BERKELEY -- At certain humidities, water condenses by first forming a single-atom thick layer of room-temperature ice. That's one of the remarkable findings made possible by a new technique that has produced the first microscopic images of water condensing and evaporating.
Lawrence Berkeley Laboratory researchers describe this novel new approach to the use of an atomic force microscope in a recent issue of Science magazine. The technique defies the conventional wisdom that says high-resolution images of liquid surfaces cannot be created. Researchers Miquel Salmeron, Frank Ogletree, X.D. Xiao, and graduate student Jun Hu report that they have been able to create maps of liquid surfaces showing features as small as 200 angstroms, or two-hundred millionth of a meter.
Imaging liquid surfaces at this resolution makes it possible to resolve questions that are of fundamental importance in physics, chemistry, and biology. Water films alter the surface properties and reactivity of solids. Likewise, films of water are critical in biology in terms of ion transport.
"Having imaged surfaces all my life," says Salmeron, "I knew that imaging liquid was taboo. We haven't done it because it has been impossible."
At the atomic scale, says Salmeron, "We really don't understand wetting, or the mechanics of how a liquid wets a surface. Problems involving washing, rinsing, corrosion -- they all rely on this mystery of how liquids interact with solids."
Water, the only substance on Earth naturally present as a liquid, solid, and a gas, is the first liquid to be studied by the research team. Models have been developed to theorize how it condenses and evaporates but these processes has never been investigated at the molecular level.
Using a unique atomic force microscope, researchers discovered that the atomic architecture underlying condensation differs depending upon the humidity.
Mica, which has a remarkably flat surface, was used as the substrate or platform for the condensation to form. Researchers controlled the humidity and gradually dialed it up as they focused the microscope on the mica.
The images captured by the microscope reveal that, at below the level of 20 to 25 percent humidity, water vapor condenses as a fluid. Having no particular shape, it covers the surface uniformly.
When the humidity rises to above 25 percent, the picture changes. Single-molecule-tall islands of condensation form. These islands are solid in character. They have a polygonal shape and possess a crystalline structure. They are room-temperature ice.
"We know this," says Salmeron, "because liquid has no shape whereas solids have shape. The angles of these polygons also are the same that ice makes. Beyond that, if you punch the surface with the tip of the microscope, you literally break the ice. And, that's what the images show. We see a system of cracks."
Condensation consists of a one molecule thick layer of ice until the humidity is increased. On mica, when the humidity rises above 40 percent, multiple layers of water molecules begin condensing.
To capture these images, researchers invented a new technique for using the atomic force microscope (AFM) called polarization force microscopy.
The AFM creates images using a radically different means than conventional light or electron microscopes. It uses a sharp tip, almost like that of a phonograph stylus, and as it scans over an object, a computer maps its path and generates a three-dimensional image.
Ordinarily, the AFM tip "touches" the sample. In a phonograph needle, about one gram of force is applied on the stylus; in the AFM, the load on the tip is about one ten-millionth of a gram, a force so slight that it does not dislodge even a single atom, except in the case of a liquid surface. Due to capillary forces, the tip of an AFM sinks into the liquid. The result is a wet tip and no image.
Unable to sink or swim and image liquid surfaces, researchers conceived of an alternative mode. They invented a way to image by flying the tip of the AFM over the surface.
First, the tip must be coated with a thin skin of metal to make it conductive. Then, an electrical voltage is applied to the tip which creates a concentrated electrical field at its apex. When the tip approaches the liquid surface being imaged, it polarizes the substrate and creates an attractive force on the tip. The tip bends toward the surface. And as if flies over the liquid, its angstrom-size displacements are recorded and translated into a topographic image.
"We fly the tip over the liquid at an altitude of 200 angstroms," says Salmeron. "We can't see individual atoms, which are separated by about two angstroms. But we can see the shape of the liquid film, and learn how a liquid wets a surface. Finally, we have tools to explore one of the enduring mysteries of nature. Now, we can see how liquids interact with solids."
LBL is a national laboratory that conducts unclassified scientific research for the U.S. Department of Energy. It is located in Berkeley, California, and is managed by the University of California.