Bulletin of the American Physical Society
APS March Meeting 2023
Volume 68, Number 3
Las Vegas, Nevada (March 5-10)
Virtual (March 20-22); Time Zone: Pacific Time
Session W09: Gel RheologyFocus
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Sponsoring Units: DSOFT Chair: Lilian Hsiao, North Carolina State University Room: Room 132 |
Thursday, March 9, 2023 3:00PM - 3:36PM |
W09.00001: The Yield Transition in Gels: Accounting for Structural Breakdown Invited Speaker: Ryan Poling-Skutvik Gels are materials comprised of a majority fluid phase, but which exhibit solid-like mechanical properties arising from a percolating network structure. When subjected to external stresses or strains, these materials undergo a yield transition in which the material no longer elastically deforms but viscously flows. This transition is often accompanied by a commensurate breakdown in the material structure to a state that depends on the shear history. For many classes of gels, the structural recovery from this yielded state back into a percolating network occurs over long time scales, resulting in time-dependent mechanical properties that obscure the yield transition and deleteriously affects the performance of gels in applications ranging from additive manufacturing to tissue engineering and drug delivery. Here, we develop a novel rheological protocol to account for structural breakdown and to precisely quantify the yield transition in a variety of materials, including colloidal gels, physical gels, and a physicochemical gel comprised of polymer-linked emulsion droplets. The gelation of these materials is characterized through standard linear oscillatory rheology, but the yield transition is measured through a series of stress-controlled creep measurements in which the time it takes to yield the sample depends on the quiescent recovery time and applied stress. From these measurements, we quantify the structural evolution of the gels through a bifurcation in the creep response and unambiguously define the yield transition according to the divergence of yield times. Our findings elucidate the unique mechanisms of structural recovery that depend on gel physicochemistry, provide insight into the origins of the yield transition, and quantifies the thixotropic recovery of mechanical properties. |
Thursday, March 9, 2023 3:36PM - 3:48PM |
W09.00002: Quenching and annealing strategies for structural and rheological control of thermoresponsive colloidal gels Matthew E Helgeson, Scott M Fenton, Tuan Nguyen, Roseanna N Zia Complex thermal processing strategies using annealing and quenching of phase instability have been a conserved motif since antiquity to generate materials with exceptional mechanical properties. However, these strategies have been largely inaccessible to colloidal gels and glasses due to their slow dynamics, difficulties with controlled quenching, and knowledge gaps in the interplay of gelation and phase separation processes. To overcome these challenges, we present studies on model colloids with thermosensitive polymer linkers, in which programmed temperature control provides a facile means to sculpt structure formation, aging and rheology of nascent colloidal gels. Using controlled rate quenches en route to gelation, we show that one can produce gels whose rheological properties vary by orders of magnitude depending on the dwell time in the region of phase instability. Rheo-microscopy establishes the quench-dependent microdynamics of coarsening that underly this control, and is used to inform the design of step-wise quench-anneal-quench protocols for fine control of the length scales of phase separation and rheology in deeply arrested gels. Our results show promise for the design of complex thermokinetic processing strategies to achieve fine-tuned or exotic rheological properties in gelling systems. |
Thursday, March 9, 2023 3:48PM - 4:00PM |
W09.00003: Hyperelastic rheology of hydrogels during compression and sliding Jing Wang, Justin C Burton Hydrogels are biphasic, swollen polymer networks where elastic deformation is coupled to nanoscale fluid flow. Examples include cartilage, gelatin, agarose, and many cosmetic products. They are ubiquitous in applications such as tissue engineering and soft robotics due to their hydrophilic nature. Here we show how sliding and shear forces produce dilation in hydrogels. First, we investigated the relaxation of the normal force in centimeter-scale polyacrylic acid hydrogel spheres subjected to strain-controlled compression. A collapse of the data revealed a multi-day relaxation timescale, and the effective poroelastic diffusion coefficient was measured to be of order of 2 x 10-9 m2/s, which is consistent with the self-diffusivity of water. Additionally, using a custom-built pin-on-disk tribometer and rheometer, we observed an increase in the normal force (or swelling) upon sliding the spherical hydrogel. This hyperelastic coupling between shear strain and dilation (Poynting effect) induces fluid imbibition in the hydrogel, as evidenced by slow relaxation upon cessation of sliding. |
