Physics reveals the secrets of slipping foam on the tongue

Do you prefer a "whipped cream" that glides quickly on the tongue or that sticks to it for a little longer before reaching the throat, or that dose that progresses on the tongue with slight vibrations?

In this question, David LaRoussery, in a Le Monde article, summarized three sensations that could soon be chosen thanks to the highly accurate work of physicists studying the surfaces and foams and how they slide.

Look in the foams
The author said that a team from the French Center for Scientific Research and the University of Paris Saclay is about to publish the results of his research in the journal Physical Review, where doctoral student Manon Marchand said, "When we study the properties of foams, the devices distort them, and on the walls they slide unfortunately."

"So researchers have worked for nearly twenty years to prevent this slipping by making contact surfaces rough, but how do you choose the correct roughness? This is the question we first asked ourselves before thinking about applying this in the food industry."

The researchers found in 1988 that the foam did not slip when the size of the grains of roughness was greater than the size of bubbles, but without clearing to them a justification at that time and if it was known today, but the surprise was that the size of the bubbles is not what matters, but rather something smaller than that. Much.

This little thing that makes the difference, according to the author, is nothing but the thin channels in which the foam made of water and any foaming liquid like dishwashing or egg whites.

These channels - as the author says - are located at the intersection of three bubbles and carry the scientific name "Plateau's Brims" with the name of a Belgian physicist in the 19th century.

The conclusions are as follows: If the roughness of the surface is wider than the diameter of these channels, this prevents slipping, and if it is narrower then it allows foam to slip. In the third case, when the roughness of the surface is similar to the size of the channels, the slip is by vibration.

Other feelings in the mouth?
In order to reach this description, the researchers pushed a two-centimeter-wide blade covered with small glass pieces to make it rough, in a well-controlled bubble soap foam by injecting air with syringes. The strength of the blade was measured in the soft material and analyzed according to the size of the bubbles and channels .

Because the upper part of the foam is drier than the bottom, the channels are finer at the top, allowing verification of slipping with bubbles with bubbles equal in size to the smaller, smaller channels.

"It is a wonderful work that solves an outstanding problem in anomalies, and it explains the various conditions for fixing foams on surfaces," says Benjamin Doulette, researcher at the French Center for Scientific Research.

After reaching this result, the researchers moved from soap to whipped cream to find that the behavior of foams is explained by engineering better than the size of bubbles or channels, more than by chemistry, that is, molecular interactions.

The scientists - as the author says - did the experiment on an artificial tongue, and they noticed that one of the foam slipped on the "tongue" while another did not slip, according to their model.

"It will be interesting to know whether changing the foam also changes the sensation in the mouth," he concluded, quoting the author, pending future experiments aimed at studying troubled foams, whose bubbles are of variable sizes, closer to reality.