Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session A15: Flow of Complex Fluids: Rheology, Structure, and Instabilities I |
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Sponsoring Units: DFD DSOFT Chair: Larry Galloway, University of Pennsylvania Room: 210/212 |
Monday, March 2, 2020 8:00AM - 8:36AM |
A15.00001: Bistability in the Flow of Polymer Solutions in Porous Media Christopher Browne, Audrey Shih, Sujit Datta Polymer solutions are often injected in porous media to improve oil recovery or groundwater remediation, but applications are limited by an incomplete understanding of the underlying physics. In a tortuous pore space, the flow becomes unstable at sufficiently large injection rates. However, how the spatio-temporal characteristics of this flow state depend on pore geometry is poorly understood. We shed light on this question by systematically varying the spacing between pores. We find that when the pore spacing is large, unstable eddies form upstream of each pore, similar to the case of an isolated pore. By contrast, when the pore spacing is sufficiently small, the flow exhibits a surprising bistability, stochastically switching between two distinct flow states. We hypothesize that this unusual behavior arises from the interplay between the retention of polymer strain between pores, hysteresis in polymer conformations, and fluctuations in the flow. Moreover, we find that while flow state is correlated between neighboring pores, these correlations do not persist long-range. Our results thus help to elucidate the rich array of flow behaviors that can arise in polymer solution flow through porous media. |
Monday, March 2, 2020 8:36AM - 8:48AM |
A15.00002: Shear melting and recovery of cellulose nanocrystal-polymer gels Abhinav Rao, Thibaut Divoux, Gareth H McKinley, John Hart Cellulose nanocrystals (CNC) are naturally-derived nanostructures of growing importance for the production of composites of attractive mechanical properties. Their fabrication involves extrusion of CNC suspensions and gels in organic solvents, in the presence of additives such as polymers and curing agents. Here, we study the rheological behavior of composite polymer-CNC gels in dimethylformamide, which include additives for both UV and thermal crosslinking. Using rheometry coupled with in-situ infrared spectroscopy, we show that under external shear, CNC-polymer gels display progressive and irreversible failure of the hydrogen bond network that is responsible for their pronounced elastic properties. In the absence of cross-linking additives, polymer-CNC gels show an instantaneous but partial recovery of their viscoelasticity upon cessation of flow, whereas, the presence of additives allows the gels to recover over much longer timescale via van der Waals interactions. By exploring a broad range of shear history and CNC concentrations, we construct master curves for the temporal evolution of the viscoelastic properties of the polymer–CNC gels, illustrating the universality of the observed dynamics with respect to gel composition and flow conditions. |
Monday, March 2, 2020 8:48AM - 9:00AM |
A15.00003: Observing phase separation in colloid-polymer mixtures with a custom light-sheet rheoscope Jing Wang, Ryan J. McGorty We study liquid-liquid phase separation (LLPS) with a colloid-polymer system subjected to shear. Our colloid-polymer mixture consists of temperature-responsive PNIPAM microgel particles and polymers acting as a depletant. This mixture separates into two phases: a colloid-poor, or “gas” phase, and a colloid-rich, or “liquid” phase. We observe the process of phase separation using a custom-built light-sheet microscope, which allows for simultaneously acquiring optically-sectioned images of our sample and shearing the sample in a Couette geometry. We measure the size and shape of elongated liquid domains that have been deformed due to flow as a function of shear rate. The temperature-responsive feature of our colloidal particles allows us to further explore the kinetics of phase separation under shear flow. We hope our study of phase separation under shear can provide fundamental insights into hydrodynamics and thermodynamics and provide novel strategies for structuring soft materials. |
Monday, March 2, 2020 9:00AM - 9:12AM |
A15.00004: Understanding and predicting flow instabilities in self-assembled polymers Patrick J McCauley, Satish Kumar, Michelle Calabrese Shear banding flow instabilities are common in wormlike micelles (WLMs). Despite reported shear banding in polymer WLMs (pWLMs), current research has focused on surfactant WLMs (sWLMs) or linear polymers. Shear banding in linear polymers is typically transient, unstable, or an artifact of slip, fracture, or geometry; conversely, sWLMs dynamically rearrange and break, enabling steady state shear banding. As breakage in pWLMs is limited, the shear banding characteristics likely fall between these limiting cases, though this behavior remains largely unexplored. Here, we use nonlinear rheology and small angle neutron scattering (SANS) to systematically evaluate pWLM shear banding in commercial triblock poloxamers, where the molecular weight, block length, and block ratios are well-controlled. Poloxamer characteristics and micelle features identified via linear rheology and SANS are then used to develop guidelines to predict shear banding a priori, where important parameters include micelle dimensions, solvent penetration, and entanglement degree, among others. Understanding the fundamental role of poloxamer subunit and self-assembled structure provides insight into shear banding mechanisms absent significant breakage, which can be widely used to predict instability formation. |
