Contact angle is the shape that a liquid takes when it is in contact with a solid. You might have listen talking about superhydrophobic and superhydrophilic surfaces and in today’s post we are going to explain in detail in which consists.

The contact angle measures the angle which a liquid takes when gets in contact with a solid. When the adhesive forces are very high the angle is less than 90º because the liquid is ‘drawn’ by the solid and it spreads. In the case of water it is called hydrophilic surfaces. In the case that the attraction forces are very low the surface will tend to “repel” the liquid and the angle will be greater than 90°, in the case of water is called hydrophobic. In the case of very pronounced find these behaviors add the prefix ‘super’ and you will have the superhydrophobic and superhydrophilic surfaces. This same behavior we can find oils, in which case you will hear the terms oleophilic and oleophobic. And if the surface repels any liquid it will be called omniphobic.

The contact angle between a solid and a liquid will depend on:

– The surface tension between solid and gas

– The surface tension between solid and liquid

– Surface tension between liquid and gas

Surface tension Surface energy

Maybe these parameters do not tell you much. A quick way to understand this is with the following parameters (found within the previous three):

– The type of liquid. It has not the same surface tension water or oil or diiodomethane. If you apply a drop of oil on a steel surface it will extend much more than if it is a water drop. The cause: the tension of the molecules of the liquid itself.

– The type of surface. It does not have the same energy a Teflon surface which tends to repel almost any liquid than glass that usually attracts everything. This is because the chemical composition of the material itself.

– The surface roughness. This case is very visual. If there are small bumps at the micro and nanometer scale on the surface, the drop is supported between the peaks of the protuberances being the contact shortest and therefore being in ‘more spherical’ shape total contact, that is, with a greater contact angle. This case was already explained in another post about the Lotus effect. These leaves repel the water because its bumps (and also by its wax – effect type of surface).

Hierarchical structures

And what is it useful for to know the contact angle of a surface?

For many things! For example:

– Knowing if my painting is going to be compatible and is going to adhere well to other surface.

– Learning if the surface that I am going to treat is it clean enough or I should make a degreasing step.

– Measuring the surface properties to know if it is self-cleaning, in which the surface will repel dirt and will not get stained.

Today we have talked about static contact angle, but in future posts we will explain also what is dynamic contact angle and sliding contact angle. See you in next posts!