Introduction to Tensiometry
An inherent property of every liquid is its surface tension, which describes the tendency of a droplet of the liquid — in contact with a gas, a second immiscible liquid, or a non-absorptive solid — to minimize its surface area. Surface tension can thus be considered as an inverse measure of the ability of a liquid to wet a solid: Higher surface tension liquids are more resistant to wetting, and produce a smaller (and taller) bead of liquid than do low surface tension liquids. Similary, lower surface tension liquids will tend to absorb into membranes or fibers more readily.
It is clear that tensiometry (the measurement of liquid surface tension) is critically important for a myriad of manufacturing processes and research endeavors. Typical applications include formulation of paints, coatings, and inks; process control in chrome, nickel, and brass plating, as well as in the wet end of paper and textiles manufacturing, petrochemical refining, and processing of dairy and other liquid consumables; monitoring of industrial and civil wastewater treatment; evaluation of surfactants, soaps and detergents; analysis of emulsions and colloidal suspensions; testing of transformer oils and other coolants; and more.
Over time, a multitude of techniques have been developed to characterize liquid surface tension. These include capillary miniscus measurement, advancing and receding contact angle measurements of an immersed Wilhelmy plate, maximum captive bubble pressure, pendant and sessile drop analysis, the spinning drop method, and the drop volume method.
These are all viable techniques, but the ring method, originally developed by DuNouy nearly a century ago, remains the simplest and most widely employed technique. It offers the advantages of low capital cost and excellent reproducibility, while allowing remarkably quick results. The ring method can be used to determine either surface tension or the interfacial tension between two immiscible liquids by measuring the maximum force required to release a precision-crafted ring from the liquid/gas or liquid/liquid interface.
The DuNouy tensiometer features a vernier scale which directly displays in mJ/m2 (equivalent to dynes/cm) the amount of force applied at the time the liquid film breaks free from the ring as it is withdrawn from the surface. The displayed reading will need to be corrected for the actual values of the radius of the ring, the radius of the wire, and the densities of the lower and upper phases that comprise the interface (liquid and gas for surface tensiometry, where the density of the gas phase can essentially be ignored; liquid and liquid for interfacial tensiometry). Extensive details on this correction factor, as well as other topics concerning DuNouy tensiometers, are available here. Our suggested test procedure for liquid surface tension testing is shown here.
Russell E. Smith, President
October 19, 2021