Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session F59: Rheology of Gels IFocus
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Sponsoring Units: GSOFT DPOLY DBIO Chair: Jacinta Conrad, University of Houston Room: BCEC 257B |
Tuesday, March 5, 2019 11:15AM - 11:51AM |
F59.00001: Failure precursors in colloidal and biopolymer gels Invited Speaker: Laurence Ramos Material failure is widespread and occurs on vastly different lengthscales, from earthquakes to the atomic level. It often involves sudden and unpredictable events, with little or no macroscopically detectable precursors. A better understanding of the mechanisms leading to failure is however highly desirable and would have deep implications, possibly paving the way to predicting failure. |
Tuesday, March 5, 2019 11:51AM - 12:03PM |
F59.00002: Gelling Colloids Through Thermally-Triggered Surfactant Displacement Li-Chiun Cheng, Zachary Sherman, James W Swan, Patrick Doyle Colloidal system undergoes gelation shows interesting rheological properties and is a powerful tool for applications in tissue engineering, food industry and porous material design. Interparticle interaction that can be finely tune results in the great possibility to engineer the material rheology and microstructures. Traditional gelation strategy replies on the manipulation of attractive interaction such as depletion, polymer bridging and dipole-dipole interaction. Here, using the nanoemulsion as a model colloidal system, we present a platform where the colloidal gelation is induced by the decrease in repulsive interaction via a thermally-triggered surfactant displacement mechanism. By adding amphiphilic oligomer surfactants into an oil-in-water nanoemulsion system, the ionic surfactants on the nanoemulsion droplets are replaced by the oligomers at elevated temperatures, leading to a decrease in electrostatic repulsion. The resulting material shows rich rheological properties, and the gelling mechanism is robust over a wide range of nanoemulsion formulations. Our stimuli-responsive gelation platform is general and provides a new degree of freedom to engineer complex soft materials. |
Tuesday, March 5, 2019 12:03PM - 12:15PM |
F59.00003: Path-dependent rheological and structural properties of binary gels from thermoresponsive soft particles Jasper N. Immink, Joakim Stenhammar, Peter Schurtenberger The interaction potential for colloidal-sized polymer microgels can be switched from soft repulsive to attractive due to a thermoresponsive collapse of the particles, and raising the temperature destabilizes these suspensions, causing aggregation and macroscopic gelation. Particularly interesting is the use of binary mixtures of microgels with different collapse temperature, where the choice of the heat ramp allows for an unprecedented control over the gel structure and the rheological properties. A fast or slow temperature ramp results in either a random binary network through homo-gelation, or a core-shell network through sequential gelation, respectively. The core-shell architecture is shown to enhance the structural integrity of the previously formed single species gel, and the final structure exhibits higher elastic and loss moduli than the compositionally identical random network. We use a combination of linear and non-linear rheology, confocal microscopy and computer simulations to investigate the relationship between kinetic pathway, multi-scale structure and linear and rheological properties, and demonstrate that binary microgel mixtures can controllably and reversibly self-assemble into particle gels with tunable structural and mechanical properties. |
Tuesday, March 5, 2019 12:15PM - 12:27PM |
F59.00004: Rheology of Dense Suspensions of Thermoresponsive Microgel Mixtures Undergoing Colloidal Gelation SAORI MINAMI, TAKUMI WATANABE, DAISUKE SUZUKI, Kenji Urayama Dense suspensions of thermoresponsive poly(N-isopropylacrylamide) (N) and poly(N-isopropylmethacrylamide) (NM) microgels with different transition temperature (TcN < TcNM) exhibit a characteristic temperature (T)-dependent viscoelasticity due to T-induced changes in the type of interparticle interaction as well as the volume fraction of microgels. In the range of T < TcN, where the swollen microgels with repulsive interparticle interactions are densely packed, the equilibrium modulus (G) decreases upon heating due to a reduction in the volume fraction of the microgels (φ). At T > TcN where the attractive interparticle interactions between hydrophobic microgels emerge, the suspensions show solid-like properties due to the network-like flocculation of microgels (colloidal gelation), even when φ becomes considerably lower than the threshold for randomly close packing. The T-dependence of G shows minimum at a characteristic temperature (TB), and TB shifts to a higher value with a decrease in N content in the mixtures. We discuss the contributions of packing degree effect and interparticle interaction effect to viscoelasticity. [1] |
Tuesday, March 5, 2019 12:27PM - 12:39PM |
F59.00005: Tunable viscoelasticity of binary colloidal gels Jae Hyung Cho, Irmgard Bischofberger The mechanical properties of colloidal gels depend strongly on the inter-particle interactions. We investigate the potential to engineer the linear viscoelastic properties of colloidal gels at low volume fractions by mixing two types of home-made polystyrene-poly(N-isopropylacrylamide) core-shell particles of largely different attraction strengths, 5kT and 30kT. We show that we can systematically tune the viscoelastic moduli of the binary mixtures by two orders of magnitude, and the spectra of the relaxation time by seven orders of magnitude, simply by altering their mixing proportions. Using confocal fluorescence microscopy and differential dynamic microscopy, we link these changes in the mechanical properties to the microscopic characteristics of the structure and dynamics of the gel networks. |
Tuesday, March 5, 2019 12:39PM - 12:51PM |
