Colloidal spheres readily attach to oil-water interfaces where their interactions can be controlled, allowing the assembly of a variety of structures including crystals and gels. The response of these systems to shear can be probed using an interfacial microrheology technique developed in the Squires lab.

I am particularly interested in the way in which local structure determines rheological response and how rheology develops as an initially crystalline system undergoes gelation. Additionally I am taking a close look at how a crystalline system reorders itself near multi-particle clusters and the resulting defect structure.

The presence of aspherical impurities in a quasi-two-dimensional colloidal polycrystal influences both the local structure near the impurity and the overall polycrystalline texture of the entire material. Locally, impurities create defects in their neighbourhoods, disrupting the hexagonal lattice. An increased concentration of impurities is associated with smaller crystalline grains and a related decorrelation in orientational order over shorter lengthscales.

This research is published in the Journal of Physics: Condensed Matter and is the result of two undergraduate research projects performed at the University of Bristol by Andrew Gray and Lizzie Mould.

During my PhD I studied two-dimensional colloidal suspensions confined by light, resulting in the development of the colloidal corral, in which particles are confined by a circular boundary created using optical tweezers. The optical traps are soft, resulting in a flexible wall. This flexibility is the source of a number of interesting phenomena not seen in similar systems confined by a hard boundary. At high density, structural transitions are observed between concentrically layered configurations and highly hexagonal organisation reminiscent of the crystal observed in bulk. These structures exhibit markedly different dynamics due to the distribution of empty space in the system giving rise to multiple relaxation regimes. By controlling the stiffness of the boundary the interior structure and dynamics can be tuned. Furthermore, by measuring displacements of boundary particles from their optical traps mechanical measurements of pressure can be made, revealing further interesting features of adaptively confined materials.

Interfacial microrheology of lung surfactants & serum proteins

When molecules such as surfactants or proteins adsorb at an aqueous-air interface, the properties of that interface are often modified. For instance, pulmonary surfactants greatly reduce surface tension, an effect which is exploited by nature to allow easy breathing. Furthermore, phase transitions between ordered and disordered phases are observed as a function of surface pressure and can be visualised using fluorescence microscopy.

Using the magnetic microbutton microrheology technique developed in the Squires lab, I am probing the response of lung surfactant monolayers to shear deformations and interpreting the observed phenomena in terms of the structure and morphology of condensed domains as observed via fluorescence imaging. Additionally, I am investigating how the rheological response of insoluble surfactant monolayers is modified by the presence of soluble serum proteins in the aqueous subphase.

Quasi-Two-Dimensional Colloidal Polycrystals

Colloids in Adaptive Confinement

I previously studied a microscopic rotational device driven by light, dubbed the colloidal washing machine. By driving colloidal spheres on a circular path torque is transmitted to additional confined spheres. Due to the soft nature of the colloidal ensemble, multiple transmission modes are observed dependent on internal density and the speed with which the device is driven. At microscopic lengthscales, thermal fluctuations are significant and so the colloidal washing machine operates very differently from a macroscopic machine. This research is published in and featured on the cover of Nature Physics.

Interfacial colloids: Crystals, Clusters & Gels

Ian Williams

Torque Transmission in Soft Materials

Postdoctoral Researcher, University of California, Santa Barbara