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

An
evaporating droplet that contains
two fluids
can
sprout fingerlike protrusions or a chain of smaller droplets around its edge,
depending on the liquids in the mixture, researchers report February 14 in Physical
Review Letters
. The researchers caught these phenomena on video using
droplets of isopropanol, a component of rubbing alcohol, mixed with either an
antifreeze ingredient called ethylene glycol or another chemical called
dodecane. Similar patterns appear in other evaporating fluid mixtures, too.

Researchers
deposited 1-microliter drops of isopropanol, mixed with either ethylene glycol or
dodecane, on a smooth surface. As each drop spread out, the isopropanol
evaporated quickly at the edge, where the puddle was thinnest — leaving a
higher concentration of either ethylene glycol or dodecane around the puddle’s perimeter.

That
distending rim ultimately splintered into a ring of smaller droplets. In pools
containing ethylene glycol, those droplets stretched outward to create fingerlike
protrusions. In the dodecane-containing pools, the droplets formed a beaded necklace
around the puddle. 

As a drop of isopropanol mixed with ethylene glycol pools on a surface, the ethylene glycol builds up at the puddle’s edge and splits into a crown of fingerlike protrusions, as seen in the first clip. Thanks to their relatively high surface tension, these ethylene glycol “fingers” can drag fluid out from the puddle’s center, coating the surface in an ethylene glycol film that gets left behind as the evaporating isopropanol recedes. The second clips shows how, in a drop of dodecane-laced isopropanol, dodecane gathers in tiny droplets around the puddle’s edge. Because of their lower surface tension, these beads cannot drag fluid outward like the fingers. So as the isopropanol evaporates and recedes, it leaves behind an array of tiny dodecane islands that eventually merge.  

The
difference in puddle edge pattern arose from the liquids’ different surface tensions — how tightly molecules on a fluid’s
surface cling to each other (SN: 12/6/18). Liquid tends to flow toward
regions with higher surface tension, where molecules exert a stronger pull on
each other. “Think tug-of-war,” says coauthor Justin Burton, a physicist at
Emory 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 starts
to move” in that direction.  

Ethylene
glycol’s surface tension is about 2.2 times as high as isopropanol’s. As a
result, ethylene glycol–rich droplets around the edge of an evaporating puddle
drag fluid from the center of the pool outward, forming fingerlike protrusions.
Dodecane, on the other hand, has a surface tension comparable to that of
isopropanol. So the droplets around the edge of dodecane-containing puddles
stay put.