Hard Sphere Nucleation Rates

Abstract

Chemistry is hard. Simulating the full quantum-chemical interactions involved in any complicated physical process is often impossible, due to the huge amount of computing power required. Instead, model systems which interact in simpler ways can be studied. The simplest model is known as the ideal gas. This model consists of point particles which only interact through elastic collisions. Even a model this abstract gives us the ideal gas law, one of the most prized equations in all of physical chemistry. Moving one step further in complexity, we can give the particles a diameter and prohibit them from overlapping. This is the hard sphere model. This extra step in complexity gives hard spheres a richer behaviour than the ideal gas. Hard spheres form the familiar liquid and solid phases, with a full thermodynamic transition between the two, just like real materials. This existence of the freezing transition has been known since the 1950s, and new simulation techniques have measured its speed (the nucleation rate). Ten years earlier, experiments on particles suspended in a solvent (colloids), had shown that the hard sphere model could be realised in experiment. However, the experimental nucleation rates disagreed spectacularly with the simulations, by up to 13 orders of magnitude. This discrepancy has been described as the second worst failure in physics.

In this thesis, we attempt to resolve the discrepancy. First, we examine the impact of the sedimentation in the experiments, and show that it can create a substantial change in the nucleation rates. Then we examine the density fluctuations in the experimental system. We demonstrate that they are significantly larger than in the simulations. A detailed investigation into the cause of the increased fluctuations is undertaken. We conclude that they are due to the particle tracking methods applied in studying colloids, revealing an important experimental limitation which has not been previously described. Finally, we develop a new method for studying nucleation rates using confocal microscopy, increasing the sample size by several orders of magnitude. This allows us to measure the most extended nucleation barriers ever seen experimentally. Unfortunately these barriers essentially agree with previous experiments, leading us to conclude that the nucleation rate discrepancy remains unresolved.

Date of Award	1 Oct 2019
Original language	English
Awarding Institution	University of Bristol
Supervisor	Cp Royall (Supervisor)