Different whiskey web patterns by brand.

When a droplet of American whisky evaporates, it leaves a residue that differs among brands such as Four Roses (upper left) and Woodford Reserve Double Oak (bottom right). Credit: Stuart J. Williams, Martin Brown VI, Adam Carrithers

Fluid dynamics

Bourbon or Scotch? A droplet’s dynamics reveal the truth

A whisky’s country of origin can be gleaned from the residue of an evaporated droplet.

American whiskies that evaporate off a surface leave distinctive, web-like patterns that distinguish them from Scotch whisky and from each other.

When a drop of Scotch whisky evaporates, it leaves behind a uniform film of particles that were suspended in the liquid and were imparted by the oak cask in which it aged. However, bourbon, an American type of whisky, is aged in charred oak containers, which impart more particles than their uncharred Scottish counterparts.

Stuart Williams and his colleagues at the University of Louisville in Kentucky found that if bourbon is slightly diluted, it too leaves a uniform mark following evaporation. But when bourbon is diluted to roughly 20% alcohol by volume, the particles that remain after evaporation assemble into strikingly intricate webs of filaments.

The particles initially form a film that is a single molecule thick on the drop’s surface. During evaporation, turbulent internal flows cause the drop to collapse into twisted strands.

Each brand of bourbon leaves its own unique pattern of filaments. These could be used to spot counterfeit bourbon whisky, the team writes.