tensiometry

Tensiometry is the measurement of surface and interfacial tension using instruments called tensiometers. It encompasses static methods that determine equilibrium tensions at liquid–gas or liquid–liquid interfaces and dynamic methods that track how tension changes with time, deformation, or flow. Surface tension is the force per unit length at an interface that tends to minimize surface area, while interfacial tension occurs between immiscible liquids.

Among the most common techniques are the Wilhelmy plate method, the Du Noüy ring method, the capillary rise (capillary suction) method, pendant drop, and maximum bubble pressure tensiometry. In the Wilhelmy plate approach, a plate of known perimeter contacts the liquid and the measured force provides the surface tension, often assuming complete wetting. The Du Noüy ring method uses a platinum or other ring pulled from the liquid surface; the maximum force required relates to the surface tension. Capillary rise measures the height a liquid climbs in a capillary to balance gravity and surface forces. The pendant drop method analyzes the droplets’ shape in a capillary or on a needle via the Young–Laplace equation to extract tension. Maximum bubble pressure records the pressure needed to form a bubble through a submerged capillary.

Applications include characterizing surfactants, detergents, emulsions, foams, polymers, coatings, cosmetics, and food products; quality control in manufacturing; and research in colloid and interface science. In biology and medicine, tensiometry has been used to study lung surfactants and cell–surface interactions.

Accurate measurements require clean apparatus, controlled temperature, and proper sample handling, as surface tension is sensitive to contamination, temperature, and time. Dynamic tensiometry also reveals adsorption kinetics and interfacial rheology.