Active Matter

“Active Brownian Ring Polymers”

Active matter is inherently out of equilibrium. By converting either internal energy into directed motion or utilizing energy from the environment, active matter agents maintain a non-equilibrium steady state, which is associated with a persistent active stress.

We consider the conformational and dynamical properties of semiflexible ring polymers consisting of chains of active Brownian particles (ABPs). The motion of the individual ABP monomers is characterized by a body-fixed propulsion direction plus orientational diffusion. An experimental realization can be chains of diffusiophoretic Janus colloids.

The fluctuation spectrum of normal-mode amplitudes is analyzed. At elevated activities, flexible (tension) modes dominate over bending modes even for semiflexible rings, corresponding to enhanced conformational fluctuations. The pertinent result is that the amplitudes of the long-wavelength modes are strongly enhanced by activity. The fluctuation spectrum exhibits a crossover from a quadratic to a quartic dependence with increasing mode number. Conformationally, the mean square ring diameter exhibits swelling qualitatively comparable to linear polymers. The ring’s diffusive dynamics is enhanced, and the mean square displacement shows distinct activity-determined regimes, consecutively, a ballistic, a subdiffusive, and a diffusive regime.

Reference: S.M.Mousavi, G.Gompper, amd R.G. Winkler, Active Brownian ring polymers. The Journal of Chemical Physics 150, 064913 (2019)

Self-organizing, Self-healing Material of Magnetic Spinners

Smart and active materials can adapt their properties spontaneously in response to changing environmental conditions; these dynamically controllable properties have already led to a wide range of applications. For example, shock absorbers containing magnetorheological fluids are able to adapt their dampening properties.  

We have developed and studied a new, active system of magnetic microparticles moving at an air-water interface. The particles are set into spinning motion by a magnetic field rotating in the plane of the water surface.

The system self-organizes in a non-equilibrium steady state. Depending on the frequency of the magnetic field, the particles cluster together to form shorter or longer rotating chains. Furthermore, the chains arrange themselves spontaneously and dynamically at the interface in a crystal-like structure. After a disturbance, e.g. by dropping a larger particle into their arrangement, the order is rapidly restored in a self-healing process.

Also, the system has tuneable transport properties. Non-magnetic particles can be actively transported through the system and that this transport can be controlled externally through the field frequency.

Reference: K. Han, G. Kokot, S. Das, R. G. Winkler, G. Gompper, A. Snezhko; Reconfigurable structure and tunable transport in synchronized active spinner materials; Science Advances 6, eaaz8535 (2020)