Local structure and dynamics of complex fluids



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There is a well-known connection between the structure and dynamics that is present in molecular and colloidal systems. Using this connection as a guide we are able to design interparticle potentials that optimizes the diffusivity of a single particle. While structure-dynamic correlations provide the insight that diffusion could be enhanced, the effect of this enhancement on the dynamics of neighboring particles is more difficult to quantify.

A novel method for calculating position-dependent dynamics is introduced that can be easily implemented into existing simulation protocols. The computational requirements are very low compared to existing methods and this technique can also be applied to a wide variety of systems, including experiments where particle trajectories can be determined. Using this method, the position-dependent diffusivity of solvent particles in the vicinity of a tracer particle can be measured. This information allows for determination of the microscopic changes that take place as a result of the optimization discussed above.

To study the effect of a non-continuum solvent, we design a system that eliminates inhomogeneous structuring near an interface. Hydrodynamic theory can predict the position-dependent diffusivity of a sphere in continuum solvent. Comparing these systems not only highlights the difference in position-dependent dynamics for continuum and non-continuum solvents, but is a starting point to study what happens to dynamics when structure is reintroduced.

This allows us to answer many other questions about the relationships between structure and dynamics. While these connections have been studied extensively for average properties, they have not been explored for their position-dependent counterparts. For bulk fluids, the insertion probability and two-body excess entropy has proven useful for predicting average dynamic properties. We develop expressions for the position-dependent versions of both of these quantities. We show that when using the appropriate reference state the position-dependent diffusivity can be qualitatively related to the insertion probability.