F59.00006: Enhanced gelation in binary mixtures of nanoparticles with tunable short-range attraction James Harden, Hongyu Guo, Martine Bertrand, Tyler Shendruk, Subramanian Ramakrishnan, Robert Leheny We report a combined experimental, theoretical, and simulation study of the phase behavior and microstructural dynamics of concentrated binary mixtures of spherical nanoparticles with a size ratio near two and with a tunable, intrinsic short-range attraction [1]. In the absence of the attraction, the suspensions behave as well mixed, hard-sphere liquids. For sufficiently strong attraction, the suspensions undergo a gel transition. Rheometry measurements show that the fluid-gel boundary of the mixtures does not follow an ideal mixing law, but rather the gel state is stable at weaker interparticle attraction in the mixtures than in the corresponding monodisperse suspensions. X-ray photon correlation spectroscopy measurements reveal that, in contrast with depletion-driven gelation at larger size ratio, gel formation in the mixtures coincides with dynamic arrest of the smaller nanoparticles while the larger nanoparticles remain mobile. Molecular dynamics simulations indicate the arrest results from microphase separation that is caused by a subtle interplay of entropic and enthalpic effects and that drives the smaller particles to form dense regions. |
Tuesday, March 5, 2019 12:51PM - 1:03PM |
F59.00007: Rheology of Colloid Gels with Depletion and Bridging Attractions Na Park, Jacinta Conrad Polymers are often added to suspensions as viscosity modifiers or stabilizers, but they can induce attractive interactions between particles that alter the suspension phase behavior and, in turn, the rheology. These attractions may be depletion attractions or bridging attractions, depending on whether the polymers adsorb on the surface of the particles. Here, we developed a model attractive colloidal suspension with depletion and bridging attractions. Co-polymer particles of 2,2,2-trifluoroethyl methacrylate and tert-butyl methacrylate were suspended in a refractive index- and density-matched mixture of glycerol and water. To these suspensions, we added poly(acrylamide) or poly(acrylic acid) to induce depletion or bridging attractions, respectively. The rheology was measured and analyzed for each set of samples with increasing concentration of polymer, corresponding to stronger attraction strengths. Significantly different rheology resulted by simply changing the polymer additive in these suspensions, suggesting that processability of suspensions may be tuned by changing the type and interactions of polymer additives in formulations. |
Tuesday, March 5, 2019 1:03PM - 1:15PM |
F59.00008: Analyzing Onset of Non-Linearity of a Fractal Colloidal Gel in the Neighborhood of Critical Point KHUSHBOO SUMAN, Yogesh M Joshi A colloidal dispersion of Laponite exhibits all the rheological features of a sol-gel transition. While undergoing gelation, Laponite dispersion passes through the critical state stress characterized by a percolated space-spanning fractal network for which the relaxation modulus shows a time dependent power law decay in the linear regime. When subjected to step strain in the non-linear regime, relaxation modulus shifts vertically to the lower values such that the deviation from linearity can be accommodated by use of a strain dependent damping function. We also perform creep-recovery, startup shear and large amplitude oscillatory shear (LAOS) experiments on the Laponite dispersion at the critical gel state and record deviation in the response as flow becomes nonlinear. We also develop a quasi-linear integral model with relaxation modulus weighted by damping function to account for the effect of non-linear strain. Remarkably the proposed quasi-linear integral model predicts the deviation from linearity in the creep-recovery, startup shear and LAOS very well, leading to a simple formulation to analyze the nonlinear rheological behavior of fractal gels. |
Tuesday, March 5, 2019 1:15PM - 1:27PM |
F59.00009: Singular dynamics in the failure of soft adhesive contacts Justin Berman, Katharine Jensen We use optical microscopy and high-speed imaging to characterize the rapid recoil of soft, silicone gels after an extreme deformation and release from a point contact during the failure of a soft adhesive contact. The gel immediately recoils away from the final contact point with a self-similar surface profile that evolves as a power law in time, suggesting that adhesive detachment is a space-time singularity. The singular dynamics we observe are consistent with a relaxation process driven by surface stress and hindered by viscous flow of the gel’s free fluid phase through its porous, elastic network. Our results emphasize the importance of accounting for both the liquid and solid phases of gels in understanding their mechanics, especially in extreme deformation. |
Tuesday, March 5, 2019 1:27PM - 1:39PM |
F59.00010: States of self stress in disordered solids: memory, heterogeneity, and yielding Shang Zhang, Vishwas Vasisht, Leyou Zhang, Emanuela Del Gado, Xiaoming Mao States of self stress (SSSs), equilibrium stress distributions in a mechanical network with zero net force on any component, govern load-bearing abilities of this network. SSSs are determined by the geometry of the network (because they span the null space of the network's equilibrium matrix), and thus encode the memory of how the mechanical network is prepared. We present our results on SSSs in disordered solids generated by molecular dynamics simulations, and discuss how SSSs are affected by the preparation history of these solids (e.g., cooling rate, compression, shear), and explore the relation between SSSs in a disordered solid and its yielding under stress. |
Tuesday, March 5, 2019 1:39PM - 1:51PM |
F59.00011: Scaling theory for mechanical critical behavior in fiber networks Jordan Shivers, Sadjad Arzash, Abhinav Sharma, Fred C. MacKintosh As a function of connectivity, spring networks exhibit a critical transition between floppy and rigid phases at an isostatic threshold. For connectivity below this threshold, fiber networks were recently shown theoretically to exhibit a rigidity transition with corresponding critical signatures as a function of strain. Experimental collagen networks were also shown to be consistent with these predictions. We develop a scaling theory for this strain-controlled transition. Using a real-space renormalization approach, we determine relations between the critical exponents governing the transition, which we verify for the strain-controlled transition using numerical simulations of both triangular lattice based and packing-derived fiber networks. |
Tuesday, March 5, 2019 1:51PM - 2:03PM |
F59.00012: WITHDRAWN ABSTRACT
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