Experimental methods in chemical engineering: Contact angles

The contact angle (CA) formed at equilibrium at the three-phase line of contact between a liquid, a solid, and a gas may be expressed as a function of both the interfacial and surface tensions. Young first derived this thermodynamic relationship in 1805. In practice, multiple CA values are observed due to kinetic phenomena induced by evaporation, vapour adsorption, or swelling, and thermodynamic ones induced by roughness and surface chemical heterogeneities, even at molecular-scale. These non-ideal conditions result into an hysteresis, i.e., a difference between wetting and dewetting behaviours, and Young's equation rarely applies. Three measuring methods stand out for their applicability and reliability. In the sessile drop method, a syringe deposits a liquid drop on a flat surface and the contact angle is measured through optical means based on the drop shape. In the Wilhelmy balance method, the force required to immerse a solid plate in a bath of liquid is indirectly related to the contact angle. In the Washburn capillary rise method, the contact angle is derived from the rate at which a liquid rises by capillarity through a packed bed of powder. Employing probe liquids of various polarities, the free surface energy of the solid may be estimated. Over the last two years, similar to 8600 published articles mentioned "contact angle" in their topic. Their main focus was either to develop the fundamental understanding of wetting science, or to assess the success of surface modification methods for the production of novel surfaces, composites, and membranes with enhanced wetting, adhesive, and filtration properties.