Instabilities in freely expanding sheets of associating viscoelastic fluids

TL;DR

This study investigates how viscoelastic fluids behave under extreme deformation during drop impacts, revealing that material rheology and relaxation times critically influence sheet stability and hole formation.

Contribution

It demonstrates that the Deborah number related to the shortest relaxation time governs instabilities in expanding sheets of viscoelastic fluids.

Findings

Highly viscoelastic fluids exhibit irregular sheet thickness and instabilities.

The Deborah number predicts the onset of heterogeneities and holes.

Sheet tearing dynamics depend on expansion rate when rupture occurs.

Abstract

We use the impact of drops on a small solid target as a tool to investigate the behavior of viscoelastic fluids under extreme deformation rates. We study two classes of transient networks: semidilute solutions of supramolecular polymers and suspensions of spherical oil droplets reversibly linked by polymers. The two types of samples display very similar linear viscoelastic properties, which can be described with a Maxwell fluid model, but contrasting nonlinear properties due to different network structure. Upon impact, weakly viscoelastic samples exhibit a behavior qualitatively similar to that of Newtonian fluids: A smooth and regular sheet forms, expands, and then retracts. By contrast, for highly viscoelastic fluids, the thickness of the sheet is found to be very irregular, leading to instabilities and eventually formation of holes. We find that material rheological properties rule the onset of instabilities. We first provide a simple image analysis of the expanding sheets to determine the onset of instabilities. We then demonstrate that a Deborah number related to the shortest relaxation time associated to the sample structure following a high shear is the relevant parameter that controls the heterogeneities in the thickness of the sheet, eventually leading to the formation of holes. When the sheet tears-up, data suggest by contrast that the opening dynamics depends also on the expansion rate of the sheet.