Session W25: Rheology, Flow and Instabilities of Soft Materials

Magnetoelastic instability induced pattern transitions in soft ferromagnetic laminates

The performance of magnetorheological elastomers (MREs) can be significantly enhanced by tailoring their microstructure. Moreover, microstructured materials when subjected to large deformations can develop elastic (reversible) instabilities; the phenomenon is often associated with dramatic microstructural transformations and can be harnessed to develop material with switchable functionalities. Here, we will present our work on instability development in soft MREs with layered microstructure, with phases exhibiting ferromagnetic behavior. To perform the magnetoelastic instability analysis, we employ the small-amplitude perturbations superimposed on finite deformations in the presence of the magnetic field. We examine the interplay between macroscopic and microscopic instabilities. We find that the layered MAEs can develop microscopic instability with antisymmetric buckling modes, in addition to the classical symmetric mode. Notably, the antisymmetric microscopic instability mode does not appear in a purely mechanical scenario (when a magnetic field is absent). Furthermore, our analysis reveals that the wavelength of buckling patterns is highly tunable by the applied magnetic field, and also by the properties and volume fractions of the phases.   

Multi-Scale Modeling of Worm-like Micelle Rheology

We model the dynamics of worm-like micellar rheology using both a slip-spring model and a “pointer algorithm,” both of which account for micellar diffusion within an entanglement network, as well as breakage/fusion of micelles. We check the accuracy of the pointer algorithm using the more highly resolved slip-spring simulations for micelles with limited numbers of entanglements (up to five per average micelle), and find that the terminal region is accurately modeled, while higher frequency processes require more care to model correctly.  Once validated, the pointer algorithm allows prediction of the rheology of more highly entangled micellar solutions, and stiffer micelles, typical of the literature and of commercial formulations. We also show that the simplified estimate of micelle length of Cates only becomes valid for highly entangled, long, micelles, and we demonstrate a simple method of improving the Cates analysis of micelle length from rheology.  We also check and confirm mean-field scaling laws of micelle length and modulus versus surfactant concentration, using micelle parameters extracted from experimental data by fits of the pointer algorithm to rheological data.   Finally, we apply the slip-spring model to the nonlinear rheology of entangled thread-like micelle solutions, including the effects of chain strength, entanglements, and breakage/fusion.

    

Mechanical resilience of biofilms towards environmental perturbations mediated by extracellular matrix

Biofilms are surface-associated communities of bacterial cells embedded in an extracellular matrix (ECM). Biofilm cells can survive and thrive in various dynamic environments causing tenacious problems in healthcare and industry. Biofilms can be considered as soft, viscoelastic materials and exhibit remarkable mechanical resilience. How biofilms achieve such resilience towards various environmental perturbations remain unclear, although ECM has been generally considered to play a key role. Here, we use Vibrio cholerae (Vc) as a model organism to investigate biofilm mechanics in the nonlinear rheological regime by systematically examining the role of each constituent matrix component. Combining rheological measurements and molecular dynamics simulations, we quantitatively characterize the mechanical behaviors of various mutant biofilms and investigate their distinct mechanical phenotypes including mechanics-guided morphologies, nonlinear viscoelastic behavior, and recovery from large shear forces and heating. Our findings provide physical insights into the structure-property relationship of biofilms, which could be potentially employed to design biofilm removal strategies or, more forward-looking, engineer biofilms as beneficial, functional soft materials in dynamic environments.   

Elucidating the effects of surface roughness-induced geometric frustration on the linear viscoelastic moduli in dense colloidal suspensions

Elastic (G') and viscous (G'') moduli in dense colloidal suspensions, interacting solely via hard sphere potential, are determined by the entropic effects and the hydrodynamic interactions between the constituent particles. In suspensions with rough colloids, these interactions are modified by the additional restriction in rotational diffusion. In this study, we probe the near-equilibrium structure of suspensions with smooth and rough colloids to decouple the effects of surface roughness on the linear viscoelastic moduli. We use smooth and rough poly(hydroxystearic acid)-grafted-poly(methylmethacrylate) (PHSA-g-PMMA) colloids of similar particle diameters (2a ≈ 1.5 μm) dispersed in the index-matched solvent squalene, at volume fractions 0.45 ≤ φ < φ_max, to study the effects of surface roughness on linear viscoelastic properties of these suspensions. Frequency sweep experiments in the linear rheological regime revealed that beyond a crossover concentration (φ > φ_c), the linear viscoelastic moduli of rough suspensions are 1000 times larger than the smooth counterparts. Furthermore, the scaling of the high-frequency elastic modulus (G'_∞) with respect to the applied frequency (ω) is system dependent: suspensions with smooth colloids show G'_∞ ∼ ω^1/2 while rough particle suspensions exhibit a plateau with G'_∞ ~ ω⁰. Combining mode-coupling formulation, dynamic localization theory, and the high-frequency moduli analysis we conclude that the higher magnitude of the linear viscoelastic moduli in rough colloidal suspensions is a consequence of the modification in the length scales and time scales associated with the “caging phenomena” in the dense suspensions, and the enhanced hydrodynamic lubrication interactions introduced by the surface roughness-induced geometric frustration.     

