Bubble entrapment and escape from sprayed paint films
Painted substrates were
weighed to determine the rate of solvent evaporation. Bubbles escaped from the
paint in 100–900 s, the time increasing with paint thickness. The Sauter
mean diameter of bubbles in films less than 300 μm in thickness decreased
with time because larger bubbles escaped faster than small bubbles, while the
mean diameter of bubbles in a 450 μm thick layer increased due to bubble
coalescence. Concentration gradients due to solvent evaporation in a paint film
create surface tension variations that drive Marangoni flows, which bring
bubbles to the paint surface. An analytical one-dimensional model of solvent
diffusion was used to calculate solvent concentration variations in the paint film
and the Marangoni number.
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