Simulation of phase separation in quiescent and sheared liquids

In this thesis we report on Molecular Dyanamics simulation of phase separation of complex liquids in sheared and non-sheared systems. Phase separation is widely observed in nature and is a building block of most of the living organisms. As for example various micro organs in the human cell can have different functionality as they are separated and structured differently. One of a very common example of phase separation can be observed in daily life in curdling of milk after addition of lemon juice. Milk separates in two liquids by process of phase separation liquid whey and solid curd. The aim of our study is enhance the understanding of rheology of these systems and to simulate the dynamics of the phenomenon of phase separation. In our simulations we use simple and polymeric liquid including liquid crystals and use Couette geometry to study the effect of shear. (Couette cell is a geometry with two concentric cylinders). Although here in our study we use simple model to represent our liquids but the results are applicable to much wider range of liquids. The principles driving the phenomenon of phase separation are very fundamental in nature and all the separation phenomenon can be classified in viscoelastic phase separation, where solid-solid and liquid-liquid phase separation being two special cases of the same.

Most important property in the study of dynamics of phase separation is the scaling of the domain size with time. Various exponents represent different stages of phase separation. Most of the previous Lattice-Boltzmann simulations show the linear scaling corresponding to the hydrodynamic regime. In chapter 2 we establish that there is a transition from diffusive to hydrodynamic driven growth regime, and domain growth before the interface formation is by diffusion. At early diffusive stage the effect of temperature is clearly visible specially the higher temperatures show prolonged diffusive regime. We also show that at higher temperature critical slowing down plays a very important role and hence is responsible for delay in collective hydrodynamic behaviour. Simulations at temperature higher than the critical temperature result in as expected no separation. We also calculate the transition time from diffusive to viscous growth which are very important in studying sheared systems.