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A less explored model of surface tension can be made by considering the local temperature at the interface. For example, consider the 2-dimensional boundry between air and water. Water molecules at this interface are constantly diffusing into the air, a process known as evaporation. Molecules with a higher kinetic energy (those bouncing around more rapidly) are more likely to evaporate while those with lower kinetic energy are more likely to remain on the surface. This kind of molecular natural selection (also known as wind chill) is the principle used by most plants and animals for keeping cool, i.e. sweat. While a macroscopic drop of water can be cooled several degrees with a fan, only those molecules at the interface are available for evaporation. Hence, even without a fan the nanoscale temperature at the interface could become low enough that a two dimensional (2-D) solid analogous to ice would form.
This model accurately predicts the inverse relation between surface tension and temperature. If the surface tension depends on maintaining local cooling, then it makes sense that a higher temperature solution would have lower surface tension. The local freezing model also explains why surfactants lower surface tension. Surfactant molecules can form a 2-D hydrophobic barrier at the air-water interface. This barrier blocks evaporation, thus halting the cooling and disrupting the surface tension. Yet another phenomena that can be explained by this model is the Lotus Effect.
Numerous plants have finely tuned surface topographies on their leaves and flowers in order to optimize their contact with soil containing water droplets. Lotus leaves in particular have the unique ability to avoid getting dirty. The technique used by lotus plants (nelumbo nucifera) was discovered by Botanists Wilhelm Barthlott and Christoph Neinhuis of the University of Bonn in Germany. They were carrying out Scanning Electron Microscopy (SEM) measurements on various plant surfaces when they noticed that plants with rougher surfaces in the SEM images were easier to clean. In particular, lotus blossoms (the easiest to clean) were coated with tiny hydrophobic particles. When water contacts these particles, it does not wet the leaves. Instead, it simply rolls off, taking with it any dirt in its path. It wasn't long before the scientists had developed various biomimetic surfaces with self-cleaning capabilities that they termed the Lotus Effect.
The evaporation cooling model requires continual contact with the air in order to maintain local temperatures low enough for freezing. If the frozen film contacts another surface, then the cooling mechanism would stop and the surface would become wet. While this is true for most surfaces, the nanoscale topography of a lotus leaf allows it to contact the drop only in very localized areas. Thus, air within the valleys of the surface could continue to cool those parts of the drop that are not in contact with the peaks. Since the peaks are so small, it is possible that the thin solid film remains frozen by transferring its vibrational kinetic energy to those regions of the surface that are exposed to the air.
Page 1 - Surface Wetting, 2.
Acknowledgements: Special thanks to Michael Gleiche (Thin Organic Films - Interface Physics Group, WWU MŸnster) for the relevant discussion.