In Part I of Measuring Surface Tension, we introduced the concept of measurement of surface tension of a liquid as an important parameter for many applications. We will continue by exploring some current applications and techniques for measuring static surface tension.
Static surface tension applications
When a new surface forms, any surface active chemicals diffuse to that surface and align. During this process, the surface tension is changing rapidly and continuously. Dynamic surface tension techniques are often appropriate to track these changes. When the process reaches equilibrium, the surface tension is static and can be monitored by the techniques described below. However, there are many applications where static techniques can be used to track surface tension changes over time. One example is the tracking of time-release chemicals, commonly employed in pharmaceuticals. Another application is to pre-determine the location-specific, temporal-specific, and dose-specific release of surface active chemicals to prepare a biological surface for the implant of devices such as stents. Static surface tension monitors can be used to study the release rates of these chemicals so that when these chemicals are employed on the implant, they are released effectively. Discrete static surface tension measurements can be taken periodically over time, thus tracking relatively slow changes in the properties of the liquid. A good analogy can be made to a time-lapse movie of a flower opening, comprised of discrete frames capturing an image at a specific time. A pure chemical or a solution in equilibrium is characterized by a single static surface tension determination.
Static surface tension techniques
The Du Noüy Ring and Wilhelmy Plate techniques are gravimetric; frequently both can be determined using a given commercial instrument. The classic measurement technique, dating from the late 1800’s, is the Du Noüy Ring technique. A precision-machined platinum/iridium ring, typically with a diameter about 2 cm, is suspended from a force measuring balance, then lowered into the liquid and gradually withdrawn (in practice, the container of liquid is raised and then lowered). As the ring is withdrawn, surface tension causes the liquid to stick to the underside of the ring. The “weight” of the ring increases due to the added weight of the adherent liquid. The maximum force increase is a measure of the surface tension. Another related method is the Wilhelmy Plate technique. A plate of metal is lowered until it just touches the surface of the liquid. In this case the weight of the liquid that “crawls” up the side (forming a meniscus) is the measure of surface tension. These techniques are considered the most precise. However, care must be taken with ring handling and storage to avoid dimensional distortion. Further, contamination of the plate can affect its wettability and therefore surface tension measurement.
The pendant drop technique measures the shape of a liquid drop suspended from a capillary needle. A liquid with high surface tension can become quite elongated before dropping off the needle. This is an optical measurement and is best done using computer control and measurement. The technique is not as precise as the force measurement methods because it depends on the eye of the operator or the sophistication of costly detection hardware and analysis software. However, several suppliers offer a system for both contact angle measurement and surface tension via the pendant drop method, potential spatial and cost advantages to those who perform both determinations.
Another static technique, the spinning drop method, is particularly suited to measuring low surface tension (down to micro-newtons/m) that might be below the limit of measurement for other techniques. In this method a drop of the liquid to be analyzed is injected into a tube containing another immiscible fluid of higher density. When the tube is spun about its long axis, the drop is forced to the center by centrifugal forces and its shape elongates; the degree of elongation is analyzed to give a measure of surface tension.
The authors acknowledge the helpful comments of Mark Coombs, Krüss USA.