Principle of Hydrophobicity: Comparison
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Hydrophobic thin films have attracted significant attention in both basic research and practical applications due to their unique properties. These thin films have undergone extensive study, and numerous efforts have been made to broaden their application fields to include areas such as oil hydrophobicity, hydrophobic anti-icing, and hydrophobic anti-corrosion. Additionally, fresh studies that explore approximate theories and fabrication techniques for hydrophobic thin films have emerged.

  • hydrophobic
  • wettability
  • magnetron sputtering
  • surfaces

1. Introduction

Hydrophobic thin films have attracted significant attention in both basic research and practical applications due to their unique properties [1][2][3][4][5][6][7][8][9][10]. Over the past few decades, these thin films have undergone extensive study, and numerous efforts have been made to broaden their application fields to include areas such as oil hydrophobicity [11][12], hydrophobic anti-icing [13], and hydrophobic anti-corrosion [14][15]. Additionally, fresh studies that explore approximate theories and fabrication techniques for hydrophobic thin films have emerged [16][17][18][19].
Hydrophobic surfaces have great potential in a number of industries and biomedical fields, among others. The protection of electrical wires and antennas from snowfall, autos with self-cleaning windows, ships with anti-corrosion coatings, metal refining, building glasses with dust-free coatings, the separation of oil and water, and textiles resistant to stains are a few examples that come to mind.
Building a micro–nano rough structure on hydrophobic substrates or chemically altering a hierarchically structured surface with a low-surface-energy material are the two main methods for creating hydrophobic thin films. Many preparation methods have been reported in the literature, including phase separation [20], plasma methods [21][22], chemical vapor deposition (CVD) [23][24], physical vapor deposition (PVD) [25], sol-gel processing [26][27], and others. However, these techniques differ in terms of their efficiency, cost, simplicity of use, and requirements for specialized reagents. Some techniques are also restricted to basic laboratory experiments, and much work still needs to be performed to prepare hydrophobic films on a commercial scale. Therefore, researchers are working to make these films easier to prepare, cheaper, more durable, and more functional.
PVD is a frequently used technique for the deposition of thin films on a substrate. PVD involves the transfer of material from a source, typically in the form of a solid or liquid, to a substrate under vacuum conditions. There are several methods of PVD deposition, including sputtering [28][29], evaporation [30][31], and pulsed laser deposition (PLD).
Magnetron sputtering is a widely used PVD method for producing hydrophobic thin films. While the creation of small-area films using magnetron sputtering has been well established, the fabrication of large-area films poses unique challenges. Achieving uniformity and precise control of deposition parameters over a large area is inherently more difficult due to factors such as edge effects, variations in gas flow, and substrate curvature. In the context of magnetron sputtering, the feasibility of preparing large-area films arises from several distinctive features of the technique [32][33][34][35]. Moreover, the precise control of deposition parameters, such as gas pressure, target-to-substrate distance, and power density, plays a crucial role in achieving uniformity and high-quality films over large areas. So, this method stands out for its feasibility of preparing large-area films and is widely used in the industry.
Some other exceptional qualities, such as capacity for mass production, environmental friendliness, low cost, and powerful adhesion between film and substrate, also draw much attention. A high voltage is applied across the target material, creating a high-energy plasma that causes atoms or ions to be ejected from the surface of the target. To achieve uniform film deposition, these particles are then deposited onto the rotating substrate in a high-vacuum deposition chamber. However, a thorough analysis of magnetron-sputtered hydrophobic thin films is still lacking.
Hydrophobic films may be affected by environmental factors (such as ultraviolet rays, high temperatures, humidity, etc.) and physical or chemical effects (such as friction, corrosion, etc.) during long-term use, resulting in a decrease in or failure of their hydrophobic ability. Hydrophobic films have a number of significant characteristics that make them suitable for a variety of real-world uses. First, hydrophobic durability describes the thin film’s capacity to retain its hydrophobic qualities over time, even after exposure to environmental factors. In many applications, particularly those where water or moisture can cause damage or impair performance, hydrophobic durability is crucial. For instance, hydrophobic thin films can extend the lifespan and increase the reliability of sensitive electronics components by shielding them from water damage. Second, the ability to withstand exposure to harsh environments and to a variety of chemicals, including acids, bases, and solvents, is the second requirement for chemical stability in hydrophobic films. Additionally, they ought to be strong enough to withstand everyday wear and tear, as well as exposure to harsh temperatures and UV rays. Third, the hydrophobic film needs to be transparent in some applications, such as optics or electronics. Finally, biocompatibility, abrasion resistance, etc., are also significant in some other applications.

