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Title: Measuring structure and interactions in colloidal fluids using test-particle insertion
Author: Stones, Adam Edward
ISNI:       0000 0004 9355 5776
Awarding Body: University of Oxford
Current Institution: University of Oxford
Date of Award: 2020
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We use the Potential Distribution Theorem to evaluate distribution functions from equilibrium configurations using test-particle insertion. We use this methodology to determine the contact value of the pair distribution function in hard-disk systems: in contrast with the conventional distance-histogram method, the insertion measurement is exact and does not require an approximate extrapolation. The resulting equations of state in both simulations and a hard-disk colloidal model system agree well with the predictions of Scaled Particle Theory. We also provide the first experimental measurement of the cavity distribution function y²(r) inside the hard core. We next develop a model-free technique for measuring the pair potential u²(r) in pairwise-additive fluids, by matching the insertion and distance-histogram measurements of g²(r) using an iterative predictor-corrector scheme. We test the method extensively in simulation, before applying it successfully in a fluid of superparamagnetic colloidal particles, obtaining the anticipated form of u²(r) and the correct dependence on the applied magnetic field. We then extend the scheme to measure the full set of pair potentials in multicomponent fluids, demonstrating its efficacy in a three-component simulation. We show that a given n-body distribution function gⁿ (n > 2) can be measured using n different insertion routes, which correspond to simultaneous insertion of between 1 and n test particles. We use these methods to measure g³ in simulation: while the noise depends strongly on the number of simultaneous insertions, the resolution is superior to that of the distance-histogram method. Finally, we consider systems with three-body interactions. We show that matching g²(r) alone gives an effective pair potential which is unable to reproduce g³ in pairwise-additive simulations. We therefore extend the predictor-corrector scheme to measure the three-body interaction u³ by matching g³, and test it in simulation. The scheme broadly recovers the correct u³, but requires further development to reduce the noise.
Supervisor: Dullens, Roel ; Aarts, Dirk Sponsor: Clarendon Fund
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Chemical Physics ; Physical Chemistry