| dc.description.abstract | Colloidal suspensions of polyballs (polystyrene spheres) are good model systems for the study of both structural and dynamical properties of solids and liquids because the pair-wise interaction between the colloidal particles is known and can be tuned easily in the laboratory. Under suitable experimental conditions, the arrangement of colloidal particles can mimic the different kinds of structures found in conventional atomic systems, such as fluids, crystals and even glasses. However, the length scales and the time scales involved in the colloids are orders of magnitude larger than in conventional atomic systems. Therefore, it becomes possible to perform easily experiments where the colloidal suspension is subjected to strong external perturbations. For example, one can subject a colloidal system to stresses comparable to its own typical energy density, or one can shear the polyball suspension with rates comparable to the inverse of its typical density relaxation time. In this way one gets to observe fascinating phenomena, such as laser?induced freezing, shear?induced melting and nonlinear flow behaviour under shear which are almost impossible to observe in conventional atomic systems. In this thesis we shall focus attention on two such interesting phenomena observed in colloids: (a) Laser?Induced Freezing and (b) the self?diffusion and the structural properties of a colloidal liquid under shear flow.
In Chapter 1 we give a brief introduction to colloidal suspensions emphasising its similarities with and its differences from conventional atomic systems. We then present a brief overview of the phenomena of Laser?Induced Freezing and the rheological properties of colloids under shear flow, and of the new works presented in this thesis, namely (1) density?functional and simulation studies of laser?induced freezing and (2) simulation studies of sheared colloids.
In Chapter 2 we elaborate our density?functional theory of laser?induced freezing, i.e., the freezing of a colloidal liquid into crystalline order in the presence of an external periodic modulating potential VeV_eVe? with its wavevectors tuned to the ordering wavevector in the liquid phase. We show definitively that the initial first?order freezing transition (at small VeV_eVe?) changes over to a continuous one (at large VeV_eVe?) via a tricritical point provided the modulation wavevectors are suitably chosen. We also apply our scheme of calculation to predict the parameter values of the tricritical point for a realistic colloidal system.
In Chapter 3 we present results from a Monte?Carlo simulation study of laser?induced freezing to investigate the phase diagram and the order of modulated liquid?to?solid transition. We find that for low values of VeV_eVe? the transition is first?order and is continuous for high values of VeV_eVe?, in agreement with the conclusions from our theory in Chapter 2. However, we find that there is a re?entrant solid?to?liquid transition with increasing VeV_eVe?, which is extremely interesting.
In Chapter 4 we investigate how the effects of fluctuations in the order parameters could modify the mean?field density?functional analysis of laser?induced freezing in the large?VeV_eVe? regime. Within the framework of simple approximations, we are able to show that the continuous nature of the freezing transition at large values of the external potential VeV_eVe? is robust even in the presence of fluctuations. We also show that the fluctuations tend to enhance the stability of the liquid phase in the large?VeV_eVe? regime, a feature that is consistent with the re?entrant behaviour observed in our simulations.
In Chapter 5 we present the results of Brownian Dynamics simulations of a charged colloidal suspension under oscillatory shear flow with both Couette and Poiseuille velocity profiles. We show that in the “steady?shear” limit, for both the velocity profiles, the enhancement of the self?diffusion coefficient in directions transverse to the flow shows a cross?over from a ??1/2\dot{\gamma}^{1/2}???1/2 dependence, for small ??\dot{\gamma}???, to a ??\dot{\gamma}??? dependence for large ??\dot{\gamma}??? as the shear rate ??\dot{\gamma}??? increases. The magnitude of the enhancement is a function of interparticle interaction strength. There is a layering tendency in the direction of the velocity gradient for Couette flow, which is not that prominent for Poiseuille flow.
Chapter 6 summarises our conclusions and indicates future directions of work. | |
