The drying pattern of the paint is affected by its pigment concentration and the drying temperature, with adjustments to these factors providing control over the final appearance of the paint.
The coffee spill leaves a black stain around the edge of the pond when it dries. But as the paint drops dry, some look like “fried eggs,” with colored “yolks” surrounded by a white halo, while others look uniform. To understand this variability, researchers report to the ACS’ Langmuir literally watching the paint dry. They found that pigment concentration and temperature affected how the liquid gelled and evaporated, information that could help control the patterns of dry paint.
Paint contains a mixture of substances, including resins, pigments, additives, and solvents, such as water. Due to the complex composition of the paint, various chemical interactions play out as the paint droplets evaporate, which sometimes leads to unwanted patterns or small cracks. Often, artists and house painters want an even, even distribution of pigment after applying paint to a surface. But it’s not so clear how to avoid patterns forming as the liquid dries. Therefore, Stella Ramos, Catherine Barentin, and colleagues wanted to investigate the factors that affect the evaporation of a water-based paint.
The researchers prepared five mixtures of a water-based acrylic paint and water, and then dropped the solutions onto heated glass slides. As the liquid evaporated, they analyzed and photographed the deposits, and observed three phenomena:
- Initially, there is a clash of internal and external fluid flow: an inward flow from the hot substrate to the cooler tip of the droplet, and an outward pull from the capillary flow.
- Eventually, the gelation of the paint suspension increases the viscosity and slows down the movement of the pigment.
- The final drying step locks the pigments in place on the slide.
The amount of pigment and the temperature of the glass surface affect the size, shape, and pattern of the dried paint droplets. The researchers found that droplets with a lower concentration of pigment or placed on the lowest temperature surface at 86 degrees Fahrenheit gathered the colorful molecules in the center, giving them a “fried egg” appearance. With more pigment and a higher temperature, up to 176 degrees Fahrenheit, the dry pattern is more uniform and has an even distribution of color throughout the circle.
The researchers say that in order to control the appearance of the dry paint, the concentration of the pigment and the temperature of the surface can be adjusted, depending on the final pattern desired.