Thapa, Anshu; (2024) Exploring the Dynamic Behaviour of Droplets at Complex Liquid-Liquid Interfaces. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Droplets are constantly interacting with their environment. A prototypical example is the case of a sessile droplet evaporating on a solid substrate. As a result, they are often in a non-equilibrium state and exhibit complex flows and behaviour that drive their motion and self-assembly processes. Droplet phenomena have been widely studied on solid (rigid) surfaces, lubricated surfaces, in a liquid bath, and on soft substrates made from polymers or gels. In this thesis, I explored droplet dynamics in a novel setting: deeply metastable aqueous droplets placed at the boundary of a planar “solid-like” oil-water interface composed of cellulose nanocrystal surfactants (NCS) assembly. By adding ethanol to water droplets, I demonstrated that the coalescence with the bulk aqueous subphase and between multiple contacting droplets can be tuned from of seconds to days. I then explored the attraction dynamics between two or more droplets placed at the solid-like oil-water interface, a phenomenon mediated by the interfacial deformation of the solid-like oil-water interface under the presence of a droplet. A modified theory based on capillary forces was developed to describe the droplet attraction quantitively, which revealed a new scaling relation for capillary-driven droplet dynamics. The droplet-droplet attraction was then exploited to build ensembles of droplets with complex 2D geometries, as well as multi-layered droplets, at the oil-water interface using a capillary-assisted droplet printing method. Applications of these ensembles in compartmentalising chemicals and encoding messages were also explored. Finally, gold nanoparticles (AuNPs) were incorporated into the system for its thermoplasmonic properties and impart additional functionality. To this effect, a laser (532 nm) was used to externally manipulate droplet motion and droplet cluster dynamics. Overall, the work presented in this thesis provides valuable insights into the dynamics of droplets situated at planar oil-water interfaces and paves the way for future studies in this novel setting.

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