Temporal stability of free liquid threads with surface viscoelasticity

TL;DR

This paper investigates how surface viscoelasticity influences the stability of free liquid threads, deriving new models and dispersion relations to understand the stabilizing effects of surface viscosities across various parameters.

Contribution

It introduces a new dispersion relation incorporating surface viscoelasticity and compares two lubrication models, extending prior stability analyses of liquid jets.

Findings

Surface viscosities stabilize the liquid thread across all parameters.

Maximum amplification wavenumber varies non-monotonically with Boussinesq numbers.

Higher-order models accurately predict stability over a wide parameter range.

Abstract

We analyse the effect of surface viscoelasticity on the temporal stability of a free cylindrical liquid jet coated with insoluble surfactant, extending the results of Timmermans & Lister (J. Fluid Mech., vol. 459, 2002, pp. 289-306). Our development requires, in particular, deriving the correct expressions for the normal and tangential stress boundary conditions at a general axisymmetric interface when surface viscosity is modelled with the Boussinesq-Scriven constitutive equation. These stress conditions are applied to obtain a new dispersion relation for the liquid thread, which is solved to describe its temporal stability as a function of four governing parameters, namely the capillary Reynolds number, the elasticity parameter, and the shear and dilatational Boussinesq numbers. It is shown that both surface viscosities have a stabilising influence for all values of the capillary Reynolds number and elasticity parameter, the effect being more pronounced at low capillary Reynolds numbers. The wavenumber of maximum amplification depends non-monotonically on the Boussinesq numbers, especially for very viscous threads at low values of the elasticity parameter. Finally, two different lubrication approximations of the equations of motion are derived. While the validity of the leading-order model is limited to small enough values of the elasticity parameter and of the Boussinesq numbers, a higher-order parabolic model is able to accurately capture the linearised behaviour for the whole range of values of the four control parameters.