Instabilities and pattern formation in viscoelastic fluids

Abstract

Instabilities and pattern formation in viscous fluids have been a major topic of non-linear fluid dynamics for several decades. The study of pattern formation in viscoelastic thin films offers the opportunity to find new fascinating structures that cannot be observed in viscous fluids. Rayleigh–Taylor and Faraday instabilities, such as the resulting patterns in thin films of viscoelastic fluids, are investigated. We use the long-wave approximation and a Karman–Pohlhausen approach to simplify the mass and momentum equations. The viscoelastic stress tensor is calculated applying the linear Maxwell model. Conditions for the Faraday instability have been found using Floquet’s theorem. It is shown that viscoelastic films can exhibit harmonic resonance under external vibration. Moreover, a simulation of the non-linear problem in 2D and 3D is conducted with a finite difference method. Unstable oscillating Rayleigh–Taylor modes occur in the 2D numerical solution. Furthermore, we find that the wavenumber changes with the relaxation time of the fluid. Faraday patterns in viscous films emerge as regular structures of the surface, like squares or hexagons. Numerical simulations of the viscoelastic fluid also show regular structures. However, they collapse into a chaotic stripe-like pattern after a certain time.