The simplest way to study the behaviour of fluid interfaces is to measure the tension that arises between them. Interfacial tension (also referred to as surface tension for the interface between a gas and a liquid) arises due to the difference in intermolecular interactions in each phase. In the bulk, these interactions are isotropic, which means that they act in the same way regardless of direction.
Different phases are composed of different molecules, so that the magnitude of intermolecular forces varies. In contrast to the bulk phases, the force felt by molecules at the interface is dependent on direction, with either stronger or weaker forces directed toward the molecules at the interface of the other phase. This ‘anisotropy’ gives rise to an energy difference over the area, equivalent to a force per unit length, which is the definition of tension. The tension can be considered as the force per unit length required to extend the interface in one direction. Accessing the surface or interfacial tension is achieved by the creation, measurement and analysis of curved menisci. The most commonly used measuring methods are:
u Force Tensiometry: Contacting the interface with a perpendicularly orientated surface of low ‘contact angle’ creates a curved interface (Fig.1). Interfacial tension can be simply calculated by using a balance to measure the mass of the liquid pulled up by the solid, which is usually in the form of a plate, rod, fibre or ring. The appropriate equations are then applied to calculate the interfacial tension. Force tensiometry is a highly accurate technique, which is quick and user friendly, but suffers the disadvantage of requiring relatively large sample volumes.
u Drop Shape Analysis: It has long been understood (Young-Laplace) that the deformation of a liquid drop in air or another liquid by gravity provides a means of accessing surface tension. The shape of the drop is described, according to Young and Laplace, by a series of three differential equations (Fig.2). The fit of the measured drop profile to the theoretical prediction must be performed numerically, and therefore requires considerable computational capacity. The relatively recent advent of automated instruments for determining drop profile can be explained by rapid improvements in computing and digital video camera technology. The advantage of automation is the possibility to perform highly accurate
time-dependent studies, not only of surface tension, but also wetting, spreading, absorption into solids and interfacial elasticity, at speeds of up to 1000 frames per second.
u Maximum Bubble Pressure: Pressing air through a capillary of known radius into a liquid gives rise to a stream of bubbles. Measuring the pressure in the capillary soon reveals that the pressure reaches a maximum when the bubble is exactly hemispherical. The pressure can be related to the surface tension according to a simple equation. The main strength of this technique is that very rapid processes can be studied. Bubble lifetimes as low as 10ms provides access to the fast adsorption kinetics of different surfactants, polymers and other surface-active substances.
u Drop Volume Tensiometry: In a similar manner to that for bubbles, a stream of liquid dropping from a capillary into air or another liquid will form drops at a particular rate, depending on surface tension, viscosity and other parameters. This method is not as rapid as maximum bubble pressure tensiometry, but is often more robust, and can provide interfacial and surface tension.
Numerous techniques exist to allow surface and interfacial tension determination. Which technique is appropriate depends on the volume of liquid(s) available, and the importance of short time scale effects. In cases where rapid kinetics is important, maximum bubble pressure is recommended.
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Dr Matthew Fielden is with KSV Instruments Ltd, Helsinki, Finland. www.ksvltd.com
