Hydrophobic materials?

Materials with hydrophobic properties have become very attractive due to the possibilities they offer and the arrival they are having in the markets with all kinds of products. If you do not know the fundamentals of this materials property, we will explain it to you.

Hydrophobicity is the ability of a material to repel water from its surface, while hydrophilicity is the property of attracting water. This property is usually measured by pouring a drop of water of a certain volume on the surface we want to measure and observing the angle that the drop forms with the surface. If the drop has an angle greater than 90 °, it will resemble a spherical drop, having a hydrophobic behavior. If, on the contrary, the drop has a contact angle less than 90 °, it will be a very flat drop and will spread on the surface; it means that the material presents a hydrophilic behavior.

For example, if we put a drop of water on a sheet of steel, the drop will spread across the surface and will remain very flat, since the contact angle of the steel is very low and it is highly hydrophilic. If, on the contrary, we pour a drop of water on Teflon, it will have the shape of a sphere and will move easily along the surface without wetting it, having a very high contact angle and therefore being highly hydrophobic.

This, as we have said, it is a materials property, but there are methods with which this property can be modified, being able to obtain a steel that repels water as it happens in your Teflon pans. In order to modify this property, two different types of techniques can be used: modify the chemical mechanisms involved, or modify the physical mechanisms.

The first option has been widely extended for many years and involves changing the chemistry of the material. The basis of this technique is to vary the bonding forces that exist between water and different materials. If a material has a very strong affinity and binding forces between the water and its surface, then the drop will be highly adhered and very hydrophilic. To understand it better, it would be as if the surface of the material and the water were two magnets that attract each other, and therefore, stay very close together. If, on the contrary, the bonding forces are very weak, then the water will have a spherical shape and will roll on the surface of the material without wetting it. In this case, it would be as if the water were in space, without any force that makes it come in contact with the surface.

Once the chemical bonding mechanism between the water and the surface of the materials is understood, the way to modify it is by applying a coating to the material. If we provide a thin layer coating (even of nanometers) to the material, we can turn a hydrophilic material into hydrophobic and vice versa. In this way, we can modify the contact angle of any material.

Coatings can be applied by dip, dip coating, by spraying or by additives in the case of polymers. The technology to apply the coatings will depend on the type and geometry of the piece.[more information]

The second option, more novel and that is beginning to be introduced in the industry, consists in changing the interaction between water and material physically. If, in the case of the chemical modification we focus on the bonding forces between the water and the surface of the material, the physical modification acts on the surface tension between a solid material and a liquid. These surface tension forces are the same than when we play soap bubbles with a hoop, the layer of soap water stays inside the hoop and then we can blow to make the bubble. In theory, because of the weight of the soapy water, it should be impossible, but surface tension is present to keep the water from falling. It is also the surface tension that causes some mosquitoes to walk above the water without sinking, since it generates an upward force that is greater than its weight.

As we have seen, there are certain materials and animals that due to the surface tension between water and them can generate hydrophobicity. In order to modify this surface tension, we only have to modify the contact surface between the material and the water, creating peaks or lines in the material, of micrometric or nanometric size, between which the water is floating in the air. The foundation is similar to a bed of a fakir. If they lay on a board with a single nail, they would nail it, but since they lie on many, the weight is distributed among many points and they remain in the air.

The same thing happens to water and it stays in the air, with a very weak interaction between the skewers of the surface and the water, giving spherical shape to the drops of water and causing them to roll on the surface without wetting it. In this case, the technology that we will use is In mold microstructuring for polymers, laser for all types of materials and Roll-to-roll for polymers and some metals.[More information]

Whether by changing the chemical properties or changing the physical properties of the materials, hydrophobicity and hydrophilicity are now attainable for any material, so if you want to modify your properties, contact us and we will help you!