Gel surface tension measurement via forced drop oscillation in an ultrasonic standing wave field

Many applications, particularly those in biology and medicine, require knowledge of the physical properties of soft matter such as gels. Surface tension is one of those properties and is particularly difficult to measure. Herein a non-contact ultrasonic levitation method is presented where the frequency response of a gel drop was used to measure surface tension. Gel drops were formed by injecting a drop of liquid agarose solution into an ultrasonic standing wave field. The drop was allowed to cool and set into a gel. The gel drop thus formed was subjected to forced shape mode oscillations achieved by amplitude modulating the ultrasonic standing wave. A frequency sweep was performed and the drop response, defined as the extent of drop deformation, was determined via laser intensity measurements. The fundamental frequency of the gel drop under forced oscillation was then determined. Using a dispersion relationship for elastic gels the surface tension was then calculated, using as inputs the gel shear modulus, the drop diameter, and the measured drop resonant frequency.