Direct Surface Tension Measurements
December 9, 2005
Chemir Analytical Services - MI can now measure liquid surface tension directly using the pendant drop method. In this method, the fluid to be tested is loaded in a syringe, and the syringe is placed into our Dynamic Contact Angle Analyzer. The test fluid is dispensed until a drop forms, detaches itself, and falls to a glass slide below the needle. A high-speed digital camera records the event from the moment the drop begins to form to the moment it falls onto the slide. The video record of the event is then reversed until a frame containing a properly shaped pendant drop is shown. Drop-shape analysis software is used to analyze the digital image and determine the shape of the drop. The two main forces that determine the shape of the drop at that moment are gravity and the surface tension of the liquid. Since gravity is a constant, the shape of the drop can be used to determine the surface tension of the liquid.
Pendent drop is the preferred method for measuring liquid surface tension because the axial symmetry of the drop can be safely assumed, and because the experimenter need not worry about errors caused by surface conditions or contamination on the substrate or test apparatus. The pendant drop method can be used on a wide range of liquids from pure solvents to pigmented paints, coatings, and printing inks.