Oscillatory Spreading and Surface Instability of a Non-Newtonian Fluid under Compression

Starch solutions, which are strongly non-Newtonian, show a surface instability, when subjected to a load. A droplet of the fluid is sandwiched between two glass plates and a weight varying from 1 to 5 kgs. is placed on the top plate. The area of contact between the fluid and plate increases in an oscillatory manner, unlike Newtonian fluids in a similar situation. The periphery moreover, develops a viscous fingering like instability, which is not expected under compression. We attempt to model the non-Newtonian nature of the fluid through a visco-elastic model incorporating generalized calculus. This is shown to exhibit a qualitatively similar oscillatory variation in the surface strain.