Strain-induced dynamical heterogeneity and local viscoelastic behavior in complex fluids

Abstract

When stress or strain is applied to the complex fluids, they exhibit unusual mechanical responses due to the geometrical hindrances that the phase coexistence induces. Their rheological properties can be attributed to characteristics such as highly disorder, caging, and clustering on multiple length scales. With it, the dynamics of complex fluids receives attentions as its deeply related to the microstructure and rheological property. To supplement the conventional rheometry, we suggest particle tracking microrheology using direct visualization as an alternative. Using this method, we can observe the local viscoelastic behavior of materials as well as the dynamics on micron length scale. As a first step, we verify the experimental setup of microrheology with totally homogeneous materials such as various polymer solutions by comparing with the results from conventional rheometer. Then, as a second step, we try to control a step of developing mechanism of biofilms by measuring rheological properties of biofilms. It composed with extracellular polymeric substances (EPS) and bacterial cells which have been known to show viscoelastic behavior and have heterogeneous microstructures. From measuring the mean square displacements (MSDs) on the micro-scale, the dynamical heterogeneities of the biofilms are evaluated using van Hove correlation function and non-Gaussian parameter. The dynamical heterogeneity of the biofilms decreased as the wall shear rate increased, analogizing the structural heterogeneity of the biofilms on the different wall shear rate. By determining the local G and G at the low wall shear rate, the structures of biofilms are characterized as void, loose and dense network structures respectively. These kinds of structural diversity in the biofilms give a strong dynamical heterogeneity at low wall shear rate. In contrast, the narrow distribution of MSDs at the high wall shear rate was caused by the dense structure of biofilms. This result clearly gives the strong point of particle tracking microrheology on localized measurement. Finally, as a third step, we modified the previous microrheological method to report the effect of dynamical heterogeneity on the theoretical modeling of nonlinear elastic modulus and Brownian stress of colloidal depletion gels that have undergone yielding in high-rate step strains by modifying previous tracking method on the open system. When we apply step strains to colloidal gels with short-ranged depletion attraction using simple shear equipment, we find the existence of a subpopulation of slow and fast particles. Within this flow regime, small aggregates of particles connected by weak bonds are broken, leaving behind a network consisting of slowly-diffusing particles. These slow clusters form rigid cores that contribute to the remnant stress supported by the sample. Based on this observation, we compare the measured rheology to the theoretical elastic modulus calculated only with the localization length of the slow clusters. We find that this approach produces a far better agreement between theory and experiment. In this thesis, the dynamical heterogeneity of complex fluids gives a vehicle to characterize the structural heterogeneity under varied shear stress. Finally, the findings in this study set the importance of dynamical heterogeneity in the rheology of complex fluids such as bacterial community biofilms and depleting colloidal gels.