Thursday, March 9, 2023 4:00PM - 4:12PM |
W09.00004: Yield surface of attractive colloidal gels under Poiseulle flow soohee bae, Deepak Mangal, Safa Jamali Short-ranged attractive colloidal systems that assemble into gels exhibit a series of exotic rheological behavior, including emergence of a yield stress. The yielding mechanism of these colloidal gels under simple drag flows have been studied extensively over the past decade or so. Nonetheless, the porous and heterogeneous structure of these gels results in presence of a yield surface under pressure-driven flows, aka Poiseulle flow. Due to heterogeneous deformation rates that develop in Poiseulle flow, the interactions between the flow boundaries and the colloids also play an important role in the structure/assembly of colloids and their resulting rheology. In this work, yielding behavior of colloidal gels at Φ=20% confined between two walls under Poiseuille flow is computationally studied for attractive and repulsive interactions between the walls and the colloidal particles. We find that the entire range of shear-induced structures: compaction, re-structuration and coarsening, and shear rejuvenation are present in the Poiseulle flow of the colloidal gels. We compare these observed regimes with the ones previously reported for the simple shear flows and benchmark the structural evolutions of the system against those previous studies. |
Thursday, March 9, 2023 4:12PM - 4:24PM |
W09.00005: Biphasic Colloidal Mixtures: Theory of Dynamic Solidification, Linear Elasticity and Yielding Subhasish Chaki, Kenneth S Schweizer We apply ideal naive mode coupling theory to treat dynamical arrest, the elastic shear modulus, and stress-driven yielding of binary mixtures of equal diameter hard and attractive spheres. A rich kinetic arrest behavior emerges from the competition between excluded volume caging forces and attraction-induced physical bond formation. A homogeneous fluid, partially localized state (repulsive spheres remain fluid but attractive spheres gel), and doubly localized attractive and repulsive glasses are predicted. The shear elastic moduli vary as a power law with total volume fraction with effective exponents that decrease with increasing sticky colloid composition and attraction strength. The partial localization scenario and aforementioned power law scaling of the elastic modulus for different sticky particle compositions are in good agreement with experiments of Lewis and coworkers, although the absolute magnitudes of the latter are overpredicted presumably due to mixture composition dependent nonequilibrium clustering effects. Predictions for the perturbative yield stress are also obtained and favorably compared to experiment. The binary mixture becomes more brittle (smaller yield strain) with increasing sticky particle composition and attraction bond strength. |
Thursday, March 9, 2023 4:24PM - 4:36PM |
W09.00006: Stress-stress Correlations Reveal Force Chains in Gels H. A Vinutha, Fabiola Diaz Ruiz, Xiaoming Mao, Bulbul Chakraborty, Emanuela Del Gado We investigate the spatial correlations of microscopic stresses in soft particulate gels, using 2D and 3D numerical simulations. We use a recently developed theoretical framework predicting the analytical form of stress-stress correlations in amorphous assemblies of athermal grains that acquire rigidity under an external load. These correlations exhibit a pinch-point singularity in Fourier space leading to long-range correlations and strong anisotropy in real space, which are at the origin of force-chains in granular solids. Our analysis for the model particulate gels at low particle volume fractions demonstrates that stress-stress correlations in these soft materials have characteristics very similar to those in granular solids and can be used to identify force chains. We show that the stress-stress correlations can distinguish floppy from rigid gel networks and the intensity patterns reflect changes in shear moduli and network topology, due to the emergence of rigid structures during solidification. |
Thursday, March 9, 2023 4:36PM - 4:48PM |
