## 1 Introduction  
Aluminum and its alloys belong to the group of materials most frequently used in production engineering, in particular in the aircraft, marine, and automotive industries. Aluminum and its alloys owe their wide application to their good strength properties in relation to low weight, good electrical conductivity, and relatively low price [1]. In order to increase the mechanical strength and corrosion protection of aluminum alloys, oxide layers are produced on their surface, usually by anodizing. The mechanical properties of oxide layers can be changed by altering their production parameters (temperature and current conditions, anodizing time, type of electrolyte) [2]. The mechanical properties of the upper layer are significantly influenced by its energy condition, which is defined by surface free energy (SFE). SFE is determined by measuring the wettability (contact angle), whose value decreases with an increasing SFE. The nanomorphology of the oxide layer surface also significantly influences the contact angle, and consequently, the SFE value. A change in the SFE value of the upper layer, in turn, has a significant effect on its abrasiveness and liquid adhesion [3; 4]. Employing the droplet deposition method enables the wettability of oxide layer surfaces to be tested using liquids with different properties (polar and non-polar), and thus enables one to determine whether the layer is hydrophobic or hydrophilic [5]. Hydrophobic materials are in great demand today because of their wide use in industry. A major problem for the aviation industry is when water droplets deposit on power lines or on aircraft wings. Water freezes at low temperatures and the ice that settles often causes serious breakdowns or may even lead to catastrophic consequences [6; 7; 8]. The use of materials with hydrophobic layers can to a large extent solve this problem. At present, the most popular way to counteract ice accumulation is manual or mechanical (by vibration) deicing, which is quite troublesome and time-consuming [9; 10]. Hydrophobic surfaces have a tendency to repel water molecules, and thus water does not freeze on their surface when the air temperature falls below 0 \({}^{\circ}\)C. A surface is considered hydrophobic when its liquid contact angle is greater than 90\({}^{\circ}\); less than this angle is considered a hydrophilic surface, i.e., those with high surface wettability. Surfaces with contact angles greater than 150\({}^{\circ}\) are called superhydrophobic. They are surfaces with very low wettability, which very strongly repel water molecules [11; 12]. Important applications of hydrophobic films also include coating clothing (to increase water impermeability and to reduce dirt), as well as vehicle bodywork and even windows, in which it prevents the adhesion of water molecules, thereby improving visibility without the need to use window wipers [13]. Hydrophobic coatings can be produced by several methods. However, most of them require a laboratory environment. The most common method used in practice is to produce oxide layers on aluminum alloys by anodizing and then modifying the layer with suitable substances. A lower SFE value of the oxide layer results in better hydrophobic properties, but in tribological applications, a higher value of surface free energy is more desirable because it better attracts lubricating fluids [14; 15]. The following studies explain how the production parameters of oxide layers affect the surface morphology (nanoporosity) and the associated wettability of oxide layer surfaces.