Two-dimensional materials are increasingly important since the advent of graphene, and the study of two-dimensional fluids is highly relevant to the behaviour of materials confined at interfaces. Interestingly, while much of physics carries over straightforwardly from three-dimensions, some properties have a strong dependence on dimensionality, including aspects of phase behaviour and criticality. In our laboratory, we use colloidal monolayers composed of relatively large (~1-10 µm ) colloidal particles, which are strongly confined to the surface of their cell and behave as a quasi-two dimensional system.
Such systems are straightforward to image and manipulate using optical landscapes, and so are an excellent playground for testing and developing concepts in liquid-state theory. In particular, COOH-functionalised MF particles have been shown to be an excellent structural model for hard disks, which represent the simplest realistic model of a two-dimensional liquid.
Pair correlation functions in an experimental hard disk model system
We have recently focussed on the phase behaviour of hard disk systems. Unlike their three-dimensional analogue, hard spheres, the fluid-solid phase transition in hard disks has been shown in simulations and experiments to proceed via an intermediate hexatic phase, in which the orientational symmetry of the fluid is broken but the full translational symmetry is retained. Further work is using a variety of static and dynamic optical landscapes to investigate material properties of 2D fluids and solids, and will also focus on the behaviour of 2D glasses.
Left: The fluid-hexatic interface in a hard-disk model system, showing capillary wave fluctuations; Right: Capillary wave analysis
Many other aspects of our research are conducted in two-dimensional systems, including experiments on grain boundaries, colloidal gels, active particles and driven systems.