Dunk (biscuit)

To dunk is to dip biscuit, bread, or cake into a beverage, usually hot, especially tea or coffee, but the popular American snack "milk and cookies" features cookies dunked into cold milk. Dunking a biscuit is said to release flavour.

The physics of dunking is driven by the porosity of the biscuit and the surface tension of the beverage. A biscuit is porous and when dunked, capillary action draws the liquid into the interstices between the crumbs.

Biscuit dunking and popular science

Biscuit dunking became prominent during National Biscuit Dunking Day in which physicist Len Fisher of the University of Bristol presented some light-hearted discussion of dunking, aiming to show that physics could be fun, accessible, and easy to comprehend.

Fisher appeared to be somewhat taken aback by the large amount of media attention, ascribing it to a "hunger for accessible science". Fisher also described his astonishment at journalists' interest in one equation used in the field: Washburn's equation, which describes capillary flow in porous materials. Writing in Nature, he says "the equation was published in almost every major UK newspaper. The journalists who published it took great care to get it right, some telephoning several times to check".

(quotes from Nature 397, 469; 1999).