Abstract:
Highly stable foams by the attachment of silica nanoparticles to bubble surfaces
For whisks
June 20, 2005
What do a glass of beer, whipped cream, dish-washing
detergent, shampoo and foam sealant have in common? They
should foam properly. Foams are gas bubbles confined by
fluid or solid boundaries. Whereas solid foams are quite
stable (foam rubber and whipped cream, for example), most
fluid foams quickly collapse: if a beer is left to stand
too long, the head eventually disappears; in a similar
manner, the best part of a bubble-bath is over. To
stabilize foams, surface-active reagents or proteins are
generally used. British researchers have developed foam
stabilizers that are more effective: highly disperse silica
nanoparticles.
Why do foams collapse? The fluid surrounding the gas
bubbles slowly flows downward and partially evaporates. As
a result, the lamellae between vesicles becomes thinner and
thinner. The bubbles at the surface eventually burst, other
bubbles fuse, and small bubbles shrink in favor of larger
ones. Bernard Binks and Tommy Horozov discovered that
miniscule silica nanoparticles can counteract this effect.
The particles attach themselves to the surfaces of the
small bubbles. Standard surface-active reagents do this as
well, but nanoparticles differ in that they do not detach
from the bubble surface. The secret to the success of the
nanoparticles is their finely balanced hydrophobicity. This
can be controlled by the manner in which the hydrophilic
silica nanoparticles are purposefully covered with a water-
repellent layer. The more hydrophobic the particles become,
the more firmly they press themselves into the air-bubble
surface. The nanoparticles cannot be completely
hydrophobic, however, as this would impede their hydration
by water altogether. Silica particles work best with
intermediate hydrophobicity.
Under the microscope, bubble surfaces appear corrugated.
The bubbles are covered with a closely packed layer of
particles. It is possible that such stable bubbles are
formed by the fusing of smaller bubbles, which themselves
are not as well-covered with particles. As a given volume
defined by many small bubbles has a larger surface area
than the same volume in fewer, larger bubbles, the process
of bubble fusion eventually creates the appropriate
available space for the nanoparticles. As the particles
cannot become detached, they move ever closer together, and
the surface corrugates. The closely packed nanoparticles
protect the air bubbles from collapse, and thus stabilize
the foam.
From: Angewandte Chemie International Edition
Contact:
David Greenberg
201-748-6484
dgreenbe@wiley.com
Copyright © John Wiley & Sons
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