Wetting, the process of water interacting with a surface, is critical in our everyday lives and in many biological and technological systems. The contact angle is the angle at the interface where water, air and solid meet, and its value is a measure of how likely the surface is to be wetted by the water. Low contact-angle values demonstrate a tendency of the water to spread and adhere to the surface, whereas high contact-angle values show the surface’s tendency to repel water. The most common method for surface-wetting characterization is sessile-drop goniometry, due to its simplicity. The method determines the contact angle from the shape of the droplet and can be applied to a wide variety of materials, from biological surfaces to polymers, metals, ceramics, minerals and so on. The apparent simplicity of the method is misleading, however, and obtaining meaningful results requires minimization of random and systematic errors. This article provides a protocol for performing reliable and reproducible measurements of the advancing contact angle (ACA) and the receding contact angle (RCA) by slowly increasing and reducing the volume of a probe drop, respectively. One pair of ACA and RCA measurements takes ~15–20 min to complete, whereas the whole protocol with repeat measurements may take ~1–2 h. This protocol focuses on using water as a probe liquid, and advice is given on how it can be modified for the use of other probe liquids.
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Reliable measurement of the receding contact angle: https://doi.org/10.1021/la400009m
Guidelines to measurements of reproducible contact angles using a sessile-drop technique: https://doi.org/10.1680/si.13.00010
Characterization of super liquid-repellent surfaces: https://doi.org/10.1016/j.cocis.2014.04.009
The protocol is the result of a procedure gradually developed since wetting research was begun in the R.H.A.R. group, and we acknowledge the contributions of H. Mertaniemi, T. Verho and M. Latikka. This work was supported by the European Research Council ERC-2016-CoG (725513-SuperRepel) and the Academy of Finland (Centres of Excellence Programme 2014–2019). X.T. is grateful for the support of the One Hundred Talents Program of SYSU and the One Thousand Youth Talents Program of China.
The minimum advancing drop volumes (= the recommended starting volume for the receding contact-angle measurement) needed to reach RCA at 10 µL (Fig. 6), were obtained by solving the Young–Laplace equation by numerical integration, using Python programming language with Scipy library. The shape and size of the drop for different pairs of contact angle and drop volume were solved using a code that is provided in this supplementary file