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Drying droplets: coffee stains and Mexican hats

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As suspension droplets lose their solvent through evaporation, the particles left behind are deposited into distinctive “dry out” patterns on the surface. Although uncontrolled experiments are simple to perform, there are many parameters to consider making a priori predictions difficult. To illustrate this I will present experimental results from two systems of drying droplets: (1) particle suspensions and (2) polymer solutions. Dilute particle suspensions typically form ring-stains, the dimensions of which are found to be robustly predicted by established scaling laws over a wide range of experimental parameters: particle size, concentration, orientation and drying rate. By contrast, the dry out patterns from polymer solutions are seen to be sensitively dependent on the underlying phase behaviour of the polymer, with significant differences observed between semi-crystalline and glass-forming polymers. The polymer results have led us to identify alternative evaporation behaviours for droplets on surfaces, which seem to be supported by literature.

This talk is part of the BPI Seminar Series series.

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