Noncontacting Techniques for Measuring the Surface Tension of Liquids
Although there is a variety of surface tension measurement techniques, most of them fail to operate in environments like open ocean, besides being contacting which means the measuring devices come into contact with the liquid surface. A reliable measurement requires utmost attention to prevent cross contaminations from the measuring device surfaces to the liquid under test. This aspect of the existing techniques make them tedious and time consuming. In this dissertation, we present novel, in-situ techniques that provide fast and accurate measurements of surface tension of liquids. In the first technique, a focused ultrasonic transducer is used to excite an acoustic pulse whose radiation pressure lifts the surface of the water. With a confocal optical microscope we measure the time that takes for the water mound to form under acoustic Langevin radiation pressure. This measurement is a sensitive indicator of the surface tension. The second technique is based on the same configuration employed in the first technique. As the liquid surface is lifted by the Langevin radiation pressure, transient capillary waves propagate radially outward from the focus of the transducer. The confocal optical microscope detects the motion of the surface at any distance away from the excitation point. From the measured capillary wave amplitude we evaluate the surface tension of the water in the presence of surfactants. The last technique consists of using a rotating glass drum to collect a top layer of water with a thickness in the range of 45-150 um, and to affix ultrasonic transducers on the inside of the drum to measure the thickness and viscosity of the water film. It is particularly suited for open ocean operation by its merits of rugged design and its incorporation into a standard microlayer sampler employed in the ocean. The thickness of the film is related to the surface tension and viscosity of the liquid, and the rotation speed of the drum. Thus, we evaluate the surface tension of the film.
