CONTENTS - L'Oréal

L’ORÉAL-UNESCO FOR WOMEN IN SCIENCE 2005
L’ORÉAL-UNESCO AWARDS 2005: The Laureates
however, than thermally excited waves, and the detection
and measurement of elasticity and viscosity are thus
more precise.
3) USING DROPS AS A MEASURING TOOL
Through a phenomenon of expansion and retraction
caused by a small motor, one can create superficial
compression waves of very fine drops formed at the tip of
a syringe. The measurement of these viscoelastic
parameters thus becomes possible.
Other optical methods are currently being developed in
Dominique Langevin’s laboratory to measure the
thickness of the interface (an average of 2 nanometers):
ellipsometry, x-ray reflectivity, and Brewster angle
microscopy.
A major discovery: the amazing properties of
micro-emulsions
Attempting to stabilize an emulsion for as long as
possible, or to lower the superficial tension in order to
improve their “detergent” power are primordial scientific
challenges. It is possible to obtain extremely fine
emulsions, called micro-emulsions, which are stable
indefinitely. These fine emulsions have the power (by
playing on the “resistance” of the interface - the
superficial tension) to penetrate porous media.
Petroleum can thus be retrieved from rocks or sand. One
of the consequences of Dominique Langevin’s research
has been discovering the origin of the low superficial
tensions within such systems (10,000 to 100,000 times
lower than those at the initial water-oil interface).
Recent studies in her laboratory have focused on the
characteristics of solutions containing blends of
surfactants and polymers that are more or less flexible,
like DNA strands that can be used in certain gene therapy
approaches.
The universe of foams
As Aesop said about the tongue, foam is both the best and
the worst of things. Foam is essential for shampoos to
effectively eliminate the oils on the hair surface, but too
much foam ruins the dishwashing soap.
The structure of foam
Upon close examination, foams turn out to be a complex
group of bubbles separated by a liquid film: foams are
gas inside a liquid. From the time they are formed to the
time they disappear, several mechanisms govern the
lives of bubbles: maturation (when gas goes from small
drops toward larger drops), the drainage of liquid
between the walls of the bubbles that dries out the foam,
and when the bubbles burst.
The study of foams draws on a number of disciplines. In
the 19th century, the Belgian mathematician Joseph
Plateau set out the rules governing the shape of bubbles
and the number of faces and the angles between the
faces. As is often the case in physics, the problem was
simplified to determine its essential characteristics, and
foams were initially studied in two dimensions, with
bubbles being polygons. The three sides of a bubble
begin at the apex of such a polygon, each side of which is
at an angle of 120 degrees to the next; bubbles with up to
six sides are under excess pressure and empty into
bubbles with a smaller number of sides. These
pressure-equalizing phenomena within a changing