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Nanophysics

Turning Water into Stone

What are the physical properties of confined liquids only a few nanometers thick? Well, they might not be liquids at all... A team of CNRS physicists is breaking new ground in the understanding of this phenomenon.

 

microscope water

© J. Chatin/CNRS Photothèque

The Atomic force microscope (AFM) has a resolution of a few nanometers.


Scientists have known for twenty years that liquids can take on the properties of solid matter when they are confined to very constricted spaces. After originally being  theorized, this knowledge has been tested empirically with the advent of extremely powerful measuring tools, such as the surface forces apparatus (SFA) and the atomic force microscope (AFM). The AFM is a device that uses a cantilever tip to probe surface properties and image them. When the cantilever is forced to oscillate, the slightest obstacle to its movement modifies the oscillation amplitude and phase of the lever. These changes are very suitable to measure the viscosity of the confined liquid–that is, its relative capacity to oppose the cantilever tip and modify its movement.

 

How the liquid's viscosity varies during the confinement, however, remains unclear. “After Jacob Israelachvili, professor at the University of California in Santa Barbara and a leading figure in this field, demonstrated an increased viscosity in confined liquids, the debate focused on how this increase happened,” explains Dr. Abdelhamid Maali, from CPMOH.1 “Some said that once a liquid reaches the equivalent of seven molecules in thickness, its viscosity increases by several orders of magnitude at once. Others said that there was a gradual increase. Our research shows that the viscosity grows in a modular fashion,” he concludes.

One of the obstacles that stands in the way of testing these hypotheses is that measuring the properties of fluids at this scale is inherently difficult. While certain configurations of atomic force microscopes (AFM) exist especially for liquid media, their sensitivity leaves much to be desired.”

 

With an AFM, the amplitude and the phase of the lever's oscillation informs on the viscosity of the medium it is going through. “It was very hard to conduct AFM readings of liquid matter until now, simply because it was very difficult to get the cantilever to oscillate at its optimal frequency and thus obtain the finest possible reading.”

There are two existing modes to get the lever moving, the magnetic mode–used widely in commercial microscopes–and the acoustic mode. We modified an existing acoustic system that is usually used in ambient air or under vacuum, so that it could be used with a liquid,” says Maali. This device, tested successfully,2 gave extremely precise readings that allowed the researchers to excite the cantilever at the very low amplitude of one angstrom (10-10 meters). The team could then get back to the issue of the “solid liquid” equipped with an instrument that might finally resolve the matter.

 

Using a liquid with characteristics similar to those of water, the experiment clearly indicated that viscosity increased and decreased in a modular fashion until the tip touched the solid surface underneath.3 That is, the liquid became quasi-solid, then turned back into a liquid, and into a solid again. “This shows that the liquid has structured itself into layers of molecules. When the tip is 7 nanometers away from the solid underneath, there are 7 densely packed molecular layers. When it is 6.5 nanometers away, there is only room for 6 layers. The liquid dilutes and the lever feels less force against it. Once the tip reaches 6 nanometers, the force against it increases once again,” explains the researcher.

 

Research into the properties of confined liquids is of great interest to scientists of several and varied disciplines: materials science, biology, geology, tribology (the study of friction), and nanofluidics (the study of fluid flow in and around nano-sized objects). “Right now, the scientific trend is geared towards the very small. It is important to know how liquids behave in such confined spaces,” says the physicist. The study of confined fluids also bears relevance to the study of living organisms. “We are wondering what impact the variation in viscosity can have on cells,” cautiously adds the researcher. “They carry nanochannels of approximately 10 to 15 nanometers. Molecular arrangement might play a role in the transport of molecules to and from cells. But we have yet to reach any conclusions. In fact, we are just starting to ask these questions.”

 

Marianne Niosi 

Notes :

1. Centre de physique moléculaire optique et hertzienne: Center for Optical and Hertzian Molecular Physics (CNRS / Université Bordeaux-1 joint lab)
2. A. Maali, T. Cohen-Bouhacina, G. Couturier, J.P. Aimé, “Oscillatory Dissipation of a Simple Confined Liquid,” Phys. Rev. Lett. 96: 086105. 2006.
3. A. Maali, C. Hurth, T. Cohen-Bouhacina, G.Couturier, J.P. Aimé, “Improved acoustic excitation of atomic force microscope cantilevers in liquids,” Appl. Phys. Lett. 88. 2006.

Contacts :

Abdelhamid Maali
a.maali@cpmoh.u-bordeaux1.fr


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