Characteristic patterns are the result of varying surface tension
When liquids containing tiny particles evaporate, they often leave fingerprints such as coffee rings or whiskey nets (SN: 10/31/19). However, liquids mixed with other liquids leave their own lip patterns.
A spray droplet containing two liquids can sprout like fingers or a chain of smaller drops around its edge, depending on the liquids in the mixture, the researchers reported on Feb. 14 in Physical Review Letters. The researchers filmed these phenomena using droplets of isopropanol, an alcohol component, mixed with an antifreeze called ethylene glycol or another chemical called dodecane. Similar patterns also occur in other vapor-to-liquid mixtures.
The researchers placed 1 drop of a microliter of isopropanol mixed with ethylene glycol or dodecane on a smooth surface. As each drop spreads, the isopropanol quickly evaporates around the edge where the puddle is thinner, leaving a higher concentration of ethylene glycol or dodecane around the perimeter of the puddle.
This expanding edge eventually relaxes into a ring of tiny drops. In pools containing ethylene glycol, these drops extend outward to create finger-like protrusions. In ponds containing dodecane, the drops form a pearl necklace around the puddle.
The variation in the pattern of the edge of the puddle results from different tensions on the surface of liquids – how close the molecules are to each other on the surface of a liquid (SN: 06.12.18). Fluid tends to flow into areas of higher surface tension, where molecules reinforce each other. “Think about drawing war,” said co-author Justin Burton, a physicist at Emory University in Atlanta. “When you have higher surface tension on one side … one tug-of-war team is stronger than the other, and then everyone starts moving in that direction.”
The surface tension of ethylene glycol is about 2.2 times that of isopropanol. As a result, droplets rich in ethylene glycol emerge from the center of the pool on the edge of a natural liquid vapor, forming finger-like protrusions. Dodecane, on the other hand, has a surface tension comparable to isopropanol. Thus, the droplets remain at the edge of puddles containing dodecane.