Understanding interfaces: Using contact angle measurements to determine surface tension, interfacial tension, and kinetic properties from contact angle hysteresis

When there is an interface between a liquid and a solid, the angle between the surface of the liquid and the baseline of the contact surface is described as the contact angle. The contact angle is a measure of the wettability of a solid by a liquid. Measurement of the contact angle helps in all situations where solids and liquids meet and there is benefit gained by control of wetting and adhesion properties. Applications where contact angle values are especially important include development of hydrophobic or hydrophilic surface coatings, paints and varnishes, cleaning agents, printing processes, and more. Contact angle goniometers are high-precision video camera-based devices which perform the optical analysis of the shape of liquid drops placed on a solid surface (sessile drop method) or the shape of drops that hang down from a dosing needle (pendant drop method). The drop shape helps in determination of different surface and interfacial parameters such as surface tension of a liquid, interfacial tension between two liquids, and surface energy of solid substrates. Contact angle hysteresis is an important physical phenomenon arising from chemical inhomogeneities, roughness, or impurities on a surface which can affect how a liquid droplet spreads across a surface. Contact angle hysteresis is the difference between advancing and receding contact angles and can be measured using the “needle-in method” or by tilting the actual contact angle goniometer or sample stage itself, allowing the droplet to roll across the surface of the substrate. This paper will describe recent advances in contact angle goniometry and will explain how surface tension, interfacial tension, and contact angle hysteresis measurements can be easily and accurately made using a modern contact angle goniometer and software.