Structure and dynamics of fluids : from molecular to colloidal perspectives

Relationships between structure and dynamics have been well studied in molecular fluids, both in computer simulations and in experiments. However, the development of simple structure-dynamics relationships would also be useful in understanding colloidal fluids. Colloidal fluids display differentiated component dynamics, are made of polydisperse particles, have soft interactions and have a separation of length and time scales. In this dissertation work, we have used computer simulations to generalize some structure-dynamics scaling laws, originally formulated for molecular fluids, in a way that successfully accounts for these important aspects of colloidal suspensions.

To begin, we examine a two-component mixture of ultrasoft Gaussian-core particles through molecular dynamics simulations. This fluid shows an anomalous dynamic crossover where the larger particles become more diffusive than the smaller particles. However, this dynamic crossover is accompanied by a corresponding structural crossover for a component-specific structural order metric. In the light of this structural order metric, the fluid is non-anomalous with respect to the relationship between static structuring and diffusivity.

Next, we show that accounting for the many-component nature of even modestly polydisperse fluids is important for accurately characterizing their structure-dynamics relationships. We demonstrate this for colloids with short-range attractions through new Monte Carlo simulation techniques and through theoretical calculations carried out in the dilute limit.

From here, we present a new generalized framework to non-dimensionalize diffusivity so that it will have an approximately one-to-one relationship with excess entropy. This method involves rescaling diffusivity with dilute-limit analyses that can be analytically and systematically executed. We tested this framework through a combination of molecular dynamics simulations, Brownian dynamics simulations and Monte Carlo simulations. The results of the simulations demonstrate that this framework can account for particle size asymmetry, particle additivity, interaction strength and some solvent effects.

Finally, we present a new, simple equation that relates non-dimensionalized forms of diffusivity from molecular dynamics and Brownian dynamics simulations. This simple relationship is tested for inverse power law fluids, as well as a suite of ultrasoft fluids that show structural and dynamic anomalies.