Colloids & polymer microrheology

vortex flow
Schematic diagram of short-time vortex flow in an inertial fluid around a sphere that suddenly starts moving (to the right). The diameter of the vortex ring increases diffusively with time in purely viscous fluids and super-diffusively in viscoelastic fluids.

Conventional soft-matter systems include technical polymers and colloidal solutions. Wide ranges of characteristic structural length and time scales are the reason for a large complexity of dynamic properties observed in these materials. We have been using our high-bandwidth microrheology techniques to study viscoelasticity in entangled synthetic polymer solutions (wormlike micelles), solutions of filamentous viruses, and colloidal suspensions. A prominent question in systems showing aging and possibly glass transitions, such as clay particle suspensions (laponite) has been how to describe, in a thermodynamic formalism, the non-equilibrium character of the systems. We have shown that, contrary to what has been claimed, an effective temperature different from the temperature of the environment is not detectable in aging laponite solutions. Even pure viscous fluids show dynamics on fast timescales that had not been explored well. We could recently directly detect inertial vortex flow around colloids moving in a fluid. When an object suddenly starts to move in a fluid a smoke ring-like flow pattern is started up near the object which then spreads into the fluid in a diffusive manner. We could see this flow pattern directly by tracking correlated and anti-correlated motions between two beads suspended by two optical traps in a fluid.