Active particles sense micromechanical properties of glasses

Despite their broad technological relevance, our microscopic understanding of glassy materials is surprisingly poor. While the transition from a viscous liquid to a glassy state hardly affects the underlying structure, the viscosity and relaxation time drastically increases and even diverges. Therefore, any evidence of an entanglement between structural and dynamical properties is an important step towards a detailed understanding of glasses.

In our experiments,  we demonstrate a close link between the structural and dynamical properties of a colloidal glass using an embedded self-propelled probe particle. Contrary to classical probe particles, which are forced along a given direction, the orientation of an active particle will undergo temporal changes. Accordingly, the direction of force exerted by an active probe particle on its environment will vary over time. We demonstrate that the rotational diffusion coefficient of an active probe particle continuously increases towards the glass transition and suddenly drops down in the glassy state, independent of the size of colloidal particles providing the glassy background. Such behavior reveals a strong entanglement between the probe’s orientational dynamics and the slow structural relaxation of the glassy environment.

Credit: Celia Lozano


Therefore, active particles might be useful mechanical micro-probes to study the properties of soft materials such as (bio)polymers, wormlike-micelles and cellular tissus. Our findings will also be relevant for numerical studies of active glasses and mixtures of active and passive particles which so far have not considered the enhancement of the particle orientation dynamics. Moreover, given that the natural environment of living microswimmers is often viscoelastic, our findings about the dynamics of active particles in such system may be useful for designing navigation strategies of microorganisms under realistic conditions.