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Drop Volume or Drop Weight IFT Measurements

An old technique for determining surface tension is the drop volume or drop weight method. This method weighs the mass of the liquid drop that falls off a dispense tip of known diameter when pumped very slowly. It is a very simple method but requires an empirical correction (fudge factor) for the wetted diameter of the dispense tip. Normally many drops are dispensed into a weighing boat and taken to a sensitive balance.

In the FTA version, we estimate the mass of the detaching drop by measuring its shape just before it falls off. Knowing its density, we can calculate the mass.

The drop volume method is useful when it is difficult to form a pendant drop with sufficient stability to use the Laplace-Young method. This can happen, for example, with paints which tend to skim over, and it often happens in liquid/liquid measurements when the interfacial tension is very small.

It should not be supposed that the drop volume method is as accurate as the Laplace-Young method, but it is handy for an approximate determination when other methods fail because of these difficulties. Because the pendant drop detachment process is irregular, it is best to average a number of measurements.

The drop volume method was added to 2.0 with Build 155.