Experimental Study of the Instability of the Viscous Flow Past a Flexible Surface

The viscous instability in the flow past a soft material is experimentally studied. The experiment is carried out using the parallel plate geometry of a rheometer, and a sheet of polyacrylamide gel of thickness about 4.5 mm is placed on the bottom plate. The fluid, silicone oil, is placed on the surface of the gel and the top plate is lowered till a preset gap of thickness between 300 and 1000$ \mu$m is attained. The rheometer is operated in the stress controlled mode, where the stress is increased at a constant rate, and the strain rate and apparent viscosity (assuming the flow in the gap is laminar) are recorded. Care is taken to ensure the Reynolds number is less than 1, so that inertial effects are negligible. The experimental results show that there is an anomalous increase in the apparent viscosity, determined assuming the flow is laminar, at a certain strain rate. This indicates that the flow becomes unstable and undergoes a transition from a laminar flow to a more complicated flow profile. This transition is repeatable if the experiment is stopped before there is irreversible damage to the gel surface. The experimental results are compared with theoretical predictions, and quantitative agreement is found with no adjustable parameters for a range of gap thicknesses and gel moduli