Overview | Example | Animations | References
Paints are a complex mixture of many components, including surfactants and solvents. Surfactants, whether from within an applied paint layer or from contamination during drying, cause surface tension gradients on the paint surface which can drive flows leading to defects. During drying, solvent is evaporated leaving only a very viscous resin. This effectively halts further flow, ``freezing'' defects in place. One type of defect, known as a crater or fish-eye, is a deep, round indentation a few millimeters across, and is believed to be due to surface tension gradients.
We have developed a model for defects of this type. An initial non-uniform distribution of surfactant, due to contamination of the paint surface, causes surface tension gradients. These gradients drive a flow which carries paint away from the contamination, creating the crater. The model uses lubrication theory to describe this flow. Meanwhile, as the paint dries its viscosity increases, until no further flow is possible. At this point, the defect has become permanent. The model requires the solution of three coupled non-linear partial differential equations for
These equations are solved numerically.
Numerical simulations are being used to provide useful insights into how these craters develop and what measures may be successful in preventing or minimizing them. Using our simulations we can easily vary properties, such as paint drying rate, surfactant diffusivity and the viscosity increase during drying, to study their effects on the final crater shape.
A typical simulation of a bolus-driven crater...
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