Experiments on superhydrophobic surfaces

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.

Functional Surfaces

We will first look at some examples of where surfaces can be designed to give a specific function.

Self-cleaning surfaces

Dipping a supherhydrophobic card in muddy water

Muddying a superhydrophobic surface is far from easy, as we can see by dipping a surface that has been given a superhydrophobic coating into a box of muddy water.

Droplet Impact: Bouncing from a bed of nails

A drop about to hit a superhydrophobic surface
A drop about to hit a superhydrophobic surface

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.

Evaporating Droplets

water droplet on a 'bed of nails' surface

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

water droplets on a surface with varying roughness

If the roughness changes across a surface, 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.

Counter-intuitive example – Liquid Marbles

liquid marble

As ‘hydrophobic’ means ‘water-fearing, we would expect a water droplet placed on a superhydrophobic surface to be repelled from the surface. However, if the surface consists of superhydrophobic grains that are loose rather than fixed (as was the case in the example of superhydrophobic sand), the grains can stick to the surface of the water droplet rather than being repelled. This creates a ‘liquid marble’ that can roll freely on solid (and water) surfaces.

Super-vortex: skating across surfaces

A jet of water flowing around a funnel coated with a super-hydrophobic surface.

A jet of water directed onto a funnel with a superhydrophobic coating goes round many times more than one without a coating.

Water collection on superhydrophobic leaves

We can look at how the superhydrophobic leaves of some plants can collect water at the stem base without the water bouncing off.

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 in water, 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

Experimental apparatus showing superhydrophobic foam, a fuel cell (to burn oxygen) and an oxygen sensor.

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.

Plastron Drag Reduction

Comparison of the rate at which spheres with plastrons and those without fall through a fluid.

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.

Leidenfrost Effect

Water droplet on a very hot surface illustrating the Leidenfrost effect.
Cryonic07 / CC BY-SA

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 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

Experimental apparatus showing two  pipes through which fluid  can flow. One pipe has a superhydrophobic coating and the other does not.

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.

Hovercraft motion would be much more efficient if a constant air layer could be achieved without the need to actively create the air layer. In principle, it is possible that this could be achieved using a superhydrophobic surface. To illustrate the principle, we have studied flow through a pipe with a superhydrophobic coating.