark-field optical microscopy to probe time variation of the density of naturally-occurring, sub-surface microstructures in PG 64-22 asphalt binders following a rapid heating (cooling) increment |ΔT| = 20 C from initial temperature T0 = 10 C (50 C). We compare these microstructure variations with time variations of the magnitude |G*(T,t)| of the bulk complex shear modulus measured for identical sample conditions with a Dynamic Shear Rheometer (DSR). The main findings are: (1) Microstructure density (inferred from intensity I(T,t) of near-infrared optical scatter) and |G*(T,t)| both continue to change appreciably long after measurable changes of binder temperature cease. Moreover, delayed time variations in I(T,t) and |G*(T,t)| (2) correlate closely with each other; (3) evolve on three distinct time scales — several minutes, ∼ 1 hour, > 1 day; (4) are more pronounced after a cooling step (ΔT = -20 C) than after a heating step (ΔT = +20 C); and (5) account for hysteresis in I(T,t) and |G*(T,t)| curves observed during heating-cooling cycles. |
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