Monday, March 2, 2020 9:12AM - 9:24AM |
A15.00005: Depletion Layer Dynamics of Polyelectrolyte Solutions under Poiseuille Flow John King, Seong Jun Park, Anisha Shakya The flow of complex fluids over solid surfaces remains an outstanding problem in fluid mechanics that is relevant for fields ranging from lubrication to nanofluidics. Direct experimental access to depletion layer dimension and composition has been prohibited due to the inherently short length scales associated with depletion layers. Here, we develop a novel adaptation of super-resolution microscopy based on stimulated emission depletion (STED) to directly measure depletion layer composition in real- space with 10s of nanometer resolution. The composition and dimension of depletion layers formed in solutions of high molecular weight poly(styrene sulfonate) at solid, non-adsorbing walls is measured at equilibrium and under Poiseuille flow. Using this novel approach, we 1) confirm concentration profile consistent with entropically driven depletion at the interface, 2) observe depletion layer narrowing at low to intermediate shear rates, and 3) observe depletion layer composition that approaches pure solvent at unexpectedly low shear rates. |
Monday, March 2, 2020 9:24AM - 9:36AM |
A15.00006: Honey bees transport pollen particles of varying shape and size by forming them into a permanent granule Marguerite Matherne, Suraj Puvvada, Ben Guy, Wilson Poon, David Hu Honey bees (Apis mellifera) carry pollen back to their hive by mixing it with nectar and forming it into a pellet, which they carry in the corbicula, or pollen basket, on their hind legs. We show that most pellets do not fluidize when subjected to vibrations or when brought into contact with a similar suspension of lower volume fraction, suggesting that it is a permanent granule. We also explore the behavior of pellets made from different size and shaped pollen particles. The bees form the pellet by squeezing small amounts of pollen and nectar through the joint of their hind leg, called the pollen press, into the pollen basket. Through many repetitions, they form a pellet of up to 2 mm3. This method allows honey bees to collect pollen of various sizes and shapes. |
Monday, March 2, 2020 9:36AM - 9:48AM |
A15.00007: Shear thickening of dense suspensions in the limit of jamming Yasaman Madraki, Guillaume Ovarlez, Sarah Hormozi In shear thickening suspensions, viscosity appears to increase when the shear rate increases. In this work, we focus on the shear thickening phenomenon that occurs in the limit of jamming. We have designed a model non-Brownian suspension to experimentally study this phenomenon. We have developed a series of comprehensive rheometry tests to provide a physical understanding of the problem. We provide a closure for shear stresses in the limit of jamming and we test this closure by studying boundary driven flows of dense suspensions |
Monday, March 2, 2020 9:48AM - 10:00AM |
A15.00008: A new dimensionless number governing dethickening in orthogonally perturbed shear thickened suspensions Meera Ramaswamy, Itay Griniasty, Abhishek Shetty, James Patarasp Sethna, Itai Cohen When concentrated colloidal suspensions are under stress, their viscosity can increase by over an order of magnitude. Previous work has shown that this shear thickened viscosity can be tuned by applying fast oscillatory perturbations orthogonal to the primary shear flows in the system. In this talk, I show that dethickening in the regime where the primary shear flow has fully thickened the suspension, is governed by a single dimensionless parameter – the ratio of the orthogonal shear rate amplitude to that of the primary shear rate. In contrast, a second parameter is required to describe the data in the primary shear flow regime where the suspension is thickening. Understanding these parameters will enable better strategies to tune the properties of shear thickening suspensions for applications ranging from 3D printing to the processing of cement. |
Monday, March 2, 2020 10:00AM - 10:12AM |
A15.00009: Connecting microscale stresses to macromolecular motion in entangled ring-linear DNA blends Karthik Reddy Peddireddy, Megan C Lee, Jonathan Garamella, Ryan J. McGorty, Rae M Robertson-Anderson Ring polymers, as well as mixtures of ring and linear polymers, are ubiquitous in nature yet still poorly understood. Due to the lack of free ends in ring polymers, the motion and dynamics of entangled rings is complex and distinctly different than their linear chain counterparts. As such, the dynamics of entangled blends of ring and linear polymers remain a topic of fervent debate. Here, we use DNA - which occurs naturally in rings and linear forms - as a model system to investigate highly entangled ring-linear blends. To elucidate the dynamics of these blends, we demonstrate a novel technique that combines optical tweezers microrheology with fluorescence imaging and differential dynamic microscopy. This technique enables us to directly image single polymers while performing active microrheology. As a result, we show that it is possible to unambiguously connect the stresses induced by both linear and nonlinear strains to the corresponding macromolecular deformations and network rearrangement in ring-linear polymer blends. |
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