Entropic forces and phase separation in binary nearly hard-sphere colloids / Anthony D. Dinsmore.

Dinsmore, Anthony D.
xxi, 255 p. : ill. ; 29 cm.
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Penn dissertations -- Physics. (search)
Physics -- Penn dissertations. (search)
Penn dissertations -- Astronomy. (search)
Astronomy -- Penn dissertations. (search)
We present the results of experimental and theoretical studies of the statistical mechanics of suspensions of hard spheres of two different sizes. We have focused on the effects of entropic depletion (or excluded-volume) effects, which play an important role in many real mixtures. In the first set of experiments, we studied the phase behavior of binary hard-sphere mixtures with diameter ratios between 2 and 12. We found that even when the volume fraction of spheres was only 0.20, separation into coexisting fluid and solid phases occurred despite the absence of attractive pair interactions. We measured the structures and compositions of the equilibrium phases both in the bulk of the sample and at the wall of the container. We also propose an original, physically-transparent model that predicts fluid-solid phase separation in hard-sphere mixtures without fit parameters, in close agreement with our measurements and with previously-published results for monodisperse hard-sphere mixtures.
Investigating the effects of hard (inert) surfaces, we demonstrated for the first time that inert walls of complex shape can induce entropic force fields that can trap, repel, or induce drift of the larger particles in a binary suspension. We measured the entropic force on a large sphere as a function of its position near the edge of a terrace and found that it is repelled by a 40-femto-Newton force. A similar mechanism confined large spheres in a corner and, inside rigid phospholipid vesicles, pushed the larger spheres along the wall in the direction of increasing curvature. We predict that, under some circumstances, a unilamellar vesicle will spontaneously envelop a large sphere. Aside from their fundamental interest, these results are likely to improve our understanding of the behavior of complex fluids inside porous media and of proteins inside cells.
We have also developed new techniques for making arrays of submicron particles for photonic applications. Two-dimensional patterns made with electron-beam lithography and three-dimensional crystallites made using an extension of the "convective assembly" technique will be presented.
Supervisor: Arjun G. Yodh.
Thesis (Ph.D. in Physics and Astronomy) -- University of Pennsylvania, 1997.
Includes bibliographical references.
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University Microfilms order no.: 98-00858.
Yodh, Arjun G., advisor.
University of Pennsylvania.
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