A recent study by researchers from the Chinese Academy of Sciences redefines how liquids maintain their contact with solid surfaces—also known as wettability—from an intermolecular force perspective.
The findings were published in Nano Research on Feb 8.
Wettability is relevant to the design of materials because it determines how layers stick together. Says study author and professor Ye Tian from the Key Laboratory of Bio-inspired Materials and Interfacial Science, it “plays a crucial role in many fields, such as the efficiency of catalytic reaction, separation, electrode materials, and the design of bionic smart materials.” For example, smart layers that change their contact depending on moisture could be used in sportswear that adapts to humidity.
Wettability models
Highwettability means that a liquid drop spreads, creating a low angle of contact with the surface, whereas low wettability describes a liquid that resists spreading. Classically, wettability, as indicated by contact angle, is characterized using Young’s equation, which models an ideal, perfectly smooth surface. If the water droplet spreads out to a contact angle lower than 90 degrees, the surface is categorized as hydrophilic or water-loving. If the water droplet makes a contact angle higher than 90 degrees, the surface is categorized as hydrophobic.
Wettability of a material is the ability of a liquid to maintain contact with a solid surface, and it is proportional to hydrophilicity and inversely proportional to hydrophobicity. It is one of the most important properties of a solid, and understanding the wettability of different substrates is essential for various industrial uses, such as desalination, coating agents, and water electrolytes.
So far, studies on the wettability of substrates have mainly been measured at the macroscopic level. The macroscopic measurement of wettability is typically determined by measuring the water contact angle (WCA), which is the angle a water droplet makes with respect to the surface of the substrate. However, it is currently very difficult to accurately measure what happens at the interface between a substrate and water at the molecular level.
Currently used microscopic measurement techniques, such as reflection-based infrared spectroscopy or Raman spectroscopy, are incapable of selectively observing the interfacial water molecules. Since the number of water molecules in the entire bulk of the liquid is much larger than the molecules that are making contact with the surface, the signal of interfacial water molecules is obscured by the signal of water molecules in the bulk liquid.
