Computational Studies of Liquid Droplet on Liquid Infused Surfaces

Abstract

Surfaces that can repel any liquid are highly desired in various aspect of life for their wide range of beneficial applications. They can be utilised to enhance production processes, to simplify maintenance processes, to preserve surfaces from fouling, and many more. Ironically, one effective way to repel a liquid from a surface is by infusing the surface with another liquid which acts as a lubricant. Such surfaces are called liquid infused surfaces (LIS). The presence of the lubricant introduces rich interplay between the interfacial properties of the solid surface and the other fluid phases, which leads to many new interfacial phenomena.

We employ two numerical methods for studying the behaviour of a liquid droplet on a LIS. First, we use the lattice Boltzmann method, which is powerful for studying the dynamic evolution of the system. Second, we use the phase field energy minimisation method, which is efficient for finding the equilibrium states of the system. In this thesis, we show how these numerical methods can be exploited to explore a wide range of LIS parameters and to confirm our theoretical predictions.

We start by examining the equilibrium properties of the LIS system and demonstrate that the droplet morphology strongly depends on the choice of liquids used for the droplet-lubricant combination. The droplet morphology, in turn, affects the translational and rotational dynamics of the droplet under the influence of an external body force. Interestingly, we found a complex interplay between contact line pinning and viscous dissipation at the lubricant ridge, which become dominant at large and small apparent angles, respectively. Our investigations further demonstrate that the relative importance of viscous dissipation at the lubricant ridge depends on the drop to lubricant viscosity ratio, as well as on the shape of the wetting ridge.

Next, we demonstrate spontaneous bidirectional motion of droplets on liquid infused surfaces in the presence of a topographical gradient, in which the droplets can move either toward the denser or the sparser solid fraction area. We show that the key factor determining the direction of motion is the preferential wetting of the droplet on the solid surface and on the lubricant film, which depends on the choice of the droplet-lubricant combination.

Finally, we study how the pinning force of droplet on LIS is controlled by the solid surface fraction, the lubricant wetting angles, and the various fluid surface tensions. We derive an analytical prediction for contact angle hysteresis and numerically test the theory. We also discuss why a droplet on a liquid infused surface with partially wetting lubricants typically experiences stronger pinning compared to a droplet on a classical superhydrophobic surface.