W09.00007: Time-resolved microstructural changes in large amplitude oscillatory shear of model single and double component soft gels Gavin J Donley, Minaspi Bantawa, Emanuela Del Gado Soft particulate gels can reversibly yield when sufficient deformation is applied, and the characteristics of this transition can be enhanced or limited by designing hybrid hydrogel composites. While the microscopic dynamics and macroscopic rheology of these systems have been studied separately in detail, the development of direct connections between the two has been difficult, particularly with regard to the nonlinear rheology. To bridge this gap, we perform a series of large amplitude oscillatory shear (LAOS) numerical measurements on a series of model soft particulate gels using coarse-grained molecular dynamics simulations. We study both a particulate network with local bending stiffness and a two-component network with a second component that provides additional cross-linking. Through the sequence of physical processes (SPP) framework, we define and track time-resolved dynamic moduli which allow us to distinguish transitions in the material behavior as a function of time. This approach helps us establish the microscopic origin of the nonlinear rheology by connecting the changes in dynamic moduli to the corresponding microstructural changes during the deformation including the nonaffine displacement of particles, and the breakage, formation, and orientation of bonds. |
Thursday, March 9, 2023 4:48PM - 5:00PM |
W09.00008: Nonlinear rheology of biologically derived interpenetrating networks Wayan A Fontaine-Seiler, Daniel L Blair The extracellular media (ECM) is a combination of different biological polymer networks that provide structure and scaffolding for the support and growth of cells in tissues, regulate function, and can play an important role in disease triggering, onset and progression. In this work, we will discuss the rheological response of biologically derived interpenetrating networks (IPNs) composed of gelatin-fibrin and collagen-fibrin with varying degrees of inter-and intra- chain crosslinking. These interpenetrating networks have demonstrated their ability to endure high strains. Using a novel framework to analyze large amplitude oscillatory rheology (LAOS) raw stress response data, we measure the equivalent transient storage and loss moduli over cycles of different frequencies and amplitudes of IPNs developed for three dimensional cell culture. Specifically, we will demonstrate that the materials transition continuously gradually from linear to nonlinear responses with an increase in the applied amplitude and strain rate. We will also discuss the impact that strain and strain rate has on the history (plasticity) of the material through the cycle as a function of biopolymer type and crosslinking concentration. |
Thursday, March 9, 2023 5:00PM - 5:12PM Author not Attending |
W09.00009: Targeting Mechanical Hysteresis of Actin Networks using Bio-Synthetic Crosslinkers Tyler Jorgenson, Margaret Gardel, Stuart J Rowan Rheological studies of in vitro actin networks provide critical insights into cell mechanics and provide design inspiration for new materials. Actin networks are dynamic and flexible yet can resist deformation through strain-stiffening. Recent studies have shown that that the magnitude of stiffening, or differential modulus, K, of actin networks is directionally increased through the application of a pre-stress that aligns the actin filaments. Computational simulations hypothesized that this mechanical hysteresis is dependent on crosslinker length, flexibility, and binding kinetics. Current in vitro actin network studies rely on proteinaceous crosslinkers that are difficult to engineer to rigorously study the mechanical impacts of crosslinker variables. In this study, we probe the mechanical hysteresis of actin networks using bio-synthetic crosslinkers comprised of polyethylene glycol (PEG) polymers end functionalized with actin binding peptides. These peptide-PEG constructs allow for finer control over crosslinker variables. Using bulk rheology, we demonstrate the effectiveness of these bio-synthetic crosslinkers in generating strain-stiffening actin networks and investigate the resulting mechanical hysteresis as a function of PEG molecular weight and relative crosslinker concentration. The results from these studies provide greater insight into cell cortex mechanics as well as a roadmap for the generation of new synthetic materials with tunable strain-stiffening properties. |
Thursday, March 9, 2023 5:12PM - 5:24PM |
W09.00010: Additive-free Gelation of Graphene Oxide Aqueous Dispersion Induced by the Mild Thermal Annealing Geon Woong Kim, So Youn Kim Graphene oxide (GO) exhibits good dispersibility and colloidal stability in an aqueous solution based on its functional groups on the basal plane and edges. These properties enable solution processing of GO for various forms of GO-based products such as fiber, membrane, aerogel, etc. In the processing of these applications, GO dispersions often require a proper phase transition to gel (network structure) to have sufficient modulus. The gelation of GO dispersion usually occurs through the addition of polymer or salt; however, these additives act as impurities that can decrease the GO dispersibility and lower the electrical properties of the final product. |
