Session Y49: Focus Session: Yielding of Colloidal Crystals, Glasses, and Other Soft Materials

Yielding, Plasticity, and Microstructure in a 2D Jammed Material under Shear Deformation

In this talk, we discuss an experimental investigation on the yielding and plastic deformation of disordered solids. Experiments are performed on colloidal particles that are adsorbed at an oil-water interface and form a dense disordered monolayer. The rheological properties (G', G'') of this dense monolayer are obtained in a custom-built interfacial stress rheometer that uses a magnetic needle within the material. This configuration allows for the simultaneous characterization of both microstructure (tracking $\sim$ 10$^{5}$ particles) and bulk rheology. Results show that for oscillatory shear below a certain strain amplitude, the microstructure becomes reversible after a transient. Above this strain amplitude, the microstructure continues to evolve through many irreversible events. We argue that this boundary between a reversible and irreversible steady state is a yielding transition, and that our experiments measure a meaningful yield stress. Further, we find that reversible plastic deformation is possible. That is, the material can reorganize itself so that the link between plasticity and irreversibility is broken: the material flows slightly, and yet at the end of each deformation cycle, it is exactly unchanged.   

Stress localization, stiffening, and yielding in a model colloidal gel

We investigate the yielding of a model colloidal gel using numerical simulations and different shear protocols. Under increasing deformation, the elastic regime is followed by a significant stiffening before yielding takes place. A space-resolved analysis of deformations and stresses unravel how the complex load curve observed is the result of stress localization and that the yielding can take place by breaking a very small fraction of the network connections. The strong localization of tensile stresses triggers the breaking of a few network nodes at around 30\% of strain and increasing the deformation further favors breaking but also shear-induced bonding, eventually leading to the damage and the reorganization of the gel structure upon yielding. In particular, at low enough shear rates, density and velocity profiles display significant spatial inhomogeneity during yielding in agreement with experimental observations.   

Structure and Rheology of Concentrated Emulsions

We construct a shearing apparatus combining light scattering and stress measurement to study the structure and rheology of concentrated monodisperse emulsions. The emulsions are subjected to oscillatory shear of variable amplitude and frequency. The light scattering data reflect droplet deformation as well as shear history-dependent inter-droplet structures. The stress measurements display pseudoplasticity near zero shear rate and shear-thinning behavior at finite shear rates. In addition, the time-resolved, synchronous measurement of light scattering and rheology reveal detailed information about the complex structure-rheology relationship of emulsions. Shear disorders the droplets at low and high shear rates but induces order at medium shear rates. Furthermore, the effective viscosity increases as the degree of inter-droplet order decreases.   

The viscous forces acting on quasi-2D emulsions under fast flow

We study the flow of dense emulsions in a quasi-two-dimensional sample chamber. Our samples are oil-in-water emulsions confined between two close-spaced parallel plates, so that the oil droplets are deformed into pancake shapes. By means of microscopy, we measure the droplet positions and their deformation, which is related to the forces on the individual droplet. Here we study the velocity dependence of the force on the droplets, and show that the main contribution is from the viscous friction between droplets rather than from viscous drag from the two confining plates. Our results can be applied to study the forces and rearrangements in fast flow in amorphous materials.   

Linear and nonlinear rheology of dense emulsions across the glass and the jamming regimes

We discuss the linear and nonlinear rheology of concentrated silicone oil-in-water emulsions, amorphous disordered solids composed of repulsive and deformable soft colloidal spheres. Based on recent results from simulation and theory, we derive quantitative predictions for the dependences of the elastic shear modulus and the yield stress on the effective droplet volume fraction [1]. The remarkable agreement with experiments we observe supports the scenario that the repulsive glass and the jammed state can be clearly identified in the rheology of soft spheres at finite temperature while crossing continuously from a liquid to a highly compressed yet disordered solid. We show that the onset of elasticity due to entropic contribution can be described by a quasi-equilibrium analytical model of linear elasticity hat includes energetic contributions from entropy and soft interfacial deformation [2]. In a second set of experiments we use confocal microscopy to monitor the structure and dynamics of emulsion droplets while crossing the glass and the jamming transition. [3]. [1] F. Scheffold et al., J. Phys.: Cond. Mat. 25, 502101 (2013), [2] T. G. Mason et al., Soft Matter 10, 7109 (2014) [3] C. Zhang et al., http://arxiv.org/abs/1411.0314   

The Role of Free Surfaces on Plastic Deformation of Colloidal Micropillars

The effect of free surfaces on the strength and deformation behavior of amorphous solids remains an area of intensive research in materials science. We present experiments on the evolution of particle-level strain in amorphous colloidal micropillars compressed uniaxially. The unique micropillar geometry allows us to study the effect of free surfaces, which are believed to be fertile sites for STZ activity, on deformation behavior. The micropillars, which are composed of fluorescent 3 um PMMA particles, are suspended in a fluid so that we can use laser scanning confocal microscopy to image through the micropillar at each increment of macroscopic strain. By employing particle-identification and tracking algorithms, we are able to track the positions of more than 100,000 individual particles during the duration of a compression experiment. Particle-level position information allows us to quantify the spatiotemporal evolution of microscopic strain with macroscopic strain and explore differences in deformation behavior between bulk and surface regions.   

