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Does pure water foam in microgravity ?


Liquid foams are omnipresent in everyday life, but little is understood about their properties. On Earth, the liquid rapidly drains out of the foam because of gravity, leading to rupture of the thin liquid films between bubbles. Several questions arise : are liquid foams more stable in microgravity environments ? Can pure liquids, such as water, form stable foams in microgravity whereas they do not on Earth ? In order to answer these questions, we performed experiments both in parabolic flights and in the International Space Station.

Liquid foams consist of gas bubbles dispersed in a liquid and stabilised by surface-active species, such as surfactants, proteins, polymers or particles. Despite the abundance in foam applications, little is understood yet about their properties. In particular their stability remains mysterious : the simple question of why a soap bubble bursts is still waiting for a clear explanation. As a result, empiricism is currently employed to estimate the operational window and design for foam handling in industrial processes.

Obtaining stable foams with large liquid fractions φ (φ > 20%) on Earth is impossible due to the gravity-driven drainage of liquid between the bubbles. This prevents the study of very wet foams, formed at the beginning of the foam life and thus an important step in foaming processes. Furthermore, wet foams show a particularly interesting transition when the bubbles are closely packed, but still spherical. For disordered foams this “jamming transition” occurs at a liquid fraction φ*= 36%. For smaller φ, the bubbles are distorted into polyhedra and the foam behaves like a soft solid, while at larger φ the foam behaves like a viscous liquid. Similar jamming transitions are encountered in other assemblies of randomly packed objects, such as sand, clays, emulsions, etc., which are presently the object of much interest.

 


Pure water foams made in microgravity with liquid/air volume ratio of 30% ; Left : ISS (a heavy bead is hand-shaken in a cylinder containing both air and liquid). Middle : PFC (liquid and air are rapidly pushed back and forth through a constriction between two syringes). Right : PFC, “mighty whipper” (porous plate moving back and forth in a cylinder containing air and liquid). With the bead-cylinder system, less gas could be incorporated in the liquid because the energy involved in the mixing procedure was low and the liquid volume fraction remained large. The “mighty whipper” is the most efficient device.

 

Simple foaming experiments were performed on parabolic flight campaigns (PFC) which simulate microgravity during 20 s, and by astronauts in the ISS. First of all and in line with observations with other liquids, solutions that are difficult to foam on Earth also require more vigorous shaking in microgravity, and water is no exception. More surprisingly, while water foams with φ>φ* are stable, it is not possible to generate a foam with φ<φ*. Only the most efficient mixing device (mighty whipper) produced foams with φ close to φ*, although never smaller.
We propose the following explanation : when φ<φ*, the bubbles are distorted and films are formed. These films drain because of capillary suction into film borders, despite the absence of gravity drainage. Thinning of pure water films is extremely fast and bubbles fuse fractions of seconds after getting into contact. As a result, stable foam with polyhedral bubbles is not formed from pure water, even in microgravity. Conversely, foams with spherical bubbles are very stable, since the bubbles do not move and never get into contact.

 

Contacts :
Dominique Langevin
Emmanuelle Rio
Anniina Salonen

Reference :
Does water foam exist in microgravity ?
H. Caps, G. Delon, N. Vandewalle, R. M. Guillermic, O. Pitois, A.-L. Biance, L. Saulnier, P. Yazhgur, E. Rio, A. Salonen, D. Langevin
Europhysics News 45, 22–25 (2014).