Davies, GB; (2015) Modelling Colloidal Particles Adsorbed at Fluid-Fluid Interfaces. Doctoral thesis , UCL (University College London).

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Abstract

This thesis concerns the modelling of the interactions between colloidal particles and fluid-fluid interfaces. Colloidal particles -- micrometre to nanometre sized solid particles -- adsorb strongly at fluid-fluid interfaces because they reduce energetically costly fluid-fluid interface area. Once adsorbed at an interface, colloidal particles interact via various physical forces. One such type of interaction, capillary interactions, emerge because particles deform the fluid-fluid interface leading to excess interface area. The particles minimise this excess interface area by overlapping their respective interface deformations. In this thesis we develop a new theoretical model describing the detachment energy of prolate and oblate ellipsoidal particles from fluid-fluid interfaces. In so doing, we verify a lattice Boltzmann based numerical method and confirm that it is a powerful tool for modelling emulsions. We investigate the rotation of magnetic prolate spheroidal particles at fluid-fluid interfaces by applying an external magnetic field directed normal to the interface. The particles experience a torque attempting to align them with the external field, but surface tension resists this rotation. Consequently, the particles tilt with respect to the interface. We show that during tilting, the particles deform the interface in a dipolar manner, providing an experimental realisation of a long sought-after capillary interaction mode. We further confirm a predicted first-order phase transition with respect to the particle's orientation as a function of the dipole-field strength. Finally, we utilise these induced dipolar capillary interactions to assemble ellipsoidal particles at fluid-fluid interfaces, and investigate the structures they form. We show that the capillary interactions and the resulting structure formation can be tuned by varying the dipole-field strength, providing one of the first realisations of truly tunable capillary interactions. Additionally, we show that by exploiting the first-order phase transition, we can switch the capillary interactions on and off by causing the particles to flip from a tilted state to a vertical state in which interface deformations and hence capillary interactions disappear. Our results have implications for the directed assembly of colloidal particles at fluid-fluid interfaces and for the creation of novel, reconfigurable materials composed of colloidal particles.

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