Rodolfo Marin Rivera
Have you ever wondered why when the rain stops, water drops remaining on the surface of the leaf do not flow or roll? Well that’s because of the surface tension of the water and the hydrophobicity of the leaf… but wait a minute! What’s surface tension and what’s hydrophobicity?
Both terms can be explained by considering Young’s principle about thermodynamics of wetting. Young described the ability of a drop to spread or not on a surface as the wettability of the surface, and he correlated this phenomenon by measuring the contact angle formed between the liquid, the solid and the gas around these two phases, as it is presented in this diagram:
If a drop of a liquid is put it on a surface (as the leaf shown in the picture) there are two possibilities:
- The liquid spreads on the surface completely, so that the red arrow squash the angle θ formed between the green and the red line (see the drop of liquid at the left), which means the contact angle θ becomes equal to zero, or
- A “contact angle” is formed between the liquid, the solid and the gas phase, so that these three phases are in equilibrium and the contact angle θ tends to 180°, as described by the drop of liquid in the right.
In other words, one can assume that the drop will wet the surface as long as the contact angle θ tends to zero, as the handsome drop of liquid in the left. Therefore, if the drop of water remains on the surface without spreading, then it is said that the surface is hydrophobic (no wet), but if the drop spread into the surface then the surface is called hydrophilic. It may occur that a liquid can be partially adsorbed on a specific surface because the wettability of a solid surface also depends on the surface tension between the liquid, solid and gas phases.
On the diagram on the left, the surface (or interface) tension is defined by the intermolecular forces that attract the liquid particles together. Along the surface, the particles are pulled toward the rest of the liquid. Surface tension (denoted with red arrows and described by the greek letter gamma, γ) is defined as the ratio of the surface force F to the length d along which the force acts:
The term “surface tension” is used when a liquid in contact with the gas phase acts like a thin-elastic sheet. This term is typically used only when the liquid surface is in contact with gas (such as the air around the leaf, or the foam formed when you’re taking a very nice bath), but If the surface is between two liquids (such as water and oil), then the term “interface tension” is used.
Young correlated the contact angle with the surface tension between the different phases with this very simple equation
where γlg represents the surface tension between the liquid and the gas phase, γsl represents the surface tension between the solid and the liquid phase and, γsg describes the surface tension between the solid and the gas phase. The location of these surface/interface tension are shown in the picture above with the two drop of liquids.
Therefore, wettability of a surface is controlled by the first term of the equation, which is known as the “adhesion tension”. If θ becomes equal to 0°, and, for instance, the left term in Young’s equation becomes equal to γlg, which means that the liquid wets the solid surface perfectly. But, if the adhesion tension becomes smaller than γlg, it means that cos θ < 1 , and therefore θ > 0°, which means partial wetting of the solid.
For more info about wettability you can check the following links: