Yield stress behavior of colloidal gels with embedded active particles

Abstract

The yield stress behavior of colloidal gels with embedded active particles is studied with three experiments: start-up of steady shear, oscillatory strain amplitude sweep, and creep testing. Activity is generated by Janus particles with a platinum hemisphere; these particles undergo self-diffusiophoretic and self-electrophoretic motion in hydrogen peroxide solutions. The free particle active motion of the Janus colloids is consistent with an active energy of 25 times thermal energy at the highest hydrogen peroxide concentration studied. Consistent with prior work, the gels with embedded active particles display enhanced microdynamics and a reduction in linear viscoelastic moduli. Furthermore, at the activity levels studied, the yield stress decreases by as much as threefold for gels with an active-to-passive particle ratio of only 1:1200. We additionally find the yield strain is independent of activity. The significant reduction in yield stress at a very low active-to-passive particle ratio is modeled by combining the theory of how activity changes the spring constant of interparticle bonds in the gel with an argument that the number of active fractal clusters in the gel—rather than the number of active particles—drives the activity-induced softening of rheological properties. We estimate that 1 in 40 fractal clusters in the gel is active due to the presence of an active particle. This approach explains how an extremely small fraction of active particles causes a substantial change in both linear and nonlinear rheological properties. The results and modeling are potentially useful for the creation of gels with multistate mechanical properties.