Stretching of viscoelastic drops by steady sliding

TL;DR

This study investigates how polymer flexibility and elasticity influence the elongation and sliding behavior of viscoelastic drops, combining experiments with numerical simulations to understand the complex interplay of forces involved.

Contribution

It provides new insights into the role of polymer elasticity and shear thinning in the steady sliding and elongation of viscoelastic drops, supported by experimental and simulation data.

Findings

Flexible polymers cause significant drop elongation during sliding.

An optimal polymer elasticity enhances drop elongation.

Elastic parameters influence the dimensionless sliding velocity.

Abstract

The sliding of non-Newtonian drops down planar surfaces results in a complex, entangled balance between interfacial forces and non linear viscous dissipation, which has been scarcely inspected. In particular, a detailed understanding of the role played by the polymer flexibility and the resulting elasticity of the polymer solution is still lacking. To this aim, we have considered polyacrylamide (PAA) solutions of different molecular weights, suspended either in water or glycerol/water mixtures. In contrast to drops with stiff polymers, drops with flexible polymers exhibit a remarkable elongation in steady sliding. This difference is most likely attributed to different viscous bending as a consequence of different shear thinning. Moreover, an "optimal elasticity" of the polymer seems to be required for this drop elongation to be visible. We have complemented experimental results with numerical simulations of a viscoelastic FENE-P drop. This has been a decisive step to unravel how a change of the elastic parameters (e.g. polymer relaxation time, maximum extensibility) affects the dimensionless sliding velocity.