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Optical Manipulation of Colloidal Interfaces, Droplets and Crystallites

Periodic Reporting for period 2 - OMCIDC (Optical Manipulation of Colloidal Interfaces, Droplets and Crystallites)

Reporting period: 2018-12-01 to 2020-05-31

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 use optical tweezing and confocal microscopy to gain fundamental insight into interfacial phenomena by actively manipulating colloidal interfaces, droplets, crystallites and liquid crystalline droplets. In particular, we plan to address phenomena like nucleation, evaporation, coalescence and capillary condensation and nematisation in colloidal liquids, crystallites and liquid crystals. We believe this will be a very rewarding approach that opens up a range of exciting possibilities to actively investigate and manipulate interfaces, droplets and crystallites, thereby providing a new perspective on interfacial phenomena in complex fluids.
To date, we have finished the design of our experimental setup combining confocal microscopy and optical tweezing, and we are finishing the details of this setup. We have also re-designed and built an optical tweezing setups for the manipulation of interfaces in colloidal crystals and for our experiments with driven colloids in optical potential energy landscapes. In addition, we have synthesised the colloidal spheres required for the colloidal gas-liquid and crystal systems and developed new colloidal rod-like and banana-shaped SU-8 particles for the experimental involving colloidal liquid crystals. We have also developed now TPM colloidal spheres with an embedded off-centre core, which allows us to study the rotational dynamics of colloids in real-space. We have started the optical manipulation of colloidal liquid-gas interfaces and the experiments on confinement-induced interfacial phenomena. Furthermore, we have developed a unique method that allows us to fully characterise the geometric properties of grain boundaries (crystal-crystal interfaces) in colloidal crystals based on particle coordinates only. In addition, we have acquired exciting data on both the growth of colloidal crystallites and the break-up of large colloidal single crystals using small-angle X-ray scattering (SAXS) and confocal microscopy. Finally, we have studied interfaces (isotropic – nematic and nematic – smectic-like) in colloidal liquid crystals using our newly developed colloidal SU-8 rods and established the phase behaviour of our new colloidal SU-8 banana-shaped particles.
With the development of our combined optical tweezing and confocal microscopy setup, our new colloidal rod- and banana-shaped and spheres with an off-centre core model particles and our grain boundary analysis method we have already clearly made progress beyond the current state of the art. Also, we have established the phase behaviour of colloidal banana-shaped liquid crystals, in which we observed the elusive splay-bend nematic phase. We are currently building onto our promising preliminary results to push our experiments further to the stage that we can take systematic date to not only significantly broaden existing knowledge on interfacial phenomena in complex fluids but also open a new field in actively manipulating and controlling colloidal interfacial phenomena.
Confocal microscopy image of the splay-bend nematic phase.
The splay-bend nematic phase with the banana particles coloured according to their orientation.