Once man-made superhydrophobic surfaces have been manufactured, we can then think about what we can do with the surfaces, both to further understand the science behind hydrophobicity and to think about potential applications.
We will first look at some examples of where surfaces can be designed to give a specific function.
Droplet Impact: Bouncing from a bed of nails
When a water droplet impacts on a solid surface, we expect the drop to splash (spread out). However, when impacting on a ‘bed of nails‘, droplets can completely rebound without any splashing.
When a water droplet rests on a microscopic ‘bed of nails’ surface, it will initially form a ball with a high contact angle (indicative of superhydrophobicity). However, as the droplet evaporates, the curvature becomes greater, which leads to a greater pressure inside the droplet. This leads to a sudden transition to a state in which the water penetrates between the ‘nails’.
Gradient Surfaces: Directing Droplet Motion
If a surface is made in which the roughness changes, the water-repellency will also change. This can lead to interesting motion of droplets on the surface.
Sensing Changes: Switching off Superhydrophobicity
While the above example used variation in surface roughness to control superhydrophobicity, hydrophobicity can also be controlled by altering external factors such as temperature. A water droplet can be switched from resting on a foam surface to being absorbed into the foam by heating it up. This could potentially be used as an indicator that the temperature had exceeded a critical level at some time in the past, which is of relevance in food storage for example.
Role of Air Layers
When looking up at the surface of a swimming pool from within the water, we see a silver sheen. Some aquatic animals can also be seen to have a silvery sheen, indicating that there is air present. Superhydrophobic man-made objects can also have air layers present, again indicated by a silvery sheen. In the following examples, we will look at the effect of these air layers around objects.
Plastron Respiration: Extracting Oxygen from Water
Some aquatic animals are able to breathe underwater because they have developed means of carrying air, such as in specially-adapted gills or, in the case of the water spider, on their abdomen. This air pocket is known as a plastron. By exploiting hydrophobicity, the animals can extract oxygen from the water into the plastron which lets them breathe underwater indefinitely. It is also possible to create man-made plastrons in the laboratory, leading to the intriguing question of whether humans could ever be able to breathe underwater using a plastron.
Plastrons aren’t only of relevance for oxygen extraction. Acrylic spheres coated in hydrophobic paint (centre) retain a layer of air around them which results in them falling faster than uncoated spheres (left) and coated spheres treated with ethanol to prevent plastron formation (right). This leads to the question of whether this plastron drag reduction can be used to make boats go faster.
We have seen that a ‘bed of nails’ can create a superhydrophobic surface. As the width of the nails become thinner (and hence the width of the air layer increases), the superhydrophobic properties increase. The ‘ideal’ superhydrophobic case can be viewed as that in which the width of the nails shrinks to zero, leaving just an air layer. Is it possible to produce this situation experimentally?
When water droplets are placed on a very hot surface, they don’t necessarily vaporise but they can remain as droplets for some considerable time. The reason for this is that a layer of vapour is created between the droplet and the surface, which insulates the droplet from the hot surface. This is known as the Leidenfrost effect. The Leidenfrost case can be viewed as being the ideal limit of a superhydrophobic surface.
Flow through pipes
Hovercraft move across water on a layer of air between the water and boat hull. This reduces drag, effectively lubricating the flow of water, in a similar way to in the Leidenfrost effect. However, hovercraft motion requires the constant active production of a layer of air.
It would be much more efficient if a constant air layer could be achieved without the need to actively create the air layer. It is possible that this could be achieved by creating a superhydrophobic surface. To illustrate the principle, we have studied flow through a pipe with a superhydrophobic coating.