Protocols for computing the contact angle on chemically- and physically-patterned surfaces by numerical simulation

Water is a fundamental element on Earth, in fact it occupies about 71% of the Earth's surface for this reason it is reasonable to study its properties and the resulting applications. At the same time, it is fundamental to analyze the way other materials behave in relation to water itself. Particularly useful and interesting is trying to understand the way in which water and solid surfaces interact, without forgetting the interactions between water and air. In this context, particular interest was aroused by those materials that show a self-cleaning behavior. This behavior allows to introduce the concept of hydrophilic and hydrophobic material: a material is defined hydrophobic or hydrophilic according to its ability to absorb or retain water inside or on its surface . In order to measure this property it was considered appropriate to analyze the angle that a drop of water forms in contact with the surface of the material under analysis: if the contact angle is less than 90 the material can be defined hydrophilic, consequently the materials that form an angle greater than 90 degrees on their surface can be described as hydrophobic. Also with reference to water, another category can be defined: the superhydrophobic materials, i.e. those that show an angle of contact greater or equal to 145 degrees when in contact with water . With the idea of imitating what happens in nature and therefore find materials that have a strongly hydrophobic behavior to exploit their technological applications, the factors that could cause an increase in the contact angle have been investigated. In general, the behaviour of a drop of water on a surface is influenced by a chemical aspect, i.e. the bond between the water molecule and the solid and a physical aspect influenced by various factors. One of the factors that certainly influences this aspect is the roughness of the surface and this is evident when analyzing the wetting phenomena considering the area of the surfaces involved and their energy values. In this regard, it was particularly useful to analyze the behavior of a drop on a surface that has pillars of micrometric size, in fact according to the theory of Cassie-Baxter and Wenzel the contact angle will increases due to the roughness of the surface. The purpose of the following discussion is therefore to analyze from a numerical point of view the behavior of a drop on a surface with micrometric pillars in order to understand how it evolves, this type of analysis is particularly useful since the size of the drops is in the order of picoliters. In order to evaluate the trend of the contact angle and surface tension, the behaviour of the drop in different situations will be investigated and sensitivity analyses will be carried out by varying some parametersby varying the size of the pillar, the volume of the drop. To study the evolution and the shape of the Drop Open Source Software developed by Prof. Brakke and called Surface Evolver has been used a tool based on C language able to calculate, starting from a specific initial situation, the lowest energy configuration by a gradient descent method. The results obtained were then compared with Cassie Baxter's theoretical values in order to evaluate the applicability of the numerical model, in fact it will be more suitable to describe the drop with certain volumes and angles of contact than others. In the final part some possible future devolopments and applicationsIn will be indicated .