Determining an Object's Consistency without Contact

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Two teams of researchers have succeeded in evaluating the rigidity of a material
without touching it! To achieve this feat, physicists from the Laboratoire de
Physique de la Matiere Condensee et des Nanostructures (CNRS / Universite
Claude Bernard Lyon 1) and the Laboratoire de Physique des Solides (Universite
Paris-Sud / CNRS) placed a liquid where they created a very weak, nanometric
scale flow between the probed object and the "tester". This technique, derived
from the latest advances in nano-mechanics, has the advantage of being non-
invasive and therefore non-destructive and could significantly improve the
testing and analysis of thin, fragile objects such as bubbles or cells. This work
is published on-line on June 18, 2012 on the website of the journal Physical
Review Letters.

A simple way of determining whether a body is hard or soft is to touch it with a
harder object. The problem with this technique is that it can destroy the item,
especially if it is extremely fragile like a bubble or a living cell. Developing a
less invasive alternative was therefore vital. To assess the rigidity of an object
without touching it, the team of physicists had envisaged blowing on it
delicately to check whether this flow of air deformed the material or not. But
precisely controlling a flow of air is difficult on account of the vortexes that can
form in the air. Hence the idea of using an easier-to-control "nano-flow" of
fluid instead.

The researchers tested their technique on a thin elastomer (rubber) film, only
several hundreds of nanometers (1) thick. In concrete terms, they placed the
film on a rigid glass support and immersed the lot in a mixture of water and
glycerol. They then created a very slight displacement of the liquid, near to the
film. To generate this nano-flow, the physicists, and more particularly Samuel
Leroy who was then working on his PhD at LPMCN (2), had to use a special
device, developed in 2000 in the same laboratory (3). It comprises in particular
a millimetric Pyrex (special glass) sphere, attached to a rod, which can be finely
moved with what is known as a "piezoelectric ceramic" system. It is precisely
this tiny glass bead that allows a nano-flow to be created at the surface of an
object...

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