The Science of Dunking

The American scientist E.W. Washburn found a similar factor of four when he studied the dunking of blotting paper – a mat of cellulose fibres that is also full of random channels. Washburn’s experiments, performed some eighty years ago, were simplicity itself. He marked off a piece of blotting paper with lines at equal intervals, then dipped it vertically into ink (easier to see than water) with the lines above and parallel to the liquid surface, and with one line exactly at the surface. He then timed how long it took the ink to reach successive lines. He found that it took four times as long to reach the second line as it did to reach the first, and nine times as long to

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reach the third line. I attempted to repeat Washburn’s experiment and the biscuits turned out to be very similar to blotting paper when it came to taking up liquid. The Washburn equation not only explains why biscuits dunked by the ‘flat-on’ scientific method can be dunked for four times as long as with the conventional method - it can also be used to predict how long a biscuit may safely be dunked by those who prefer a more conventional approach. Only one assumption is needed – that the biscuit will not fall apart so long as a thin layer remains dry and sufficiently strong to support the weight of the wet bit. But how thin can this layer be? There was only one way to find out, and that was by measuring the breaking strength of dry biscuits that had been thinned down. I consequently ground down a range of biscuits on the physics department’s belt sander, a process that covered me with biscuit dust and which caused much amusement among workshop staff.

Whole dry biscuits, I found, could support up to two kilogram’s of weight when clamped horizontally at one end with the weight placed on the other end. The thinned-down dry biscuits were strong in proportion to their weight, and could be reduced to two percent of their original thickness and still be strong enough to support the weight of an otherwise saturated biscuit (between ten and twenty grams, depending on the biscuit type).

All that was needed now was to calculate how long the biscuits could be dunked while still leaving a thin layer, either in the mid-plane of the biscuit for a conventional dunk or on the upper surface of the biscuit for a ‘scientific’ dunk. The calculation was easily done using the Washburn Equation plus the values of the effective channel radius for different biscuits. For most biscuits, the answer comes out at between 3.5 seconds and 5 seconds for a conventional dunk, and between 14 and 20 seconds for a ‘scientific’ dunk.