2. Principle of Hydrophobicity

2.1. Nature Inspiration

Numerous distinctive natural surfaces, such as lotus leaves, butterfly wings, cicada wings, and rose petals, offer fresh concepts for human designers of hydrophobic thin films. The lotus leaf primarily displays excellent superhydrophobicity; when it rains, water beads form on the leaves. The water beads roll away from the leaves as long as they are slightly tilted. The “Lotus Effect,” also known as the self-cleaning effect, was first discovered in the 1970s by a group of German botanical classification scientists led by W. Bartroot. ThereFigure 1 are shows some natural hydrophobic surfaces for a better understanding. Two popular techniques for producing the “Lotus Effect” on surfaces are nanoimprinting and anodic aluminum oxide techniques. With the help of pressure and heat, a mold having nano-scaled patterns is replicated onto a polymer substrate using the nanoimprinting technique. With this method, a surface topography that resembles the roughness of a lotus leaf is produced. The fabrication of the nano mold can be performed in a number of ways, but the most popular way to create a superhydrophobic surface is to copy a natural leaf or use deep reactive-ion etching on a silicon or silicon oxide substrate [36]. On the other hand, the anodic aluminum oxidation technique is a method that uses anodized templates as molds to grow a highly ordered nanotube structure on an aluminum surface. The surface of the resulting structure is hydrophobic due to its surface roughness [37]. The “Lotus Effect” is a remarkable phenomenon that displays superhydrophobicity. This effect has inspired researchers to develop hydrophobic thin films that exhibit self-cleaning properties [38][39][40][41][42].
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Figure 1. Natural hydrophobic surfaces. (a) Water beads formed on a lotus leaf shows the “Lotus Effect”. Reprinted with permission from ref. [7]. Copyright 2015 American Chemical Society. (b) Scanning electron microscope (SEM) image of the lower surface of the lotus leaf. Reprinted with permission from ref. [38]. Copyright 2002 Wiley. (c) Water strider legs show superhydrophobicity. Reproduced with permission from ref. [39]. Copyright 2006 Wiley. (d) SEM images of a water strider leg showing numerous oriented spindly microsetae. Reprinted with permission from ref. [40]. Copyright 2004 Nature Pub-lishing Group. (e,f) SEM images of the surface of a red rose petal, showing a periodic array of micropapillae and nanofolds on each papillae top. Reproduced with permission from ref. [41]. Copyright 2008 American Chemical Society. (g,h) Water droplets with spherical shape pinned on the irregular surface of peanut leaves and SEM image shows the top of nanoslices are covered with nanostructured papillae. Reproduced with per-mission from ref. [42]. Copyright 2013 Wiley.
As a result, when creating a superhydrophobic thin film, a micro–nano rough structure and lower the surface energy of the substance is typically created, because it has been discovered that a nano multilayer structure and low-surface-energy wax together form surface superhydrophobic properties. The main reason why water drops on surfaces is due to high-energy molecules on the surface that have a strong affinity for the liquid, as illustrated in Figure 12, depicting different wetting behaviors with different contact angles. To fully comprehend how liquids adhere to solid surfaces, the contact angle must be accurately measured. The sessile drop method, the Wilhelmy plate method, the pendant drop method, and axisymmetric drop shape analysis (ADSA) are some of the techniques that have been developed to measure the contact angle. In the sessile drop method, an optical instrument, such as a goniometer, is used to measure the contact angle between a small droplet of the liquid and the solid surface. By measuring the angle between the liquid droplet’s tangent line and the solid surface at the three-phase contact point (where liquid, solid, and air meet), the contact angle can be determined. In the Wilhelmy plate method, a solid plate or rod is submerged in the liquid, and the force necessary to separate the liquid from the surface is measured. The contact angle is determined using the pendant drop method, which suspends a droplet from a needle or capillary tube. The angle between the droplet’s tangent line and the needle or capillary tube is then calculated. The shape of a suspended droplet is lastly captured with ADSA using a high-resolution camera, which is then examined using mathematical models to determine the contact angle. Using a contact angle analyzer, the water contact angles of the untreated and treated substrates can be measured to determine whether the treated substrate is hydrophilic or hydrophobic [43].
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Figure 12. Wetting behavior of a liquid droplet on solid substrates with different contact angles. (a) superhydrophilic: θ < 5° in 0.5 s. (b) Hydrophilic: θ < 90°. (c) Hydrophobic: θ = 90°~150°. (d) Superhydrophobic: θ = 150~180°.
Based on the contact angle, surface wettability is also divided into four categories: superhydrophilic (completely wetting the surface at 0 degrees), hydrophilic (partially wetting the surface at 90 degrees), hydrophobic (wetting the surface partially at 90 degrees but not fully at 150 degrees), and superhydrophobic (completely wetting the surface at >150 degrees). Superhydrophobic surfaces (SHSs) is the term used for these substances. Since the lotus leaf, which is mentioned above, is a type of SHS in nature, the droplets simply roll on the surface without any wetting effect.

