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Colloids Symposium 2010

William B. Russel

84th Colloid and Surface Science Symposium Plenary Paper

The mechanics of drying colloidal dispersions: fluid/solid transitions, skinning, crystallization, delamination, and cracking

William B. Russel
Department of Chemical Engineering
The Graduate School Princeton University

A number of technologies depend on the removal of solvent from a colloidal dispersion to create a structured solid.  Production of magnetic tapes, photographic film, and architectural coatings are the most familiar but depositing highly porous coatings on ink jet papers, forming sol-gel glass into fiber optic cable, adding anti-reflection coatings to eyeglass lenses, encapsulating vitamins in beads, fabricating photonic crystals from silica sols, and spray depositing thin film oxide fuel cells are also important.  The drying process generally is driven by mass transfer limited evaporation, which can generate surface tension driven flow, often toward the edges of the film.  Eventually enough solvent is lost to create a close packed layer of particles. Further evaporation produces a negative capillary pressure that either deforms the particles and closes the voids or pulls the air-water interface into the layer.  van der Waals/dispersion forces take over once all the water evaporates and can play a role in the wet state with small particles.  In some cases the lateral stresses generated in the packed bed by capillary pressure cause cracks to appear, nucleated either in the bulk or at the edge of the film.  The technology developed to accomplish the specific goals requires both synthesis of suitable particles and additives as well as control, where possible, over the process.  This review will highlight the fundamentals controlling the aspects of the process through a range of theoretical and experimental approaches and some of the novel formulations and materials that have emerged.

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