The science of watching paint dry

March 18, 2016
Nacho Martín-Fabiani is preparing a paint sample for analysis with the Atomic Force Microscope (AFM). Credit: University of Surrey

New research published today in the journal Physical Review Letters has described a new physical mechanism that separates particles according to their size during the drying of wet coatings. The discovery could help improve the performance of a wide variety of everyday goods, from paint to sunscreen.

Researchers from the University of Surrey in collaboration with the Université Claude Bernard, Lyon used computer simulation and materials experiments to show how when coatings with different sized , such as paints dry, the coating spontaneously forms two layers.

This mechanism can be used to control the properties at the top and bottom of coatings independently, which could help increase performance of coatings across industries as diverse as beauty and pharmaceuticals.

Dr Andrea Fortini, of the University of Surrey and lead author explained:

"When coatings such as paint, ink or even outer layers on tablets are made, they work by spreading a liquid containing onto a surface, and allowing the liquid to evaporate. This is nothing new, but what is exciting is that we've shown that during evaporation, the small particles push away the larger ones, remaining at the top surface whilst the larger are pushed to bottom. This happens naturally."

Dr Fortini continued, "This type of 'self-layering' in a coating could be very useful. For example, in a sun screen, most of the sunlight-blocking particles could be designed to push their way to the top, leaving particles that can adhere to the skin near the bottom of the coating. Typically the particles used in coatings have sizes that are 1000 times smaller than the width of a human hair so engineering these coatings takes place at a microscopic level. "

The team is continuing to work on such research to understand how to control the width of the layer by changing the type and amount of small particles in the and explore their use in industrial products such as paints, inks, and adhesives

Explore further: Scientists show there's nothing boring about watching paint dry

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not rated yet Mar 18, 2016
Interestingly, particles that are loose in a container tend to push the large pieces toward the top (the brazil nut effect in a can of mixed nuts). I might wonder if the two processes could be used to tune where the particles are located? That leads me to realizing that nuts in a container are in a fluid, just as paint particles in solvent are in a fluid. Densities and viscosities are different. Do we dissolve particles in air? Would it have the same effect as the paint particles in solvent?
not rated yet Mar 19, 2016
In this context, particles aren't "dissolved" (if using the common definitions). Density of particles vary with size until surface effects are negligible. A paint generally is a polymer solution (although there're some exceptions), which means that viscosity and polymer particle interactions increase as solvent evaporates. Depending on the cure situation, convection currents may occur, mixing the film. Particles are generally fillers & pigments, and usually have wide distributions of size, shape, surface roughness, surface chemistry, etc. Also, its well known that a drying paint film has a compositional gradient; meaning the solubility & surface interactions vary thru the depth of the coating which acts to drive the structure of the film towards its lowest energy state.

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