Optical Tweezers Tensiometry

Abstract

The interfacial tension is a fundamental property of liquid-liquid systems and governs the mixing potential of two immiscible fluids. For two fluids to form a stable mixture, their interfacial tension needs to be very low. Applications of systems with a low interfacial tension find a plethora of applications: for example, in the food industry, for generating oil-water emulsions, or in the pharmaceutical industry, where characterising the mixture is important for the drug delivery. Despite the numerous industrial applications, characterising low interfacial tension remains a challenge. Traditional direct force measurement techniques, such as the Wilhelmy Plate method, have difficulties resolving the small forces involved. Other conventional drop distortion techniques, such as spinning drop tensiometry, require long calibration time and can have error margins up to 60%. This thesis investigates the potential of utilising the optical tweezers for studying low interfacial tensions and developed a method to do so. In this set-up, a highly focused laser beam holds a micron-sized sphere: similar to a pair of tweezers. As light refracts through the bead, momentum is transfered and thus exerts a force. The light refraction is also used for position detection, allowing for a spatial resolution of 0.1-2 nm. As a micro-sphere, which is hold by the optical tweezers, is pushed against an interface, the forces exerted on the bead are probed allowing for the interfacial tension to be determined. Combining the excellent spatial resolution of the optical tweezers with fitting a minimal surface model, the set-up has the potential to measure the interfacial tension over a range of decades. We have researched the dodecane-water interface, where we added glycerol to the aqueous phase to match the index of refraction. The interfacial tension was lowered by adding Span-80 or the combination of Span-80 and SDS. We have been able to conduct several measurements on interfaces with estimated values of O(-6) - O(-3) N/m. We find that the system is highly dependent on the different in index of refraction between the two fluids as well as the curvature of the interface. However, discrepancies between literature values and measurements arise due to the altering of the fluid properties and interfacial dimensions. We conclude that the use of optical tweezers to measure low interfacial tension is promising, however its speed and accuracy are reduced due to the strong requirement to match the index of refraction.