Echoes in x-ray speckles track nanometer-scale plastic events in colloidal gels under shear

Any solid under applied stress possesses an elastic limit above which it yields. The microscopic signatures of yield are irreversible changes to the material's structure. We describe x-ray photon correlation spectroscopy experiments on a concentrated nanocolloidal gel subject to in situ oscillatory shear strain that provide information about the spatial character of rearrangements above yielding at the nanometer scale. The oscillatory strain causes periodic echoes in the x-ray speckle pattern, creating peaks in the intensity autocorrelation function. The peak amplitudes are attenuated above a threshold strain, signaling the onset of irreversible particle rearrangements. The gel displays strain softening well below the threshold, indicating a range of strains at which deformations are nonlinear but reversible. Above the threshold strain, the peak amplitudes decay exponentially with the number of shear cycles, demonstrating that all regions in the sample are equally susceptible to yielding and that the probability of a region yielding is independent of previous shear history. The wave-vector dependence of the decay rate reveals a power-law distribution in the size of rearranging regions, suggesting a nonequilibrium critical transition at yielding.   

Yielding of colloidal gels under steady and oscillatory shear

The structural and rheological properties of intermediate volume fraction colloid polymer gels are examined during and after steady and oscillatory shear flow using rheometry, confocal microscopy, light scattering and Brownian Dynamics simulations. Our main objective is to rationalize the microscopic mechanisms through which one can tune the mechanical properties of such metastable colloidal gels by imposing different types of external shear and flow. Experimentally, the gels consist of model hard sphere particle dispersions of $\varphi =$0.44 with the addition of non-adsorbing linear chains, while BD simulations are conducted for hard spheres with the superposition of an AO potential for depletion attractions. Structural analysis shows that variation of the applied shear rate produces strong changes in the structure of the gels both when under shear and during gel reformation at cessation. Larger rates are characterized by disperse particles and the total breakage of structures at rest, which after cessation evolve with time into strong solids with relatively homogeneous structures. However, smaller rates show large inhomogeneous structures under flow, which do not evolve after cessation and additionally exhibit reduced elasticity and as such are weaker solids. Furthermore oscillatory shear is far more efficient than steady shear creating gels with stronger differences in their elastic modulus. Thus by tuning the way a gel is sheared, one may vary the final strength and structure of the resulting gel. Work in collaboration with R. Besseling, W. C. K. Poon and J. F. Brady   

Matrix polymer species have distinct effects on the mechanics of bacterial biofilms

Biofilms are aggregates of microorganisms embedded in a self-produced extracellular polymer matrix. The matrix confers protection to these microorganisms against mechanical and chemical stresses that they may experience in their environment. The bacterium \textit{Pseudomonas aeruginosa} is widely used as a model biofilm-forming organism because it is an opportunistic human pathogen common in hospital-acquired infections, in chronic wounds, and in cystic fibrosis lung disease. \textit{P. aeruginosa} strain PA01 forms biofilms that are primarily structured by the extracellular polysaccharides Pel and Psl. Using bulk rheological measurements, we show that these polysaccharides each play a unique role in the mechanical robustness of the biofilm. Psl increases the elastic storage modulus while Pel increases the ductility of the biofilm. Increased expression of either Psl or Pel increases the yield stress by about the same amount. Identifying the mechanism(s) by which these polymers contribute to the mechanical toughness of the biofilm could allow new approaches to effective biofilm clearance, by revealing targets for disruption that would weaken the biofilm.   

A simple feature of yielding of dense suspensions of soft micro-hydrogel particles

The highly dense suspensions of soft micro-hydrogels with a narrow size distribution, which form a regular lattice structure, exhibit a simple feature in the yielding behavior: the yield strain $\gamma_{\mathrm{c}}$ [ca. 2.5{\%} and ca. 4.8{\%} for PNIPMA) and PNIPA hydrogel particles, respectively] is nearly independent of the cross-link concentration, particle diameter, and particle concentration (c) in the limited c range examined here, and $\gamma_{\mathrm{c}}$ is almost constant in a wide range of equilibrium shear moduli over two orders of magnitude. Further, no appreciable difference in $\gamma_{\mathrm{c}}$ is observed in the dense pastes with crystalline and glassy structures which are formed by mono- and bidisperse microgels, respectively. In addition, the highly dense suspensions of NIPA core\textbf{--}NIPMA shell microgels are similar in $\gamma_{\mathrm{c}}$ to those of NIPMA microgels. These results indicate that $\gamma_{\mathrm{c}}$ for the highly dense suspensions of soft micro-hydrogels depends primarily on the kind of constituent polymer near the particle surface. The yield strain $\gamma_{\mathrm{c}}$ is expected to be governed by short-range interactions such as adhesion and friction. [Reference] K. Urayama, T. Saeki, S. Cong, S. Uratani, T. Takigawa, M. Murai, Suzuki, \textit{Soft Matter}, DOI: 10.1039/c4sm01841a.   

Plasticity and fracture of curved colloidal crystal shells

Crystalline shells display peculiar equilibrium properties resulting from the interplay between geometrically necessary topological defects and curvature induced stresses. Here we report the results of large scale numerical simulations of the deformation of colloidal particles arranged in crystalline shells showing that the dynamics of topological defects exhibits a rich and non-trivial phenomenology. Depending on the mode of deformation, we observe intermittent plastic deformation with collective particle reorganization mediated by the proliferation of disclinations pairs and grain boundary reorientation or abrupt structural failure induced by crack nucleating at defects. Our work clarifies the role of topology and curvature in the mechanical deformation of crystalline shells.