Collapsing Colloidal Gels: How and When?
Russel, William B.
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Drying colloidal dispersions by evaporating the liquid to create particulate solids, porous coatings, or continuous films is common to a number of important technologies, ranging from applying latex paint and manufacturing photographic film to depositing highly porous coatings on ink jet papers and fabricating photonic crystals from silica sols. The objective is generally to create a layer of specified thickness and controlled porosity with permeability, strength, transparency, or other physical properties. Both the understanding and implementation of drying processes have advanced considerably in the past two decades. Yet processing still raises a number of interesting and difficult issues because of conflicting constraints and performance properties. The focus of this talk is the complex phenomena that emerge as evaporation drives fluid flow in the thin film. Rapid evaporation can segregate binary mixtures or create an impermeable skin at the surface. Slower evaporation produces a porous packing subject to a rising capillary pressure that deforms the particles. Elastic deformation can cause cracking and peeling, while a viscous response can produce a pore-free solid.