Review of Contact Angle: From Advanced Surface Characterization to Liquid Assays

The contact angle of a liquid drop on a solid surface is a simple yet powerful experimental technique for characterizing the surface properties of materials. This method uses a liquid drop as a probe to investigate the chemical and physical properties of a solid surface, but it can also be used in reverse—a well-characterized surface can be employed to study the properties of a liquid. The versatility of the technique can also be extended to submerged systems, where a gas bubble serves as the probe. The contact angle is the angle measured at the solid–liquid and liquid–gas interfaces at the triple-phase point where a solid, liquid, and gas meet. This angle reflects the balance of forces at the three interfaces, and because this balance is highly sensitive to surface properties, contact angle is one of the most surface-sensitive techniques available. The interface can be analyzed from multiple perspectives: empirically through the measured angle (hydrophobic/hydrophilic), thermodynamically using principles such as the Young–Dupré equation, from a physical point of view (the Cassie–Baxter and Wenzel models) and from a chemical standpoint by considering intermolecular forces and the specific chemical nature of the substances involved. Despite its experimental simplicity, contact angle analysis can provide deep insights, ranging from a surface’s wettability (hydrophilicity or oleophobicity) to estimation of van der Waals interaction on a surface. Thanks to its ease of use, its flexibility, and a century of established physical and chemical theory, contact angle measurement has a wide range of applications. These include determining the surface tension of materials, determining the presence of a hierarchical nanostructure at the surface (superhydrophobicity), measuring the percentage of alcohol in a hydroalcoholic solution, assessing surface stability, and many other applications. This article presents a comprehensive review of the diverse applications of contact angle measurements, aiming to provide researchers with a detailed overview of its potential uses and the valuable information that can be obtained.