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A versatile new approach for calculating the depletion potential in a hard sphere mixture is presented. This is valid for any number of components and for arbitrary densities. We describe two different routes to the depletion potential for the limit in which the density of one component goes to zero. Both routes can be implemented within density functional theory and simulation. We illustrate the approach by calculating the depletion potential for a big hard sphere in a fluid of small spheres near a planar hard wall. The density functional results are in excellent agreement with simulations.82.70. Dd, 61.20.Gy Typeset using REVT E X 1

Based on density functional theory the influence of curvature on the depletion potential of a single big hard sphere immersed in a fluid of small hard spheres with packing fraction η s either inside or outside of a hard spherical cavity of radius R c is calculated. The relevant features of this potential are analyzed as function of η s and R c . There is a very slow convergence towards the flat wall limit R c → ∞. Our results allow us to discuss the strength of depletion forces acting near membranes both in normal and lateral directions and to make contact with recent experimental results. 82.70.Dd, 61.20.Gy, Typeset using REVT E X

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A hard sphere fluid confined by hard, structureless, and parallel walls is investigated using a certain version of weighted density functional theory. The density profile, the excess coverage, the finite size contribution to the free energy, the solvation force, and the total correlation function are determined as function of the slit width L for various bulk number densities ρ b . In quantitative agreement with rigorous results the present version of density functional theory yields a constant and large but finite number density profile for the limiting case that L is reduced to the diameter of the hard spheres. Within the Derjaguin approximation the results for the slit geometry allows us to obtain the solvation force between two large hard spheres immersed into a fluid of much smaller hard spheres. 61.20.Ne, 68.15.+e, 68.10.Cr Typeset using REVT E X