Dynamic wetting behavior of soluble surfactant-laden droplets under shear: A phase field based lattice Boltzmann method

Abstract

Surfactants can reduce the surface tension of fluid interface and significantly change the wetting characteristics, demonstrating versatility in droplet manipulations. In this work, a phase field based lattice Boltzmann method (PF-LBM) is employed to investigate the dynamics behavior of soluble surfactant-laden droplets on solid surfaces subjected to a linear shear flow. We propose to extend the Yokoi dynamic contact angle model to the scenarios containing surfactant effects. Comparisons between the Yokoi model with the Young equation-based model demonstrate that the dynamic contact angle predicted by the Young equation-based model remains constant on the neutral surface. It implies that the Young equation-based model has limitations in accurately describing the dynamics behavior of droplet with soluble surfactant. By contrast, the Yokoi model reproduces more reasonable dynamic variation of contact angle, demonstrating good universality and reliability. Furthermore, we employ the Yokoi model to further investigate the shearing behavior of soluble surfactant-laden droplets, focusing on the influence of surfactant concentration on droplet deformation and identifying the critical conditions for droplet rupture. The results reveal that for a certain effective capillary number (Ca_e), there exists a critical surfactant concentration, beyond which the droplet rupture occurs. Moreover, the critical surfactant concentration for droplet rupture decreases monotonically with increasing Ca_e.