Rheological Response of a Colloidal Glass with Large Size Bidispersity

Soft glasses, in the form of colloids, emulsions etc., have widespread applications in our day-to-day life. In reality, many of these systems have large size dispersity among constituent particles. It is known that such systems have consequently large separation of relaxation timescales, among the constituents. The rheological response of materials like these have not been much explored. We use large-scale molecular dynamics simulations using a model bidisperse colloidal soft-spheres mixture with large size ratio, to study the response to external shear, across a wide range of densities. The system shows complex macro rheological response, and we try to understand these observations by probing the microscopic structural and dynamical  properties of the system. We demonstrate that the interplay of the time scales associated with applied shear and the intrinsic relaxation of the species leads to the observed density-dependent response. Also, by varying the composition of the mixture, we illustrate that increasing the concentration of smaller species leads a softer material, which thus provides a pathway to control the rheology.   

Rheology of Charge-Modified, Soft Phytoglycogen Nanoparticles

Phytoglycogen (PG) is a natural polysaccharide produced in the form of compact, 44 nm diameter, electrically neutral nanoparticles in the kernels of sweet corn. Its highly branched, dendritic structure and soft, compressible nature leads to interesting and useful properties that make the particles ideal as unique additives in personal care, nutrition, and biomedical formulations. To tailor the particles to specific applications, it is often desirable to modify their properties. We consider the effect of covalently attaching positively charged glycidyltrimethylammonium chloride (GTAC) chemical groups to PG on the rheology of the particles, focusing on the zero-shear viscosity of GTAC-modified PG dispersed in water at different concentrations C. Dispersions of GTAC-modified PG were significantly more viscous than those of native PG and showed a much steeper increase in the zero-shear viscosity with increasing C. Additionally, the viscosity of GTAC-modified PG dispersions was sensitive to the addition of salts and decreased significantly with added NaCl. These results show that electrostatic interactions have a significant effect on the strength of the interactions between the particles and suggest new applications for GTAC-modified PG.   

Rheology of Dilute Poly-Disperse Granular Flow

Previous poly-disperse granular studies have largely involved discrete number of species. However, the majority of granular flows are composed of particle whose characteristics, such as size and mass, follow continuous probability distributions. The relation between the general rheology of dilute granular flows behave and the shape of particle-size distribution is currently unclear. We implement the direct simulation Monte Carlo method to study continuously distributed particle sized granular flows. The effect of poly-dispersity on the stresses for an unbounded shear flow is investigated.   

A multiscale investigation of reconstituted intermediate filament hydrogels under compression

Cell migration is an essential process in many biological functions, including tissue maintenance and disease progression. The mechanical forces behind these functions are mediated through the cells via the cytoskeleton, a complex network of (semi-) flexible biopolymers that include actin, microtubules (MTs), and intermediate filaments (IFs). What role IF mechanics have in cell motility is yet to be fully understood. Here, we first study the mechanical response of reconstituted polymer networks comprised of the IF proteins vimentin and keratin using a parallel-plate rheometer. We investigate the mechanical response of these IF networks to uniaxial compression, knowing that cells stiffen under compression and this function may protect cells during migration. We first demonstrate that vimentin and keratin networks also stiffen under axial compression, in contrast to actin and MTs which soften. Further, we demonstrate this stiffening is divalent cation concentration dependent, a property regulated by the cell. We further investigate the microscopic compression behavior of these IF gels by real-time imaging their compression and directly analyzing network motion.   

