These experiments are motivated by results in the Group of Dan Goldman (GaTech) with granular media. Among other things, our colleagues found that at a certain packing fraction, the drag exerted by sand on a bar as it displaced at constant speed exhibits oscillations related to formation of slip planes. Our goal is to see whether there is an analog to this behavior in colloidal microgel suspensions, which are an example of a viscoelastic fluid.

Experiment:

Figures 1 and 2 display our set-up. The brass par is connected to the linear actuator that moves across the stage at a constant velocity. The drag force is measure by a strain gauge placed on the bar. The bending of the bar due to the drag force is measured as a voltage and converted into a force.

We are currently calibrating the system and removing possible sources or error (such as noise). The movie below demonstrates a typical run (1 mm/s for 1 cm in water)