Staying dry
(appeared on 27th Nov 2013)

(link to main website)

Geometry of a surface is found to matter as much as the material, in shrugging water away, says 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 water that falls on them simply runs off and does not stick, or wet the surface. The surface thus stays dry and firm, which is useful for a plant that grows in water and for the butterfly, which needs its wings to get about. But the water does wet any dirt particles on the surfaces, and it takes them away, leaving the surface not only dry but 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 structure at the very fine level and 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 main agency which keeps 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 a sphere 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 more than very small in size, it would hence spread out, to minimize energy, and the drop would be said to have 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, of course, the tendency of the surface to minimize itself, which is called surface tension, cannot support the greater weight and the drop collapses. A material that behaves like this, examples being oils or a wax covered surface, are called hydrophobic.

Creating surfaces like this, where drops of water do not adhere to them but bounce off, is of great interest. Surfaces like this stay dry, stay clean and, as they do not wet, also avoid getting covered with ice, in cold weather.

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. When the drop strikes the surface, it is the motion energy of the drop that is able to spread it out over the surface, creating tension in the surface of the deformed drop. When the movement stops, the surface tension draws in the drop into a spherical shape again, and the reaction against the surface propels it back, to rebound.

Theoretical considerations would suggest that it is in symmetrical deformation, equally in all directions, that the energy of impact would get converted and reversed in the most efficient and hence fastest way. The least contact time, between the water and the surface would help prevent development of attractive forces, called ‘pinning forces’, which promote wetting. Studies so far have

focused on properties such as the angle of contact of the interface of a drop on a surface, to delve into ways to increase water repellence. The picture shows the shape of a drop of water on a surface that it does not readily wet, like a greasy (as opposed to a clean) sheet of glass. It can be seen that there is a large angle of contact in the case of the largest drop, where the surface tension has to support the largest weight of water.

The angle of contact also comes into play when attractive, wetting forces between glass and water support a column of water by capilarity when an narrow glass tube is dipped in water.

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 or strongly water resistant material, like the lotus petal. The experiment was then to let fall a drop of water, 2.66 mm in diameter, from a height of just over a meter, 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 photographs show the progress, from 0 millisconds, when the drop just strikes, to 2.7 ms, when it has spread to the maximum, at 4.7 ms, when it drawing inwards, at 7.8 ms, when the drop is forming again and at 12.4 ms, when it starts bouncing off the surface. The upper pictures are the side view and the lower pictures are the top view. It can be seen that the while the rim of the drop spreads out and recoils, the centre stays stationary.

The second step in the experiment was to introduce non-symmetric spread, where the centre also moves. This 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 results of the second part of the trials are in the second picture, where we can see that the recoil is complete by 7.8 ms, while it took 12.4 ms in the first case – a saving of 37%.

The effect of the ridge is to start the recoil in both directions along the ridge, for faster recovery and also resulting in smaller droplets, which would be less likely to wet the surface in subsequent contact too.

The group tried out the effect of the ridge-like macrostructure on waterproofing other surfaces, like one of anodized aluminium and copper, coated with flourosilane, to obtain similar results. It was also seen that in nature too, the wings of the butterfly and the leaves of the nasturtium contain a macrostructure of ridges, and these surfaces showed less water drop contact time than the legendary lotus leaf.

Creating materials that resist wetting would have industrial value. The property would prevent corrosion and save maintenance. The last property, of not icing in cold weather, would be useful in high altitude aircraft components, for instance.

Do respond to : [email protected]