Microdynamics and Elasticity of Fractal Gels with Embedded Active Colloids

Colloidal gels are a state of soft matter with attractively bonded particles in a network; these networks display rheological properties such as linear elasticity at low-frequency and a yield stress. Controlling these properties is important in industries where gels are used, such as pharmaceuticals, agricultural formulations, and consumer products. We present a method to control the microscopic dynamics of colloidal gels through active motion. We study how the introduction of active particles affects the dynamics of fractal colloidal gels comprised of passive, Brownian particles. Platinum coated Janus particles are embedded within the gel network and activated by the addition of hydrogen peroxide through a porous hydrogel membrane. We visualize gel structure and dynamics through two channel confocal microscopy. Through image analysis, we determine the positions of particles within the gel and compute their mean squared displacement (MSD). We find that the MSD of particles within the gel increases after the embedded Janus particles are activated. We analyze the MSD enhancement as a function of hydrogen peroxide concentration and the ratio of active to passive colloids. The active to passive gel MSD ratio increases as we vary the peroxide concentration from 0% to 12% at a fixed active to passive colloid ratio of 0.05. At a fixed peroxide concentration of 5%, the gel MSD increases as we increase the amount of active particles within the gel. We explore the implications of the enhanced microscopic dynamics for the macroscopic linear elasticity of the active gels.