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Together with my 5 years old assistant, after endless observations breakfast after breakfast, we finally decided to be serious about it, and quantify a remarkable property of biscuits in milk.

Surely, we are humbly trying to climb on the shoulders of a giant. In his book, Len Fisher explains a lot of the physics of dunking doughnuts, but I couldn't find this particular feature:

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So, we see a big difference in weight gained by a biscuit during full immersion in milk, depending on its orientation. When immersed vertical, the milk permeates much faster than when the biscuit is immersed horizontally. Which makes the vertical orientation not only more practical but also unfortunately useful in our world where breakfast time tends to be minimized. You are invited to reproduce it with your biscuits!

What is the mechanism that explains this?

(Methods: the numbers in the picture correspond to a dozen of biscuits [yum] of the same batch, mean weight = 8g, held in milk using kitchen tweezers without breaking. The error is the st.dev. The scale has 1g sensitivity. My assistant left at biscuit number 3 ...)

scrx2
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1 Answers1

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I contend that if the biscuit goes in one-end-first, there exist air pathways that allow the escape of air up until the point where the upper end of the biscuit is submerged. After that point, the escape of air relies on bubble formation and the ability of the air to pass through the "wet biscuit membrane".

The horizontal biscuit, in contrast, is only able to expel its internal gasses for an comparatively short amount of time, and so fluid stands a much lesser chance of permeating the biscuit material.

To test this hypothesis, I propose comparing the absorption of vertical biscuits that are submerged over a range of dunking speeds. If dunked very quickly, the absorption should approach that of the horizontal biscuit.

I also predict that since the horizontal biscuit is submerged faster and holds more air under the current experimental design, another inspection should reveal more and greater bubbles than the vertical biscuit currently exudes. More bubbles, however, are not enough to match the air that escapes the vertical biscuit.

LegitimateWorkUser
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