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Title: Colloids in optically defined confinement
Author: Williams, Ian
Awarding Body: University of Bristol
Current Institution: University of Bristol
Date of Award: 2013
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Materials under spatial confinement are subject to volume exclusion conditions differing from those of a bulk material. On reducing a system to a lengthscale of the order of a few particles onc can observe new structures, modified dynamics and phase behaviour dramatically differing from that of the bulk system. Through understanding and controlling the boundaries confining a system one can alter the energy landscape it experiences. Three colloidal model systems are realised using holographic optical tweezers to investigate confinement phenomena in quasi two dimensions. ~Whereas the bulk of existing rescarch is concerned with confinement by hard boundaries, the systems presented here employ an adaptive, deformable wall or a soft confining potential. The first system, dubbed the colloidal corral consists of a circular boundary of optically trapped colloids confining additional colloids to its interior. Despite the fact that boundary curvature inhibits hexagonal ordering within this geometry, a bistability is observed between locally favoured hexagonal structures and globally preferential configurations mimicking the symmetry of the confinement. Such behaviour is not observed with hard wall confinement and is entirely due to the adaptivity of the boundary. Additionally, a novel technique for the mechanical measurement of pressure is presented, facilitated by the deformable wall. The second system introduces shear to the colloidal corral through rotation of the boundary and is known as the colloidal washing machine. The flow behaviour in this rotating confinement is investigated for a range of rotation rates. Rigid-body- like and shear melted regimes are identified and their structural behaviours characterised. The final system confines colloids within a circularly symmetric optical potential without a boundary. The result is the assembly of two-dimensional clusters, which are characterised for a range of potential widths and depths. The absence of a curved wall allows enhanced hexagonal ordering compared to the colloidal corral.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available