Physics Takes the Biscuit

commentary Physics takes the biscuit Why did a light-hearted experiment attract so much attention from the media? The episode is an interesting lesson for those wanting to explain science to the wider public — equations do not always scare people away. to dunk the biscuit so that part of it can could use the Washburn equation to predict Len Fisher remain dry (and mechanically strong) and how long different biscuits could be safely Scientists wanting to share their picture of support the weight of the wet bit. Hence, dunked by the physicist’s method, the the world with a wider audience have a famil- instead of holding the biscuit vertically when longest dunkers generally giving the best iar problem — the knowledge gap. One dunking, a physicist grips it at the edge, slid- flavour release (to my palate at least). doesn’t need to be a writer to read and under- ing it into the tea or coffee at a shallow angle, Washburn will be turning in his grave to stand a novel, or know how to paint before so that the lower surface is wetted but the learn that the media have renamed his work being able to appreciate a picture. Some upper surface remains dry. This explanation the “Fisher equation”. The equation was knowledge of what science is about, though, is so simple that I was able to talk radio inter- published in almost every major UK news- is a prerequisite for both understanding and viewers through it, and have them perform paper. The journalists who published it took appreciation. experiments on air. Yet simplicity alone great care to get it right, some telephoning Our intended audiences can often be cannot explain the high degree of interest. several times to check. Some even did their trained on the spot, if we can persuade them I decided to ask the interviewers them- own experiments, extending my results. to stay around for long enough. One effective selves why they found the story so interest- Only one journalist published without approach is to use the ‘science of the familiar’, ing. An important criterion for most was checking, provoking the following letter: exemplified by my recent exercise on the that the topic is slightly daring, but the scien- “Dear Sir, I think there is something wrong physics of biscuit (or cookie) dunking. The tific gloss added an objectivity that legit- with your biscuit dunking equation. Please project was a publicity agent’s idea, and the imized public discussion. Most interviewers send me some biscuits for noticing this. results were presented at a press conference confessed to a strong interest in science, cou- Chao Quan (aged 12).” in London on “National Biscuit Dunking pled with a fear of looking foolish when ask- Such excitement over an equation contra- Day”. Local publicity, fuelled initially by the ing questions. This fear barrier is much lower dicts the normal publisher’s advice to authors eccentricity of the story, rapidly spread when discussing the science of the familiar, — that every additional equation halves the worldwide — I am still receiving requests for as the questioner feels on a more equal foot- sales of a popular-science book. Why was this radio and TV interviews from countries as ing with the scientist. so? Let me suggest an answer, relevant to the far away as Australia and South Africa. These criteria are well known to experi- sharing of more serious science. Scientists are What gave the story such global appeal? enced science popularizers, but there was seen by many as the inheritors of the ancient Even the Nobel prizes don’t receive such another attraction that seemed counter- priestly power of the keys, the owners and coverage. Yet this minor bit of science is on intuitive. Journalists were enthralled to dis- controllers of seemingly forbidden knowl- everyone’s lips, metaphorically speaking. Its cover that there is an equation to describe edge. Equations are one key to that knowl- success, I believe, reflects a hunger for acces- biscuit dunking. Newspapers published it; edge. The excitement of journalists in gaining sible science. The lessons from this success, TV programmes showed it. More than one control of a key was surely a major factor in some of them surprising, are important for radio interviewer even insisted I describe their sympathetic promotion of the story. By those who wish to share more serious science it on air. making the Washburn equation accessible, I with a wide audience. All I had done, in fact, was to write down was able to ensure that journalists unfamiliar What is dunking? It is simply dipping the Washburn equation, derived2 in 1921 to with science could use the key to unlock your biscuit, cookie, doughnut or pastry into describe capillary flow in porous materials: Pandora’s box. a drink such as tea or coffee, a process shown Len Fisher is in the Physics Department, 2 gDt by chemists at Firmenich (Switzerland) to L 4 ___ University of Bristol, Bristol BS8 1TL, UK. 4h enhance flavour release by up to ten times. e-mail: [email protected] Although scientifically acceptable, dunking where t is the time for a liquid of viscosity h 1. Deegen, R. D. et al. Nature 389, 827–829 (1997). is often socially frowned upon, which is and surface tension g to penetrate a distance 2. Washburn, E. W. Phys. Rev. 17, 374–375 (1921). probably part of its attraction. L into a fully wettable, porous material The physics of dunking is straightforward. whose average pore diameter is D. The equa- A biscuit is porous, with interconnecting tion is strictly true only for capillary flow in hollow channels between the crumbs. When a single cylindrical tube in the absence of the biscuit is dunked, capillary action draws gravitational effects, but the liquid into these channels: a similar can be extremely ac- FORTE RODNEY process occurs when a piece of blotting paper curate for more is dipped into ink, or when ring stains form complex from dried liquid drops1. materials, The problem for serious dunkers is that including, the wetted part of the biscuit becomes very as I found soft, especially when the tea or coffee is hot. A experi- biscuit is basically dried-up starch grains mentally, glued together with sugar: the hot liquid biscuits. swells and softens the starch grains and dis- Why this solves the sugar. The wetted biscuit eventual- should be ly becomes so soft that it collapses under its so is a very own weight. interesting ques- The physicist’s answer to this problem is tion. In practice, I NATURE | VOL 397 | 11 FEBRUARY 1999 | www.nature.com © 1999 Macmillan Magazines Ltd 469.

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