A key concept in current fluid dynamics and its applications to biology and technology is a phenomenon known as wetting. Wetting is familiar from everyday life and is simply the ability of a liquid to stay in contact with a solid surface. The wettability depends on the properties of the liquid and the solid and can be characterized by the static equilibrium contact angle θ (the angle at which the liquid–gas interface meets the liquid–solid interface). A contact angle below 90° indicates favorable wetting such that a drop of the liquid would spread over a large amount of the flat solid surface, whereas a high contact angle indicates that very little of the solid is wetted (this can be seen in Fig. 1, which shows various stages of surface wetting in terms of the equilibrium contact angle). Nevertheless, this theory generally sounds quite dry or difficult to visualize when explained to students for the first time. The theory of the contact angle also contains some controversies and has undergone some recent developments. We propose a simple classroom demonstration with superhydrophobic sand that gives a concrete visualization of “superhydrophobicity” and outline how the phenomenon can be explained macroscopically with wetting theory. There are several interesting physical effects that are due to superhydrophobicity: experimental studies have found, for example, that superhydrophobic spheres always splash when they impact a body of liquid. In terms of applications, there are various possibilities for water storage with superhydrophobic sand outlined in the chemistry literature.

A key concept in current fluid dynamics and its applications to biology and technology is a phenomenon known as wetting. Wetting is familiar from everyday life and is simply the ability of a liquid to stay in contact with a solid surface. The wettability depends on the properties of the liquid and the solid and can be characterized by the static equilibrium contact angle θ (the angle at which the liquid–gas interface meets the liquid–solid interface). A contact angle below 90° indicates favorable wetting such that a drop of the liquid would spread over a large amount of the flat solid surface, whereas a high contact angle indicates that very little of the solid is wetted (this can be seen in Fig. 1,1,2 which shows various stages of surface wetting in terms of the equilibrium contact angle). Nevertheless, this theory generally sounds quite dry or difficult to visualize when explained to students for the first time. The theory of the contact angle also contains some controversies and has undergone some recent developments.3 We propose a simple classroom demonstration with superhydrophobic sand that gives a concrete visualization of “superhydrophobicity” and outline how the phenomenon can be explained macroscopically with wetting theory. There are several interesting physical effects that are due to superhydrophobicity: experimental studies have found, for example, that superhydrophobic spheres always splash when they impact a body of liquid.4 In terms of applications, there are various possibilities for water storage with superhydrophobic sand outlined in the chemistry literature.5,6
Fig. 1.

Droplet A has a large contact angle with the surface (corresponding to little wetting), and droplet S has a very small contact angle (corresponding to favorable wetting). Reproduced from Ref. 1, accessed from Ref. 2.

Fig. 1.

Droplet A has a large contact angle with the surface (corresponding to little wetting), and droplet S has a very small contact angle (corresponding to favorable wetting). Reproduced from Ref. 1, accessed from Ref. 2.

Close modal
In Fig. 2, we show the classroom demonstration that we have in mind (this should be fascinating for those students who have never seen the phenomenon before). There are some existing YouTube videos that do an excellent job of demonstrating the behavior of superhydrophobic sand in water.7 Teachers may note that the sand can be purchased cheaply online by the name of magic sand.8 In fact, the trick of lifting dry hydrophobic sand from water has been done for many years in stage magic shows, where it is sometimes referred to as the “Sands of the Nile” trick. When the superhydrophobic sand is under the water, it looks to be “wet” and can be manipulated like the usual wet sand that is used to make sand castles. However, as soon as some of the sand is lifted out of the water, it instantly becomes dry. It then clumps up and becomes wet sand again as soon as it is dropped back into the water. Students might like to think of other physical phenomena that they have observed that relate to this effect. Certain leaves, such as the lotus leaf, are hydrophobic, and water droplets are repelled away from the surface of the leaf after heavy rain, preventing it from becoming soaked. This principle is mimicked in the design of hydrophobic clothing. Another famous example is the failure of oil to mix with water, which is sometimes attributed to oil being hydrophobic (although the full picture is slightly more complicated).9 
Fig. 2.

Demonstration of the wetting properties of superhydrophobic sand.

Fig. 2.

Demonstration of the wetting properties of superhydrophobic sand.

Close modal
The complete theory behind hydrophobicity is somewhat complicated and involves intermolecular interactions between the solid and the liquid.10,11 However, as mentioned before, we can give a simple macroscopic explanation by relying on the concept of the contact angle. Roughly speaking, if a droplet of liquid spreads easily over a flat solid surface, then the adhesive forces involved in the intermolecular interactions are stronger than the cohesive forces (that is to say, it is chemically favorable for surfaces composed of different particles to stick together). If a droplet has difficulty spreading, then the cohesive forces are stronger (that is to say, it is chemically favorable for molecules of the same type to stay together). In both cases, the favorability for wetting is characterized by the static contact angle via the Young equation (or some modification of this equation) after rearranging for cos θC. The most basic possible version of the Young equation is as follows:
γSGγSLγLGcosθC=0,
where θC is the contact angle in equilibrium, γLG is the surface tension, γSG is the solid–gas interfacial tension, and γSL is the solid–liquid interfacial tension.
The Young equation is not one that will be familiar to most high school students, so we attempt to demystify this equation by showing in Fig. 3,2,12 a diagram of a liquid droplet on a flat solid surface with the above quantities labeled. This diagram should hopefully clarify what the static equilibrium contact angle θC represents and the fact that the Young relation is essentially just the net force relation that one obtains by considering Fig. 2 and the three different forces of surface tension that are involved in the balance of forces to keep the droplet in static equilibrium. From classical mechanics, students should be familiar with the concept that the net forces all need to add up to zero in order to have a system in static equilibrium. They could also compare to similar mechanical setups involving several different tensions in an equilibrium system of ropes and pulleys.
Fig. 3.

Contact angle for a liquid droplet on a solid surface. Reproduced from Ref. 12, accessed from Ref. 2.

Fig. 3.

Contact angle for a liquid droplet on a solid surface. Reproduced from Ref. 12, accessed from Ref. 2.

Close modal

A low contact angle (below 90°) generally indicates favorable wetting, a contact angle between 90° and 150° indicates a hydrophobic surface that repels water, and a contact angle at or above 150° indicates that the surface is superhydrophobic and that wetting is impossible. In reality, the angle that we describe is only the static contact angle, and there is also a “dynamic” contact angle that comes into play in certain applications (note that the dynamic contact angle is always larger than the static contact angle).3 This equation is the most basic possible for understanding the contact angle and wettability, although there are many more sophisticated ones in the literature.2 The exact chemical treatment of the sand that makes it superhydrophobic is something that we will not go into here and involves some chemistry. An example of a chemical protocol that is used to make aluminum beads superhydrophobic (θ ≈ 150–170°) can be found in Ref. 13.

The sand grains in the demonstration are superhydrophobic, so they repel water on entrance into the liquid. Since the sand fails to be wetted by the liquid, it entrains air, which can be seen as a thin layer around the sand. It could now be a discussion point with students what causes the sand to form clumps that can be molded and moved about. This can be explained if a mass of sand grains of heterogeneous shape is held together by the liquid–gas interfacial tension γLG. The sand cannot wet, so there are obviously no adhesive forces between the molecules of the sand grains and the water molecules. At the interface between the thin air film and the water, the cohesive forces between the water molecules are stronger than the adhesive forces between the water and air molecules, so the water layer around a blob of sand acts like an elastic membrane that keeps a number of sand grains held together in one piece (an effect known as surface tension).

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Published open access through an agreement with University of Warwick