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Whiskey dries into uniform stains

Dried whiskey droplets: understanding the stain-forming process may be useful in creating new industrial coatings

Ernie Button Dried whiskey droplets: understanding the stain-forming process may be useful in creating new industrial coatingsErnie Button

The way whiskey evaporates and leaves a relatively uniform residue on the bottom of a glass can provide important clues for the creation of new types of industrial paints and coatings.  With the help of professional photographer Ernie Button, researchers at Princeton University (USA) discovered that some types of molecules – such as surfactants (compounds that reduce the surface tension between two liquids) and plant polymers – play decisive roles in making whiskey droplets dry in a uniform spatial arrangement on the bottom of a clean glass (Physical Review Letters, March 24, 2016).  The researchers used fluorescent markers to measure the movement of fluids inside droplets of whiskey during evaporation, and observed a flow from the edges towards the center.  This internal current at least partially counteracts the tendency for droplets to evaporate faster at the edges than at the center, a phenomenon seen in many liquids, such as coffee, which dries into irregular stains.  In the case of whiskey, surfactants accumulate at the edges and create a surface tension gradient (in what is known as the Maringoni effect) that pushes the liquid towards the center of the stain.  The role of polymers in creating uniform stains is a little different: they stick to the bottom of the glass and attract particles to the substrate to which they adhered.

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