Foam Optics and Mechanics - Stability (FOAM-Stability)
Research Area: Fluid Physics
Expedition(s): 19- 22
Principal Investigator(s): ● Nicolas Vandewalle, Université de Liège, Liège, Belgium
RESEARCH OBJECTIVES
Foam Optics and Mechanics - Stability (FOAM-Stability) studies the behavior of wet foams in
microgravity conditions. Foamability is an important property of liquids. Most liquids do not
stabilize into foam structures because
liquid films are drained by gravity until
the liquid films break. Amphiphilic
(surfactant) molecules are commonly
used to prevent rupture of the liquid
films. These additives stabilize the films
by forming layers of electrostatic charges
at the molecular level along the liquid/air
interfaces.
RESULTS
The main process that controls the
stability of foam on Earth is the gravity
drainage, which eventually causes film
thinning of the bubbles toward a critical
thickness at which they become unstable
and burst. This process is not a factor in microgravity, and foams containing large amounts of
liquid can be studied for a longer time. Foam experiments in space allow researchers to explore
a new sort of very “wet” foam with liquid fractions around 30%. The high liquid fraction enables
bubbles to keep their spherical shape. On Earth, such materials are unstable and most of the
time not called foams but bubbly liquid. In space, drainage is suppressed, such that liquid films
remain thick, but bubble coalescence events are nevertheless seen. However, after some time,
bubble motions become rare events such that the foam is more stable even when shaken. In
most cases, the foamability (the volume fraction occupied by the foam only) is almost two
times larger on the International Space Station than on Earth. A striking and unexpected result
is that a non-foaming solution on Earth makes significant foam in space.
It was discovered that super stable aqueous foams can be created in microgravity conditions.
On Earth, coarsening and film ruptures are always present for a solution even with foaming
agents. In microgravity, the foam still evolves but the amount of foam does not appear to
change significantly. Surprisingly, antifoaming agents have a reduced effect in microgravity, and
the resulting foam appears to be stable. This behavior was completely unexpected since anti-
foaming agents are meant to avoid foam creation and stabilization. These observations raise
new fundamental questions that should be investigated in future works.
ESA astronaut Frank De Winne pictured near the Foam Optics
and Mechanics - Stability experiment in the Columbus
laboratory of the International Space Station in September- NASA image.