Evaporating mixtures of two liquids create hypnotic designs

Droplets containing liquids with different surface tensions, like isopropanol and ethylene glycol (shown), bloom into intricate patterns as they evaporate.

A.P. Mouat et al/Physical Review Letters 2020

By Maria Temming

March 2, 2020 at 6:00 am

Whenliquids containing small particles evaporate, those fluids often leave behind fingerprintslike coffee rings or whiskey webs (SN: 10/31/19). But liquidsmixed with other liquids leave their own distinct residue patterns.

Anevaporating droplet that containstwo fluids cansprout fingerlike protrusions or a chain of smaller droplets around its edge,depending on the liquids in the mixture, researchers report February 14 in PhysicalReview Letters. The researchers caught these phenomena on video usingdroplets of isopropanol, a component of rubbing alcohol, mixed with either anantifreeze ingredient called ethylene glycol or another chemical calleddodecane. Similar patterns appear in other evaporating fluid mixtures, too.

Researchersdeposited 1-microliter drops of isopropanol, mixed with either ethylene glycol ordodecane, on a smooth surface. As each drop spread out, the isopropanolevaporated quickly at the edge, where the puddle was thinnest — leaving ahigher concentration of either ethylene glycol or dodecane around the puddle's perimeter.

Thatdistending rim ultimately splintered into a ring of smaller droplets. In poolscontaining ethylene glycol, those droplets stretched outward to create fingerlikeprotrusions. In the dodecane-containing pools, the droplets formed a beaded necklacearound the puddle.

Thedifference in puddle edge pattern arose from the liquids’ different surface tensions — how tightly molecules on a fluid'ssurface cling to each other (SN: 12/6/18). Liquid tends to flow towardregions with higher surface tension, where molecules exert a stronger pull oneach other. "Think tug-of-war," says coauthor Justin Burton, a physicist atEmory University in Atlanta. "If you have a higher surface tension on one side… one tug-of-war team [is] stronger than the other, and then everything startsto move" in that direction.

Ethyleneglycol's surface tension is about 2.2 times as high as isopropanol's. As aresult, ethylene glycol–rich droplets around the edge of an evaporating puddledrag fluid from the center of the pool outward, forming fingerlike protrusions.Dodecane, on the other hand, has a surface tension comparable to that ofisopropanol. So the droplets around the edge of dodecane-containing puddlesstay put.

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A.P. Mouat et al. Tuning contact line dynamics and deposition patterns in volatile liquid mixtures. Physical Review Letters. Vol. 124, February 14, 2020. doi:10.1103/PhysRevLett.124.064502.

Previously the staff writer for physical sciences at Science News, Maria Temming is the assistant editor at Science News Explores. She has bachelor's degrees in physics and English, and a master's in science writing.

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