When running some experiments about the properties of water, researchers at the Pacific Northwest National Laboratory became frustrated that ice was melting too fast. Seeking a means to slow this process down, the scientists uncovered something interesting about the substance that is essential to life.
Water is composed of two atoms of hydrogen and one of oxygen, drawn together by covalent bonds. Water is a transparent fluid. It is found in streams, lakes, oceans and rain (covering 71 percent of our planet); water is also the most important constituent of the fluids in our bodies.
At a standard temperature and pressure, water is a liquid. At lower temperatures it forms ice, and at higher temperatures or pressures it transforms into a gaseous state, either as steam or water vapor. Variant forms of water include snow, fog, dew and cloud.
The U.S. Department of the Environment, working out from the Pacific Northwest National Laboratory, has been examining the properties of water. One area being investigated was with ice, and one problem, involving ultra-high vacuums, was that the ice was melting too fast, giving insufficient time for the experiments to run.
The new method allows ice to remain for longer, acting almost like a “stop-action movie.” This involved digitally capturing the motion of water every 10 nanoseconds (one hundred millionth of a second.)
The new method was proposed by Dr. Greg Kimmel. The method has revealed that water droplets do “wet” surfaces. Some theories have proposed that water does not actually wet all surfaces. In relation to water, some surface are considered “wettable” and are termed hydrophilic and some are considered “non-wettable,” or hydrophobic. The reason for this distinction comes down to whether water stays in puddles on a surface and then rolls off or evaporates (which is not considered as true “wetting”) or if water flows across a surface (which is “wetting.”) This difference is sometimes called the “lotus effect“; the word derives from the self-cleaning properties (or “superhydrophobicity”) of the lotus plant.
The lotus effect can be seen in the video below:
Even here water, to some scientists, is not “wet.” Wetness is the description of experience of water, based on sensations of temperature or movement. With people, the sense of wetness relates to the sensation caused by the movement of a fluid over the skin. For example, if you hold your hand perfectly still in a bowl of water at ambient temperature your hand does not feel wet. Much comes down to how wetness is defined. In relation to the research discussed, wetness relates to whether water flows along a surface or not.
The new results show water droplets spread out and cover any surface. This has been revealed by studying how water interacts with surfaces on the nanoscale. Here the research was hampered by the rapid melting of ice on different surfaces, which was due to the vacuum chamber.
The solution, the researchers found, was to rapidly melt ice and then quickly refreeze it. This allowed the research team to analyze the effects using different instruments. The study was undertaken using a platinum surface. The heating (and thus melting) was achieved using quick bursts from a laser.
On heating the water molecules moved and then reorganized. The act of refreezing stopped the liquid water’s motion, allowing patterns to be studied down to the level of nanoscale crystalline ice forms.
The key research finding was that the heat pulse turns the ice crystallites into nanoscale droplets of water that do not initially wet the surface. However, the droplets eventually spread out and cover the surface, creating a film that’s several layers of molecules thick. This was reproduced across a variety of surfaces.
The findings are important because insights into the water responds to a surface could offer clues as to how to design different coatings and how to conserve energy. This could extend to such areas as new fuel cells; studying weather patterns and cloud formation; to making biofuels. The key to achieving greater control, the researchers argue, comes from controlling how water behaves at the nanoscale rather than, as previous research efforts have done, by focusing at the macro-level.
The research is published in the journal Chemical Research. The paper is headed “How Does Water Wet a Surface?”
This article is part of Digital Journal’s regular Essential Science columns. Each week we explore a topical and important scientific issue. Last week we examined developments with dissolvable medical devices, to minimize risks associated with surgery. The week before we outlined the latest research into naturally occurring antimicrobial compounds.
