Surfactants on liquid-infused surfaces

QC 260720

Abstract

Liquid-infused surfaces (LIS) have emerged as a promising bio-inspired technology for drag reduction and anti-fouling applications. By trapping a lubricating liquid within a micro-textured substrate, LIS replace part of a rigid solid wall with a liquid-liquid interface. If this interface remains mobile, the external flow can experience slip and reduced viscous resistance. However, experiments often show significantly lower slip than predicted by ideal clean-interface models. This licentiate thesis investigates the physical mechanisms underlying this performance degradation, with particular focus on surface-active agents (surfactants).

Using local experimental measurements and direct numerical simulations (DNS), this work shows how trace amounts of surfactants at the liquid-liquid interface can generate Marangoni stresses that strongly reduce slip. The thesis is based on two appended papers. The first study uses Doppler optical coherence tomography (D-OCT) to extract local slip velocities on longitudinal LIS. The measured slip lies far below clean-interface predictions and agrees closely with simulations of an immobilised interface. To explain this immobilisation, the second study uses DNS to resolve surfactant transport coupled to the fluid flow. The results show that shear-driven interfacial motion redistributes surfactants along the interface, creating concentration and surface tension gradients. These gradients generate Marangoni stresses that oppose the external flow and can reduce the interfacial velocity to nearly zero. Three-dimensional simulations further show that, at high Marangoni number, non-uniform surfactant distributions can drive recirculating interfacial motions even when the mean slip is suppressed. Finally, surfactant leakage from the interface is shown to partially recover interfacial mobility.

Together, these findings bridge the gap between ideal clean-interface theory and experimental observations. The thesis establishes surfactant-induced Marangoni stresses as a central mechanism for slip degradation on liquid-infused surfaces. Consequently, the practical design and evaluation of LIS cannot rely on clean-interface theory alone. Even trace contamination can reduce the interfacial velocity to nearly zero, making the liquid-liquid interface behave effectively as an immobilised boundary.

Link to DiVA