Interfaces and interfacial phenomena like nucleation, droplet coalescence and capillarity are amongst the most widely experienced phenomena in life, with relevance to areas as diverse as oil recovery, food and inkjet printing. The importance of interfacial phenomena has been further prompted by the recent and rapid emergence of micro- and nano-fluidics, because at these small length scales, interfacial phenomena are particularly dominant and interface dynamics strongly affect the transport and response of fluids in such devices. Actively controlling interfacial phenomena is therefore becoming increasingly desirable, both for providing new and fundamental insights into interfacial phenomena and for the successful implementation of micro- and nano-fluidic devices. The interfacial roughness in atomic and molecular systems is, however, typically in the (sub)nanometre range, making the relevant interfacial phenomena experimentally very hard to access. Colloidal systems, which consist of particles with a size between roughly one nanometre and several micrometres dispersed in a molecular solvent, provide a unique model system to explore interfacial phenomena in great detail due to their ultralow interfacial tension.
In this project, we have used optical tweezing and confocal microscopy to gain fundamental insight into interfacial phenomena by actively manipulating colloidal interfaces, droplets, crystallites and liquid crystalline droplets. The main scientific aims were:
1. Active deformation of interfaces in colloidal systems
2. Nucleation and evaporation of droplets, crystallites and tactoids
3. Coalescence and detachment of droplets, crystals and tactoids
4. Heterogeneous nucleation and capillary phenomena
The most important conclusions from this project are: (i) that colloidal nematic liquid droplets can be generated using optical tweezing of specially developed SU-8 colloidal rods; (ii) that banana-shaped colloidal particles can form a splay-bend nematic phase – and thus that this phase does exist, as this is the first experimental observation of this phase since its theoretical prediction more than 40 years ago; (iii) that polydispersity and entropy alone can result in hierarchical self-assembly (in a vortex phase); (iv) that solid-solid interfaces (grain boundaries) move according to an adaptation of the geometric framework called the O-lattice, which predicts the motion of both particles and dislocations during grain boundary migration; (v) that optical tweezing can be used to nucleate and manipulate colloidal liquid droplets and that we can study their interface fluctuations; (vi) that the emergence of interparticle friction is important in attractive colloidal systems and can be understood in terms of the coordination number of particles.
