Spreading of Non-Newtonian and Newtonian Fluids on a Solid Substrate under Pressure

TL;DR

This study investigates the unique spreading and deformation behaviors of non-Newtonian starch gels under compression, revealing oscillatory contact area changes and instabilities, and models these phenomena using a visco-elastic framework with generalized calculus.

Contribution

It provides the first detailed experimental analysis of non-Newtonian gel spreading under pressure and introduces a visco-elastic model that captures observed behaviors.

Findings

Non-Newtonian gels exhibit oscillatory contact area during compression.

Instability patterns similar to Saffman Taylor fingers are observed.

A visco-elastic model with generalized calculus qualitatively reproduces experimental results.

Abstract

Strongly non-Newtonian fluids namely, aqueous gels of starch, are shown to exhibit visco-elastic behavior, when subjected to a load. We study arrowroot and potato starch gels. When a droplet of the fluid is sandwiched between two glass plates and compressed, the area of contact between the fluid and plates increases in an oscillatory manner. This is unlike Newtonian fluids, where the area increases monotonically in a similar situation. The periphery moreover, develops an instability, which looks similar to Saffman Taylor fingers. This is not normally seen under compression. The loading history is also found to affect the manner of spreading. We attempt to describe the non-Newtonian nature of the fluid through a visco-elastic model incorporating generalized calculus. This is shown to reproduce qualitatively the oscillatory variation in the surface strain.