Marangoni driven spreading of thin liquid films on liquid substrates

Abstract

This thesis uses experiments and numerical or theoretical studies to explore two distinct instabilities that can occur during the surfactant-driven spreading of thin liquid films on liquid substrates. Thin liquid films are significant in many processes, from coating flow technology and environmental applications to biological systems. Understanding the physico-chemical mechanisms at play in spreading thin liquid films in the presence of surfactants is thus essential for developing medical and industrial solutions. In Chapters 2 and 3, the fragmentation into droplets of a spreading film containing surfactants on a liquid viscous substrate is investigated experimentally and analytically using lubrication theory. Due to Marangoni forces, as the film spreads axisymmetrically over the liquid substrate, a rim is formed at the liquid-liquid-gas contact line of the film. Ultimately, this rim becomes unstable and breaks down into a myriad of droplets. The ratio of surfactant concentrations present initially in the spreading film and at the surface of the liquid substrate plays a key role in the instability. By varying the surfactant concentration present initially in the spreading film or on the liquid substrate, we find that the fragmentation can occur for intermediate values of this ratio of surfactant concentrations ranging from 10 to 1000. The trends in the dynamics of the contact line of the film are also directly related to the occurrence of the fragmentation instability. A model comprising a confined liquid-liquid bilayer system lying over a solid substrate and containing surfactants at the liquid-liquid interface is solved using a Newton-Raphson solver. The model allows for transfers of surfactants from the film-subphase interface before the contact line of the film to the air-subphase interface beyond the contact line. We find that depending on this transfer of surfactants, surfactant concentration and velocity shocks can be observed at the contact line. Additionally, the numerically predicted power-law-like dynamics for the surfactant front show quantitative agreement with the experiments in both the intercept and exponent of the power law, while the predicted dynamics of the contact line agree with the experiments qualitatively. In Chapter 4, the spontaneous formation of holes, which we call dewetting, at the surface of a spreading film on a liquid substrate is investigated experimentally. We test the hypothesis that the dewetting phenomena observed is made possible by the attractive van der Waals forces acting on the thin film. A Minkowski functionals-based morphological analysis of the hole distribution observed on the film shows that the dewetting observed is not due to a growing instability at the surface of the film but rather to the presence of defects. Dewetting velocities, i.e. velocities at which the holes grow, are found to be uniform across the spreading film and constant through time. In Chapter 5, the dewetting of the upper film of a liquid-liquid bilayer lying on a solid substrate is studied through linear stability analysis. The system is unstable for long wavelengths when van der Waals forces are attractive and dominant over short-range repulsive intermolecular forces. For shorter wavelengths, the system is stabilised by capillary forces. Gravitational effects considered in the liquid subphase can have a damping effect on the perturbations, but within the experimental parameter space, this effect is negligible. Surfactants can slow down the onset of rupture but cannot prevent it. Additionally, increasing the Marangoni number Ma or the Péclet number Pe of the system leads to weaker growth of the perturbations. Through the study of dewetting and fragmentation instabilities, this thesis contributes to the understanding of Marangoni-driven flows and the impact surfactant may have on the dynamics of spreading liquid films by proposing new theoretical elements and by quantitatively studying experimental configurat

Date of Award	1 Aug 2024
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Andrew Hazel (Supervisor) & Julien Landel (Supervisor)