Focus: When a Liquid Is Not a Liquid
For years physicists have suspected that near the edge of its container a liquid is not a liquid–that its structure differs from the rest of the liquid. Direct evidence for this deviation has now come from an experiment described in the 15 March PRL. The analysis of x-ray reflections from an ultra-thin liquid film atop a silicon surface has shown that near the solid-liquid boundary the liquid’s molecules form layers, much like those of some liquid crystals. The work supports a fundamental theory of liquids in confined geometries, conditions encountered in processes such as lubrication and filteration.
Because liquids are rarely found floating in free space, it is important to understand how they are confined by solids. There has been clear evidence for some time that thin film liquid properties, such as viscosity and thermal conductivity, are different from those of the same liquid en masse. Because structure determines properties, the internal arrangement of molecules in these films–less than 10 nanometers in thickness–must be special. No internal ordering has ever been seen at the free (liquid-gas) surface of a nonmetallic liquid film, but indirect evidence suggested a layering, or oscillation, in the arrangement of the liquid molecules next to a solid-liquid border. “At a solid-liquid interface, the liquid layering depends on the roughness of the solid,” says Pulak Dutta of Northwestern University in Evanston, IL. “While you can never get a perfect solid, we used a polished silicon that comes close.”
To directly detect this layering near the interface, Dutta, graduate student Chungjong Yu, and their colleagues bounced x-rays off of a thin film of TEHOS, an ordinary liquid with desirable properties for such experiments. TEHOS molecules are roughly spherical, with a diameter of about 1 nm. Using x-ray beam facilities at Brookhaven National Laboratory in New York and Argonne National Laboratory in Illinois, they varied the incident angle of the x-rays and observed their scattering from the TEHOS. “You get reflection from the top of the film, you get reflection from the bottom of the film, and you also get reflection from elsewhere in the film,” says Dutta. “Starting from the observed interference pattern, we can work backwards to figure out where these reflections are coming from.” They found that the electron density within the liquid oscillates with distance, so that peaks are separated by about the size of one molecule. They concluded that the layered TEHOS molecules were in an intermediate physical state between bulk liquid and solid.
“Everyone expected this to happen, but no one had been able to measure it before in such a direct way,” says Steve Granick of the University of Illinois at Urbana-Champaign. “Other [experiments] will become possible by natural extension.”
David Appell is a freelance science writer.