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Depletion Force In Colloidal Systems

Y. Mao tex2html_wrap_inline24 , M. E. Cates tex2html_wrap_inline26 and H. N. W. Lekkerkerker tex2html_wrap_inline28

tex2html_wrap_inline24 Cavendish Laboratory, Madingley Road,
Cambridge, CB3 OHE, UK.

tex2html_wrap_inline26 Department of Physics and Astronomy,
University of Edinburgh, King's Buildings,
Mayfield Road, Edinburgh EH9 3JZ, UK.

tex2html_wrap_inline28 Van't Hoff Laboratory, University of Utrecht,
Padualaan 8, 3584 Utrecht, The Netherlands.


The entropic depletion force, in colloids, arises when large particles are placed in a solution of smaller ones, and sterically constrained to avoid them. We calculate the interaction between large spheres (of radius R) in a dilute solution of mutually avoiding small spheres (of diameter tex2html_wrap_inline38 and volume fraction tex2html_wrap_inline40 ), to third order in tex2html_wrap_inline40 . In addition to the well-known attractive force for tex2html_wrap_inline44 , we find a repulsive barrier at larger separations, and beyond that a secondary minimum. Except for unusually large size ratios (perhaps abetted by relatively high density tex2html_wrap_inline40 ), these features of the interaction potential are too small, compared to tex2html_wrap_inline48 , for kinetic stabilization (arising from the barrier) or flocculation into the secondary minimum, to be widespread, although such effects are possible in principle. For feasible size ratios, the same features should have observable consequences for the radial distribution function of the large spheres. Such effects can be viewed as precursors, at low density, of liquidlike structuring (solvation forces) expected at higher tex2html_wrap_inline40 . Our third order calculation gives satisfactory agreement with a recent computer simulation at moderate density and size ratio ( tex2html_wrap_inline52 ).

Yong Mao
Tue Apr 22 14:30:30 BST 1997