From Binary to Ternary Fluid Systems

Abstract

This thesis focuses on key interfacial phenomena where the presence of three fluid phases or components lead to new effects which are absent in their binary system equivalents. We study three ternary fluid phenomena related to the formation of capillary bridges, de-wetting of droplets and phase separation of mixtures.

Firstly, we numerically study two-component capillary bridges formed when a liquid droplet is placed in between two liquid-infused surfaces (LIS). Two-component liquid bridges can exhibit a range of different morphologies where the liquid droplet is directly in contact with two, one, or none of the LIS substrates. We also characterize the capillary force, maximum separation, and effective spring constant and find that they are influenced by the shape and size of the lubricant ridge. Importantly, we argue that LIS are not only “slippery” parallel to the surface, but they are also “sticky” perpendicular to the surface.

Secondly, we investigate a novel de-wetting phenomenon whereby droplet lift-off is driven by an incoming film of another immiscible liquid. This mechanism exploits the lifting force arising from the triple contact line between the droplet, film and surrounding air. We find droplet detachment from the substrate can occur with film thickness that is comparable to the droplet size. As such, we believe this mechanism is potentially interesting for developing a sustainable and more environmentally friendly way to clean.

Thirdly, we explore the spontaneous phase separation of ternary fluid mixtures, both when all the surface tensions are equal and when they have different values. By combining systematic computer simulations over the full range of the composition space and theoretical analysis on the eigenvalues and eigenvectors of the unstable modes, here we identify four fundamental phase separation pathways. In particular, we highlight a dominant but so-far overlooked mechanism involving enrichment and instability of the minor component at the fluid-fluid interface.

Another key contribution of the thesis is the development of suitable computational schemes for modelling ternary fluids, required to pursue the phenomena described above. Here, we employ a combination of Surface Evolver and Lattice Boltzmann Method.