Dipping into liquid imaging.

By dragging the tip of a microscopic needle back and forth across a solid surface, an atomic force microscope can detect and image surface features as small as individual atoms. However, applying the same technique to liquid surfaces merely wets the tip and disturbs the material.

Now, Miquel Salmeron and his coworkers at the Lawrence Berkeley (Calif.) Laboratory have developed a novel technique for surveying the microscopic structure of liquid films. Called scanning polarization force microscopy, the method depends on detecting slight variations in the attractive electric force between the microscope's electrically charged needle and a liquid's molecules.

Normally, it's hard to image liquids, and their structures are largely unknown, Salmeron says. This technique offers a way of obtaining information about such processes as corrosion and the evaporation and condensation of thin films of water on solid surfaces.

The researchers coat a conventional silicon nitride cantilever and needle with a thin layer of platinum, then connect the unit to a battery. This produces an intense electric field at the needle's sharp tip. Brought no closer than about 200 angstroms from a liquid's surface, the needle induces changes in the distribution of electric charge and orientation of the liquid's molecules, creating an attractive force. By scanning a surface without touching it, the researchers can create a map showing variations in the electric force, which generally correspond to surface topography. The method can detect features larger than about 200 angstroms across.

Salmeron and his colleagues have used the technique to monitor the wetting of an extremely smooth mica surface. Freshly cleaved in a humid environment, mica readily acquires a one-molecule-thick layer of water. As the humidity increases, a second layer begins to form, creating distinctively shaped patches, or islands, of water atop the first layer (see illustration). The researchers find a strong correlation between the straight edges of these water islands and the geometry of the underlying mica crystal structure.

This type of microscopy may prove useful for contamination and environmental chemistry studies, Salmeron says. His team has already examined the microscopic details of how tiny droplets of sulfuric acid attack aluminum. "It tells you where things are happening," he says.