For the research, scientists ran a computer simulation and ran some materials science experiments, looking at how coatings, with different sized particles, dry the coating spontaneously forms two layers. This action happens as paint, applied to a wall, begins to dry over time.
Interviewed by Controlled Environments, lead researcher Dr. Andrea Fortini indicated: “When coatings such as paint, ink or even outer layers on tablets are made, they work by spreading a liquid containing solid particles onto a surface, and allowing the liquid to evaporate.”
Paints are a type of mixture called a colloid. In a colloid, particles of one substance are mixed and dispersed with particles of another substance, but they are not dissolved in it.
While this in itself is well established, Dr. Fortini explains the importance of the core discovery: “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”
Understanding this mechanism should allow scientists to better to control the properties at the top and bottom of coatings independently. Building in such variations should increase the performance of different coatings and could be applied to sectors as diverse as building, beauty and pharmaceuticals.
For example, self-layering with a suntan lotion could result in the majority of the sunlight-blocking particles being designed to move to the top of the solution. The consequence of this would be those particles that adhere to the skin being located at the bottom of the coating. This would improve the efficiency of the sunscreen. A further application is with the environment, in helping to create paints that do not contain damaging volatile organic compounds.
Designing this requires precise methods, since many of the particles are on the micro- or nano-scale. The trickiest part is with controlling the thickness of the layer, and with varying the different types of particles to reach optimal numbers.
The European Union funded research was conducted at the University of Surrey and it has been published in the journal Physical Review Letters. The paper is titled “Dynamic Stratification in Drying Films of Colloidal Mixtures.”
