Fingering instability in Marangoni spreading on a deep layer of polymer solution

TL;DR

This study investigates fingering instability during Marangoni spreading on deep polymer solutions, revealing how molecular weight and concentration influence pattern formation and proposing a generalized spreading law considering viscoelastic effects.

Contribution

It introduces a detailed analysis of fingering instability in complex fluids, incorporating viscoelastic and shear thinning effects into a new spreading model.

Findings

Fingering wavelength depends on polymer molecular weight and concentration.

Normal stresses at high shear rates cause deformation leading to fingering.

A generalized law for spreading of viscoelastic fluids is proposed.

Abstract

Spreading on the free surface of a complex fluid is ubiquitous in nature and industry, owing to the wide existence of complex fluids. Here we report on a fingering instability that develops during Marangoni spreading on a deep layer of polymer solution. In particular, the wavelength depends on molecular weight and concentration of the polymer solution. We use the Transmission Lattice Method to characterize the finger height at the micron scale. We model the evolution of spreading radius, involving viscoelastic and shear thinning effects, to suggest a more generalized law than the spreading of Newtonian fluids. We give physical explanation on the origin of the fingering instability as due to normal stresses at high shear rate generating high contact angle and deformation at the leading edge, and so selects the wavelength of the fingering instability. Understanding the spreading mechanism has particular implication in airway drug delivery, and surface coating with patterns.