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[Surface treatment] About hydrophilicity, hydrophobicity and water repellency
10/03/2025
In the surface treatment industry, we often hear relevant technicians mention the three words hydrophilicity, water repellency and hydrophobicity for surfaces. When we mention these terms, we think of "lotus". As we all know, when rain falls on leaves, the water does not disperse, but appears on the surface as water droplets. The main reason for this can be understood by using the picture below:
After magnification, we observed that the leaf surface is covered with nanoscale surface cells and cilia. When water forms a large drop angle with the surface cells, the water cannot cover the surface and appears in the form of water droplets.
Before we explain these three properties, let's first introduce what the "water drop angle" is.
Water drop angle (contact angle)
The water drop angle (also known as the contact angle) uses θ as its symbol and is used to determine whether a material surface is hydrophilic or hydrophobic. When a liquid adheres to a solid surface, the angle between the liquid surface and the solid surface is called the water drop angle θ. When θ is closer to 0 degrees, it represents hydrophilicity, and when θ is closer to 180 degrees, it represents hydrophobicity. (As shown in the figure below)
In addition, we often hear the triple point, which represents the point where solid, liquid and gas are in contact. It is the starting point and forms an angle along the tangent direction of the liquid and gas interface.
Hydrophilicity, hydrophobicity and water repellency
Hydrophilicity
Hydrophilicity refers to an object's attraction and affinity to water. When water contacts a solid, it immediately adsorbs to the surface to form a thin water film. When the water drop angle is < 10 degrees, we call it superhydrophilicity, and when the water drop angle is 40 degrees, it is called hydrophilicity. In the surface treatment process, hydrophilicity is extremely important for the post-process. When the surface hydrophilicity is poor, it will affect the coating effect of the post-process (such as electroplating) on the workpiece. In severe cases, the coating will peel off. Therefore, the pre-treatment process (cleaning) is very important. Cleaning the surface of the plated part is of great help to the post-process.
Hydrophobicity
Hydrophobicity refers to the repulsion or incompatibility of a surface material with water. In this case, water can be observed in the form of water droplets on the surface. The water droplets will roll and easily slide off the surface. Usually, the angle of the water droplets is about 70 degrees. In order to avoid surface oxidation, some plated parts in the process will use chemicals to form a protective layer on the surface to prevent oxidation of the metal surface due to water vapor.
Water Repellency
Water repellency means that the material repels water more obviously, the water droplets are not easy to adhere and are more likely to slide off, and the angle of the water droplets is close to about 90 degrees. The water repellent function is commonly used on car windshields. In order to reduce the adhesion of rainwater to the glass and increase visibility when the car is driving in heavy rain, the water drop angle is greater than 90 degrees or higher.
In recent years, the topic of waterproofing has also been mentioned and tested in the 3C industry. The waterproofing function is used to prevent water from penetrating into 3C products and causing short circuits. However, the wear resistance of the treated waterproof coating is quite poor. Its waterproofness will decline significantly after friction, which is one of the reasons why it cannot be widely used at present. But I believe there will be better technology to overcome it in the future, let us wait and see!