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Title: Computer Simulation Studies of Inhomogeneous Fluids: The Depletion Force and the Disjoining Pressure of Colloidal Physics
Author: Herring, Adam Russell
Awarding Body: University of Leeds
Current Institution: University of Leeds
Date of Award: 2008
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Investigations have been carried out via Monte Carlo simulation of simple, inhomogeneous fluids into two important quantities of colloidal systems; the depletion force and the disjoin'': ing pressure. Simulations of a hard-sphere solvent confined to the annular wedge formed between a spherical hard colloid and a planar hard wall were performed in order to shed light on the recently discovered disagreement of several results for the depletion force in the nanD-colloidal regime at solvent density pa3 > 0.6. Emphasis is placed on attempting to understand the limits of validity in terms of colloid size for the Derjaguin approximation applied to depletion forces and fundamental-measures-theory density functional theory (FMT-DFT), and the manner in which the depletion force scales between these two results at intermediate colloid sizes. The depletion force was evaluated via an exact statistical mechanical sum rule requiring only knowledge of the integral of the one-body density of solvent at the planar hard wall from the apex of the wedge to a large distance from the colloid. Simulations were performed for a colloid/solvent size ratio of 8 = 20 for several colloid-wall separations, h, between physical contact and the hard-sphere solvent diameter, a, at pa3 = 0.764, the results for the depletion force appearing to be consistent with a recently proposed theoretic model suggesting a (curiously non-analytic) 8-1/ 2 correction to the linear scaling behaviour of the depletion force with colloid size between the FMT-DFT and Derjaguin results, with the Derjaguin result valid in the large colloidal limit 8 --t 00 and FMT-DFT only as colloid size approaches solvent size. Further simulations, restricted to h = a for 8 = 10, 30, 50 and 100 though reveal that at least for this special separation 8-1/ 2 scaling does not hold, suggesting that to confirm scaling behaviour requires simulations over the entire range 0 ~ h ~ a for several values of 8. The disjoining pressure profile has been simulated through the three-phase contact line formed between the liquid-vapour interface of a square-well fluid at bulk liquid-vapour coexistence and a planar, square-well wall for three different depths of the wall-fluid potential. The disjoining pressure is found to follow a smooth, downward curve across the contact line that is well fit by a Gaussian. The simulation method used to make these disjoining pressure measurements has been validated using a statistical mechanical sum rule linking the integral of the disjoining pres- . . sure across the contact line to the liquid-vapour surface tension and macroscopic Young's contact angle, both measured from the interface far from three-phase contact.
Supervisor: Not available Sponsor: Not available
Qualification Name: University of Leeds, 2008 Qualification Level: Doctoral
EThOS ID:  DOI: Not available