2.2. Young Equation

In 1805, Young was the first to propose the concept of contact angle to describe surface wettability, and he was the pioneer of research on wetting [44]. A liquid rests in a droplet on an ideal flat surface, as shown in Figure 23a. The surface free energy of a solid surface is expressed by the Young equation: γ s v = γ s l + γ l v cos ⁡ θ Y where γsv, γsl, and γlv stand for the interfacial energy values for solid–vapor, solid-liquid, and liquid–vapor, respectively, and θY is the contact angle in the Young model. The Young angle is the result of the thermodynamic equilibrium of the surface free energy at the solid–liquid–vapor interface.

2.3. Wenzel Model

However, there are not many perfect flat surfaces in the natural world. Wenzel developed the following equation in 1936 to establish a relationship between the macroscopic roughness of a solid surface and the contact angle, explaining how surface roughness increases hydrophobicity [45]: cos ⁡ θ W = r cos ⁡ θ Y where θW is the apparent contact angle in the Wenzel model, r is the factor of surface roughness, and θY is the contact angle in the Young model. According to Wenzel’s theory, when a liquid comes into contact with a rough surface, it completely fills the voids and grooves of the surface, as seen in Figure 23b; as a result, the static contact angle is decreased, and the sliding angle is increased. Wenzel’s theory states that as roughness increases, hydrophilic surfaces become more wettable, while hydrophobic surfaces become less wettable.

2.4. Cassie–Baxter Model

Cassie and Baxter expanded this theory to include rough and porous surfaces in 1944 [46]. It specifies that there are air pockets among the rough grooves and that the rough surface is inherently uneven. As seen in Figure 23c, water droplets adhere to the surface rather than penetrating it. The Cassie–Baxter equation is given by cos ⁡ θ C B = f 1 cos ⁡ θ 1 + f 2 cos ⁡ θ 2 where θW is the apparent contact angle in the Wenzel model, r is the factor of surface roughness, and θY is the contact angle in the Young model. According to Wenzel’s theory, when a liquid comes into contact with a rough surface, it completely fills the voids and grooves of the surface, as seen in Figure 23b; as a result, the static contact angle is decreased, and the sliding angle is increased. Wenzel’s theory states that as roughness increases, hydrophilic surfaces become more wettable, while hydrophobic surfaces become less wettable.

2.4. Cassie–Baxter Model

Cassie and Baxter expanded this theory to include rough and porous surfaces in 1944 [46]. It specifies that there are air pockets among the rough grooves and that the rough surface is inherently uneven. As seen in Figure 23c, water droplets adhere to the surface rather than penetrating it. The Cassie–Baxter equation is given by cos ⁡ θ C B = f s cos ⁡ θ s + 1 − 1 where fs is the solid fraction, which means that a fraction of the solid surface is wetted by the liquid. The relationship between contact angle and surface roughness can be successfully explained by the Wenzel model as well as the Cassie–Baxter model. However, it is important to note that roughness and surface material chemistry should work together on different scales to produce hydrophobicity. Hydrophobic surface preparation, characterization, and application have recently received a lot of attention.
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Figure 23. (a) Young model. (b) Wenzel model. (c) Cassie model.

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