Stability of a photosurfactant-laden viscous liquid thread under illumination

• Published in: Journal of fluid mechanics 2024-03, Vol.983, Article A10
• Main Authors: Mayer, Michael D., Kirk, Toby L., Papageorgiou, Demetrios T.
• Format: Article
• Language: English
• Subjects: Approximation; Dynamic stability; Eigenvalues; Exact solutions; Growth rate; Illumination; Incident light; Interface stability; Interfaces; Interfacial properties; JFM Papers; Light effects; Mathematical models; Navier-Stokes equations; Numerical methods; Phase transitions; Rheology; Stability; Surfactants
• ISSN: 0022-1120; 1469-7645
• DOI: 10.1017/jfm.2024.88

This paper investigates the effects of a light-actuated photosurfactant on the canonical problem of the linear stability of a viscous thread surrounded by a dynamically passive fluid. A model consisting of the Navier–Stokes equations and a set of molar concentration equations is presented that capture light-induced switching between two stable surfactant isomer states, trans and cis. These two states display significantly different interfacial properties, allowing for some external control of the stability behaviour of the thread via incident light. Normal modes are used to generate a generalized eigenvalue problem for the growth rate which is solved with a hybrid analytical and numerical method. The results are validated with appropriate analytical solutions of increasing complexity, beginning with a solution to a clean interface, then analytical solutions for one insoluble surfactant, one soluble surfactant and a special case of two photosurfactants with a spatially uniform undisturbed state. Presenting each of these cases allows for a holistic discussion of the effect of surfactants in general on the stability of a liquid thread. Finally, the numerical solutions in the presence of two photosurfactants that display radially non-uniform undisturbed states are presented, and details of the impact of the illumination on the linear stability of the thread are discussed.