Thursday, March 9, 2023 5:24PM - 5:36PM |
W09.00011: Re-entrant dynamics in polymer-linked colloidal networks Taejin Kwon, Tanner A Wilcoxson, Delia Milliron, Thomas M Truskett Linked colloidal gels composed of functionalized building blocks and complementary linker molecules can form an equilibrium gel phase at low volume fractions. Here, we use computer simulations of a coarse-grained model to establish that polymer-linked colloidal networks show dynamic hallmarks of equilibrium colloidal gels with re-entrant behavior as a function of the linker-to-colloid ratio. The simulations reveal the link between colloidal structural relaxation and the decorrelation time of linker-mediated colloid-colloid bonds. The latter relates to the number of effective bonds per colloid, which varies nonmonotonically with linker concentration in a way that can be predicted from a thermodynamic perturbation theory which accounts for linker looping and redundant bonding motifs. Our results provide a basis for controlling macroscopic properties of the gel by tuning the linker-to-colloid ratio. |
Thursday, March 9, 2023 5:36PM - 5:48PM |
W09.00012: Probing the Interstitial Pore Structure of Packed Granular Microgels Christopher S O'Bryan At relatively low polymer concentrations, packs of highly swollen granular hydrogel particles, commonly called microgels, undergo a jamming-like transition. Under small applied stresses, these packed microgels behave like soft elastic solids but will flow like viscous fluids when the applied stress exceeds the yield stress. By tuning the rheological properties, support baths of packed microgels have been used to 3D-print soft materials in their fluid phase. Recently, this 3D-printing approach has been extended to 3D-bioprinting by swelling the microgel particles in cell growth media. However, long-term assays in this 3D-environment require the rapid exchange of cell growth media to provide fresh nutrients and the removal of cellular waste. Understanding the relationship between the interstitial pore structure of packed microgels that controls the permeability and the rheological properties that enable embedded 3D-bioprinting will further enhance the ability to systematically study cell behavior in a 3D-environment. Here, we investigate the interstitial pore structure of uncharged, polyacrylamide microgels with increasing cross-linker and polymer concentrations through a combination of cryo-SEM and single particle tracking. In addition, we compare the changes in the pore structure to the rheological behavior of the packed microgels. We believe that these studies will guide the development of microgels as sacrificial support materials for embedded 3D-bioprinting applications. |
Thursday, March 9, 2023 5:48PM - 6:00PM |
W09.00013: Compression Induced Syneresis and Yielding in Fibrous Oil-in-Hydrogel Emulsions Benjamin G Thorne, David A Weitz Emulsion gels stabilized by fibrous networks are found in cosmetic, pharmaceutical, and food industries where oil droplets are dispersed throughout a continuous water phase and fibers form a continuous network. While the droplets and fibrous network work together to impart unique mechanical properties, the release of water and subsequent collapse of the gel under compression is less understood. In this work, we probe the effects of emulsion droplets on hydrogel syneresis in a fibrous gel network to control and optimize water flow and retention within the gel. Sodium myristate surfactant, silicone oil, and water are mixed to form an emulsion that gels when cooled to room temperature. Compressing these oil-in-hydrogel structures causes the release of water from the highly permeable gel phase and eventually creates a reinforced compressed emulsion with a storage modulus that scales with the Laplace pressure. Further compression of the sample results in a buckling-like failure of the material as either the emulsion breaks due to increased droplet coalescence and the oil is released as a single phase, or the fibers fail, and the gel deforms plastically. The failure stress of the material depends on the radius of the emulsion droplets, and is greatest for oil volume fractions of around 40%. The results have implications for the understanding of complex gels such as those of food and tissue. |
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