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.

Surface wettability is usually evaluated by the contact angle between the perimeter of a water drop and the surface. However, this single measurement is not enough for proper characterization, and the so-called advancing and receding contact angles also need to be measured. Measuring the receding contact angle can be challenging, especially for extremely hydrophobic surfaces. We demonstrate a reliable procedure by using the common needle-in-the-sessile-drop method. Generally, the contact line movement needs to be followed, and true receding movement has to be distinguished from "pseudo-movement" occurring before the receding angle is reached. Depending on the contact angle hysteresis, the initial size of the drop may need to be surprisingly large to achieve a reliable result. Although our motivation for this work was the characterization of superhydrophobic surfaces, we also show that this method works universally ranging from hydrophilic to superhydrophobic surfaces.

Awareness of instrument inaccuracies will boost the development of liquid-repellent coatings

Contact angle measurements on superhydrophobic surfaces can have uncertainties of many degrees due to difficulties in positioning the baseline. The uncertainty depends on the goniometer image resolution.

IntroductionSuperhydrophobic surfaces comprise a class of materials characterized by extreme water repellency, a low roll-off angle and self-cleaning ability.1-5 These ultimate properties are achieved by combining roughness with a low-energy surface.1-3,6-9 The composite CassieBaxter wetting state 10 is connected to superhydrophobicity through high contact angles and low roll-off angles. Industrial applications concerning superhydrophobicity are currently only slightly behind the academic state-of-the-art, which makes the field interesting from the point of view of basic research as well as applications, provided the structures are able to cope with both chemical and mechanical wear and tear. 11An interesting approach using 1D nanostructures, denoted as silicone nanofilaments (SNFs), has shown excellent properties as well as resilience toward chemical and also mechanical wear. 12,13 In addition, SNFs have been shown to retain the CassieBaxter wetting state even when subjected under pressure.9 In 2006-2007, three groups independently reported on the first successful synthesis of the 1D polysiloxane nanostructures [14][15][16] and later more studies emerged to deepen the understanding of the process parameters, 12,13,17-25 and recently, also larger tubular polysiloxane structures were introduced. 26A few growth models for the SNFs have been proposed until this day. [18][19][20]24,27,28 However, these models are challenged by our observation of hollow polysiloxane nanostructures. Gao and McCarthy 27 were the first to propose a growth mechanism for SNFs, which they synthesized in solution phase from a trimethylchlorosilane/tetrachlorosilane (TMCS/TCS) azeotrope. In their model, TCS molecules cross-linked and formed 3D structures, yet occasionally TMCS molecules terminated the growth. Finally, the lateral expansion was fully blocked and growth continued in one direction. Rollings and Veinot 18 systematically expanded their original study 16 using the atmospheric gas-phase process and introduced a growth model hypothesis. They reasoned

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