The researchers found that the pattern of dry pigments was affected by pigment concentration and drying temperature. A study in the American Chemical Society's Langmuir journal showed that paint droplets with lower pigment concentrations or placed on cooler surfaces dried like "fried eggs," while those with higher pigment concentrations or dried at higher temperatures looked more uniform. The results show that by adjusting the pigment concentration and drying temperature, the final appearance of the dried paint can be controlled.
Coffee spills can leave dark stains around the edges of puddles as they dry. However, when the paint drops dry, some look like "fried eggs," with the "yolk" color surrounded by a halo of pure white, while others look uniform. To understand this change, the researchers, reporting in the journal ACS Langmuir, observed the pigment drying process. They found that pigment concentration and temperature affect the gelation and evaporation of liquids, and that this information helps control the morphology of dry paint.
Paint contains a mixture of substances, including resins, pigments, additives, and solvents (such as water). Due to the complex composition of paint, various chemical effects occur when paint droplets evaporate, sometimes resulting in undesirable patterns or small cracks. Generally speaking, artists and home painters alike want an even and consistent distribution of pigment once paint is applied to a surface. However, it is not yet clear how to avoid pattern formation when the pigment liquid dries. So Stella Ramos, Catherine Ballentine and colleagues wanted to study the factors that influence the evaporation of water-based coatings.
When paint droplets dry, they may look like "fried eggs" (left image, graticule is one millimeter), or they may develop a more even distribution of pigment (right image). Image source: Adapted from "Langmuir2023", DOI:10.1021/acs.langmuir.3c01605
The researchers prepared a mixture of five aqueous acrylic paints and water, then dropped the solutions onto heated glass slides. As the liquid evaporated, they analyzed and photographed the sediment and observed three phenomena:
Initially, internal and external flow of liquid coexist: internal flow from the hot base to the cooler top of the droplet, and external flow from capillary flow.
Ultimately, the gelling of the pigment suspension increases the viscosity and slows down the flow of the pigment.
The final drying step locks the pigment onto the surface of the slide.
The amount of pigment and the temperature of the glass surface both affect the size, shape and pattern of dry pigment droplets. The researchers found that paint droplets with lower pigment concentrations or deposited on surfaces where the temperature was as low as 86 degrees Fahrenheit accumulated colorful molecules in their centers, giving them a "fried egg" appearance. The higher the pigment concentration and the higher the temperature, which can be as high as 176 degrees Fahrenheit, the more uniform the pattern will be after drying and the more even the color will be distributed throughout the circle.
To control the appearance of dry paint, adjust the pigment concentration and surface temperature according to the final desired pattern.