Optical tweezers microrheology of transiently crosslinked actin networks

Cells dynamically change their viscoelastic properties by restructuring networks of actin filaments, one of the most abundant proteins in the cytoskeleton. This restructuring is modulated, in part, by actin-binding proteins (ABPs) that regulate the assembly of actin filaments (F-actin) into networks and bundles, leading to the structural integrity of the cell. Myosin II, best known for its ATP-driven restructuring of actin networks, also functions as a transient cross-linker at low  ATP concentrations. While the mechanics of cross-linked actin networks with common ABPs have been studied extensively, there is a lack of understanding of the mechanics and structural evolution of transiently cross-linked actin networks. Here, we use optical tweezers microrheology to impart local nonlinear strains and measure the mechanical properties of actin networks cross-linked via myosin-II. Specifically, we drag a microsphere 10 micrometers through cross-linked actin networks at a constant speed using optical tweezers and measure the network's response on the bead during and following strain. Furthermore, we simultaneously image the network via fluorescence laser scanning confocal microscopy to correlate the microscale mechanics with the structural evolution of the networks. Our measurements shed new light on how varying the transient cross-linker densities tune the mechanical properties of actin networks.   

Micromechanical origin of plasticity and hysteresis in nest-like packings

Disordered packings of unbonded filaments form a unique class of meta-materials, where the mechanics derive from the bending of constituent elements between frictional contacts. We probe the mechanical responses of one such instance, a tangle of wooden sticks in a cylindrical container under cyclic compression, both experimentally and in-silico. We find two prominent features in stress-strain curves' evolution: initial cycles of irreversible plasticity followed by a repeatable steady state with finite, velocity-independent hysteresis. Upon validating simulations by comparing bulk responses and spatial distribution of contact points with those of experiments, we trace the prominent bulk responses to their micromechanical origin in the motion of inter-element contact points   

No silver bullet: compositional ripening in water-in-oil emulsions

One approach to achieve low calorie foods is to substitute regions of high calorie content with water droplets, forming water-in-oil emulsions. In complex food systems consisting of multiple species of dispersed phases, compositional ripening may occur in which the water undergoes mass transfer to regions filled with other less soluble species. 

Here we present a model system to study compositional ripening for water-in-oil Pickering emulsions. Water-in-dodecane emulsions stabilised by PMMA particles were prepared and combined with similar emulsions that included sugar in the water. Overtime the pure water droplets appear to crumple due to the loss of water; in extreme cases they eventually ‘explode’. Simultaneously, the sugar-filled droplets slowly coalesce. Evidently, our interfacial coating of particles is unable to suppress this effect. Using particle tracking, quantitative analysis of the individual droplets shows a decrease in the concentration of the sugar solution results in a reduction in the rate of change of water droplet size. Observations of droplet ‘explosions’ suggest that the driving force is vastly stronger than that of Ostwald ripening. Forthcoming experiments will further modify the system including the continuous phase and the liquid-liquid interface.   

Capillary control of composite rafting

Elastic plates floating on a fluid substrate, and subjected to axial compression, display a sinusoidal pattern in their vertical displacement, called wrinkles. We examine the wrinkles of inhomogeneous soft composite thin sheets lying on a liquid. In particular, we consider effective medium behaviour theories to predict the bending stiffness of a composite plate consisting of a soft host with liquid inclusions both large and small relative to the elastocapillarity length. By imposing a gradient of the volume fraction or varying the inclusion size, we can devise elastic sheets with a varying stiffness and therefore deliberately manipulate the nature of the emerging wrinkles for different lengths of the plate.   

Crack patterns on soft substrates containing a wrinkled layer inside

We study crack patterns in thin films coated on tri-layer substrates that contain a wrinkled layer in the middle. Examples of such complex substrates can be found in the epidermis layer of the skin and laminated fabrics. Due to the geometry and residual stress formed during the wrinkling process, cracks occur in a unique way because of uneven stress distribution. We first fabricate the wrinkled surface by pre-stretching a thick soft polymer plate, spin-coating a thin rigid film on it, and then releasing it. Then, we coat another polymer as the top layer by a push-coating process which fills the valley of the wrinkled surface and becomes widespread. Finally, brittle material is coated on the tri-layer substrate and uniaxially stretched to acquire cracks. We combine the experimental results with finite element simulations and find uneven stress distribution appears on the surface inducing cracks at certain locations. Furthermore, we systematically vary the wrinkling shape and push coating thickness to understand the crack-inducing mechanism.