The effort behind staying dry

Geometry of a surface is found to matter as much as the material, in shrugging water away, writes S Ananthanarayanan

"The material of the duck’s feathers and the lotus petal are renowned for not getting wet. The lotus effect, as it is called, is the property of these surfaces to strongly repel water, so that the water that falls on them simply runs off and does not wet the surface. This is useful for a plant that grows in water and for the butterfly, which needs its wings to get about. Also, the water does not wet any dirt particles on the surface, and takes them away, leaving the surface clean.

The surface of the lotus petal and also of other plants, like the nasturtium, the prickly pear, and some grasses, have been found to have  a structure at the very fine level with a coating of waxes. The lotus petal has tiny hair-like protuberances, just 10-20 microns in size and these are covered with waxes that are hydrophobic or water-repelling. The waxes have been considered to be the main agents which keep these surfaces dry. But the work of James C Bird, Rajeev Dhiman, Hyuk-Min Kwon and Kripa K Varanasi at Boston University and MIT, reported in the journal, Nature, finds that veins and ridges, or a larger dimension structure of the surface, plays an important role.

Water repellants

The property of being water repelling has to do with the molecular structure of the material and with how it relates to the structure of water molecules. A drop of water, free of other forces, takes a spherical shape because this is the energy efficient form, as it presents the least surface area for a given volume. But if the drop is placed on a sheet of glass, for instance, the molecules of water are strongly attracted to the glass surface and retaining the spherical shape, against both gravity as well as the attraction of the surface, takes energy. If the drop is heavier, it would spread out, to minimise energy, and hence wet the glass.

This happens because the material of glass has affinity for water, or is hydrophilic. But if the material were a layer of oil or grease, for instance, the material has a closed molecular surface and is unable to blend with water molecules, The droplet of water is hence not drawn to spread out and is able to retain its spherical shape and shrink away from the surface even when it a reasonably large drop. When the drop grows very large, it’s surface tension cannot support the greater weight and the drop collapses.

Bouncing off

The authors of the paper in Nature note that the current understanding of what transpires when a drop of water strikes a surface suggests that it is important that the time of contact of water and the surface, till the drop bounces off, be kept as short as possible. “A drop striking a non-wetting surface of this type will spread out to a maximum diameter and then recoil to such an extent that it completely rebounds and leaves the solid material,” they note in the paper. 

 The least contact time, between the water and the surface, would help prevent development of attractive forces, called ‘pinning forces’, which promote wetting. But in respect of water bouncing off a surface, the Boston/MIT group tested the assumption that uniform, ‘axisymmetrical’ deformation of the drop promotes fastest recoil. The hydrophobic surface they used was a sheet of silicon, coated with flourosilane, a known super-hydorphobic. The experiment was then to let fall a drop of water, 2.66 mm in diameter, from a height of just over a metre, on to the coated silicon surface. The spreading of the drop on the surface, and then the recoil and bounce back were recorded, from the side and from above, by high speed cameras.

The second step in the experiment introduced non-symmetrical spread, which was achieved by criss-crossing the silicon surface with a macrostructure of ridges. The principle was that if the drop is divided and the portion near the centre also gets into the act of promoting the recoil of smaller drops, then the time of contact may be reduced. The group tried out the second experiment on surfaces like anodised aluminium and copper, coated with flourosilane, to obtain similar results.

Creating materials that resist wetting would have industrial value as they would prevent corrosion and save maintenance. Since they do not ice in cold weather, it would be useful in high altitude aircraft components too.

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