Hydrophobicity refers to the physical property that a molecule (hydrophobe) and water repel each other. Generally speaking, most substances containing alkanes, oils, fats and greases are hydrophobic molecules. Hydrophobic molecules tend to be non-polar, so they are easier to dissolve in neutral and non-polar solutions (such as organic solvents). Hydrophobic molecules usually congregate into a mass in water, and when the water is on the surface of a hydrophobic solution, it will form a large contact angle and form a drop shape. Super-hydrophobic polymers generally refer to polymers with a contact angle of greater than 90° with water. Such substances have the characteristics of preventing snow, pollution, oxidation and current conduction.
Figure 1. The surface contact angle of hydrophobic materials
- Industrial field: The super-hydrophobic surface coating film of a lotus leaf with a micro-nano double microstructure can be prepared on the metal surface by traditional mechanical treatment and dispersion of nano-silica. The contact angle of the coating film is up to 173°, which can be widely used in large-scale facilities such as building exterior walls. In addition, the superhydrophobic coating film is widely used in gene transmission, microfluidics, lossless liquid transportation, anti-pollution and anti-oxidation with its unique properties.
Figure 2. Hydrophobic materials based on silane and silicate clay minerals
- Agricultural field: The rapid development of greenhouse vegetables urgently requires a large area of the hydrophobic film. Using the principle of phase separation, a general-purpose plastic can be used as a raw material to prepare a large area of a hydrophobic film with good adhesion. In addition, using a sol-gel process at room temperature, a hydrophobic surface can also be constructed with super-large molecular silicone.
- Optical field: For some optical instruments, the self-cleaning function is particularly important, so a lot of researches has emerged on super-hydrophobic, non-reflective or highly reflective super-hydrophobic surfaces. In order to obtain the light transmittance of the surface, the particles constituting the rough surface structure must be smaller than the wavelength of visible light. Some researchers have prepared a super-reflective silver mirror surface with super-hydrophobic properties by controlling the silver mirror reaction, making it possible to prepare a super-hydrophobic base surface while ensuring high reflectivity.
- Biomedical field: In the biological field, bioadhesion on the surface of the substrate is a complex phenomenon, which includes many different interactions between organic matter and the interface. There are some researchers immersed the prepared hydrophilic/superhydrophobic surface in the protein solution, and the superhydrophobic part formed an air layer. This "separation wall" prevents the cell from contacting the surface, forming a separation area, and living cells can grow freely on the hydrophilic surface. In clinical treatment, the hydrophobic surface exhibits anti-cell adhesion properties. For example, a partially fluorinated and biocompatible polyurethane surface is used to test the adhesion to platelets. Experiments have shown that compared with the strong adhesion of ordinary polyurethane surface to platelets, the surface with super-hydrophobic properties has almost no adhesion to platelets.
- Life field: The hydrophobic fabric not only requires high water repellency, but also ensures the principle of non-toxicity and comfort. Therefore, super-hydrophobic fabrics can be woven into fabrics, for example, block polymers formed from polycaprolactone, styrene and dimethylsiloxane can be made into fibers. Because of their excellent water repellency and low rolling angle, they are the preferred spare materials for textiles with hydrophobic and self-cleaning functions.
Figure 3. Preparation process and morphology of hydrophobic and durable polyionic liquid-based nanofiber membranes.
- Wang S T, Feng L, Jiang L.(2006) "One-step solution-immersion process for the fabrication of stable bionic superhydrophobic surfaces." Adv. Mater. 18(6):767-770.
- Zhang X Y,Li Z,Liu K S,et al(2013). "Bioinspired multifunctional foam with self-cleaning and oil/water separation." Adv.Funct. Mater. 23(22):2881-2886.