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To dunk is to dip biscuit, bread, cake, or doughnut into a beverage, especially tea, coffee, or cold milk as in the popular American snack milk & cookies. Dunking releases more flavor from confections by dissolving the sugars,^[1] while also softening their texture.

A popular form of dunking in Australia is the "Tim Tam Slam", also known as "tea sucking". 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 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.

To save serious dunkers from having to type variables into a spreadsheet as they prepare to dunk their biscuit, or work out the dunking equation by hand on their napkin, by which time their tea would be cold, Fisher suggested that he could helpfully provide a table with the dunking characteristics of popular biscuits. Dunking technique is also important and the team also designed a prototype dunking holder to aid in this tricky task. A chocolate biscuit should be dunked flat, with the chocolate side up, to minimise the chance of chocolate bleed into the beverage.

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).

It is observationally true that hot water pentrates biscuits faster than hot tea with cold milk.

1. ^ Lee, Laura. The Pocket Encyclopedia of Aggravation. New York: Black Dog & Leventhal, 2001.