# Physics of dunking biscuits

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:

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 ...)

• When putting them into the milk do you put them in the same way (and then rotate one inside the milk) or do you immerse one vertically and one horizontally? It could be interesting do the experiment such that you separate the process of immersion from the soaking time. Commented Feb 2, 2017 at 21:38
• @user1583209 I immerse them already in their final orientation. I can try what you suggest, but my guess is that things won't change if the immersion is fast. Commented Feb 2, 2017 at 21:48
• That is a very strange property indeed. Unless the structure of the biscuit is to blame, what matters here are the time to reach the final configuration, and the pressure of the fluid. Let's assume you've adequately controlled for the first. The pressure in such a fluid goes proportional to depth and therefore if the horizontal biscuit indeed descends to the midpoint of depth of the vertical biscuit, then they have the same average pressure and this would seem to mean that the milk-penetration is a nonlinear function of pressure. Commented Feb 2, 2017 at 22:18
• You could confirm this by inserting the horizontal biscuit down to the same lowest-depth as the vertical biscuit, in which case it should gain weight faster; if that's not the case then I would expect that it's because the biscuit itself has a structure which prefers one orientation over the other somehow -- perhaps as @LegitimateWorkUser suggests, it expels air faster one way rather than the other. Commented Feb 2, 2017 at 22:22
• How many biscuits did you test? What type of biscuit? Commented Feb 3, 2017 at 5:24

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.

• Speed: for the measurements, I tried to keep the time of immersion as short as possible (less than a second) for both orientations. Commented Feb 2, 2017 at 22:27
• +1 for the prediction and to point towards formation of air bubbles Commented Feb 2, 2017 at 22:29
• @scrx2 - I would point out that if the horizontal biscuit took 0.2 seconds, and the vertical biscuit took 0.4 seconds, then up to twice the amount of air would have escaped the vertical biscuit. Commented Feb 2, 2017 at 22:35
• sure! I'll try then (what also @user1583209 suggested in a comment) to immerse them all in the same direction. But my guess is that it will give the same result, because in the first second or so after immersion, both orientations gain very little weight. So, I think, the difference is to be found in the long term Commented Feb 2, 2017 at 22:44
• +1 Possibly a minor point, but for air, substitute $CO_2$, produced by the yeast in the mix.
– user140606
Commented Feb 3, 2017 at 17:11