Antifoam products such as those used in washing up liquid tablets are ineffective in zero gravity conditions. Physicists at the University of Liege have just demonstrated this fact by means of an experiment conducted during parabolic flights. The conclusions to this study which were published in the new journal NPJ Microgravity, are of interest for industry but they also answer important fundamental questions of physics: how is foam formed, how does it become stable, how does it disappear?

What would happen if the astronauts aboard the International Space Station decided to take some washing up liquid into orbit? “It would result in a ‘foam party’ aboard the ISS” says an amused Hervé Caps, professor of optofluidics at the University of Liege and a member of GRASP (Group for Research and Applications in Statistical Physics).  In the absence of gravity, antifoam products cannot burst soap bubbles. To demonstrate this, the researchers at ULg came up with an original experiment which involved testing detergents and antifoams in microgravity conditions. The experiment was conducted on board the Airbus Zero G used by the company Novespace. This scientific aircraft which is often used by the CNES or the ESA, carried out flights during which it executed around thirty parabolas: Initially, the aircraft climbs with a pitch angle of 45 degrees using engine thrust and elevator controls. The sensation of weightlessness is achieved by reducing thrust and lowering the nose to maintain a neutral, or "straight and level" configuration (0 degree angle of attack). Weightlessness begins while ascending and lasts all the way "up-and-over the hump", until the craft reaches a downward pitch angle of 30 degrees. At this point, the craft is pointed downward at high speed, and must begin to pull back into the nose-up attitude to repeat the maneuver. The forces are then roughly twice that of gravity on the way down, at the bottom, and up again. This lasts all the way until the aircraft is again halfway up its upward trajectory, and the pilot again reduces the thrust and lowers the nose.

This aircraft is used to train astronauts in zero-g maneuvers, giving them about 20 seconds of weightlessness out of 65 seconds of flight in each parabola; hence the name of the Airbus : Zero G. During these 20 precious seconds (repeated around thirty times during the course of the same flight), scores of scientists busy themselves with various experiments. These experiments cover all areas, in particular, human physiology but also physics, chemistry, engineering sciences, agronomics etc.

Making foam in microgravity conditions

The experiment carried out by ULg is simple in principle. The main part of the experimental apparatus is a cylinder, a kind of large syringe, in which a liquid is agitated by a piston (photo). “In order to create foam, explains Hervé Caps, two conditions are necessary. Firstly, a source of energy capable of distorting the liquid. This is the purpose of the piston. When you wash your hands with soap, it is the rubbing motion that constitutes this source of energy; if you pour yourself a beer, it is the fall of the liquid into the glass that constitutes the source of energy. This distortion allows air to penetrate the liquid. And the second condition is what is referred to as a “surfactant agent”, that is to say a molecule which favors the formation of stable bubbles that capture air”. Soap is a surfactant agent just as yeast is a surfactant agent in beer.
For the requirements of this experiment, the researchers at ULg used a constituent of traditional detergent, sodium dodecylsulfate that foams very well, and an antifoam marketed under the brand name Silcolaps RG22, a product that can be found in most dishwasher tablets.

During seven different parabolic flight sessions, and a total of 92 parabolas, the researchers tested four hypothetical cases: 1.The detergent alone under normal gravity conditions (stationary flight) 2.The detergent alone in microgravity conditions (during 20 parabolic seconds) 3.The detergent and the antifoam in normal gravity 4.The detergent and the antifoam in microgravity conditions

A high-speed camera, recording 200 images per second filmed the liquid inside the transparent cylinder, in such a way as to measure the volume of foam produced in each of the above four experimental conditions as well as the stability of the foam and the amount of time it lasts.

“The results are very clear, considers Hervé Caps. “In microgravity conditions, despite the antifoam, the foam does not disappear after 20 seconds while it disappears after 10 seconds under conditions of normal gravity. Conclusion: gravity is the explanation for the effectiveness of the antifoam”. It still remains to be explained why.

(1) P.Yazhgue, D.Langevin, H.Caps, V.Klein, E.Rio and A.Salonen, How antifoams act : a microgravity study, NPJ Microgravity, 2015, 1, 15004.