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Bubble Clustering in Drying Paint Films

Saranjam, N., Chandra, S.
Industrial & Engineering Chemistry Research 2016 v.55 no.50 pp. 12825-12835
air, bubbles, butanol, contact angle, convection ovens, drying, engineering, evaporation, glass, hydrophilicity, hydrophobicity, liquids, models, photographs, solvents
Paint films with uniform thicknesses (∼1 mm) were applied on glass substrates using a model paint consisting of a resin dissolved in butanol. Small air bubbles were introduced into the liquid, and test samples were cured in a natural convection oven. Bubbles in paint grew larger as the evaporating solvent diffused into them. Photographs of the liquid film surfaces were taken during drying, and the distance between bubbles was measured. Bubbles were observed to move toward each other and form clusters. When a vertical surface was introduced into the liquid film, bubbles moved toward the surface if the liquid–solid contact angle θ < 90° but moved away if θ > 90°. A floating bubble or hydrophilic surface creates an upward curving liquid meniscus near itself so that neighboring bubbles experience buoyancy forces that drive them up the rising liquid surface. A hydrophobic surface creates a downward-curving meniscus, so that bubbles move away from it. Bubble motion was modeled using an analysis of the shape of liquid meniscus and buoyancy forces acting on floating bubbles.