Tidy, Nicholas James; (2019) The Investigation of the Properties of Soft Matter using Optical Traps; an Investigation into Microemulsions using Optical Tweezers. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

We investigated the properties of a microemulsion of Heptane water and bis(2-ehtylhexyl) sulfosuccinate also known as Aerosol-OT (AOT). To achieve this two types of optical traps were created; Dual beam optical trap (DBOT) and Optical tweezers. We constructed two successful DBOT’s, the first was 3D printed and to our knowledge is the first demonstration of a successful 3D printed DBOT. The second used laser etching to etch the features into the microscope slide. This proved a more successful design as it reduced fluid flow. Both set-ups trapped 4 µm polystyrene particles for a period of tens of minutes using 1064 nm light. // The bending rigidity of the droplet boundary (κ) and its interfacial tension (γ) were found by modelling the boundary thermal fluctuations. We recorded values similar to those previously published. // Optical tweezers were used to investigate the radius of the droplet as a function of trapping power. A dimensionless quantity Nd describes the shape of the droplet. Below the value Nd = 1 the droplet has an ellipsoid shape and above that value a dumbbell shape. Using an 850 nm diode laser with a maximum power 3.6 mW we saw the droplet shape approach values of Nd = 1 as predicted in the theory. Using an Nd:YAG 1064 nm laser with a maximum trap power 28 mW we found the energy from the optical tweezer was absorbed by the microemulsion, resulting in it reaching its phase inversion temperature (PIT). The power required to reach a given PIT scaled positively with the PIT. To our knowledge it is the first time this behaviour has been observed. // Finally we measured γ using a relaxation technique. We trapped the droplet in an optical tweezer and then turned the tweezer on and off periodically. From the relaxation time the droplet interfacial tension can be determined. This value was one order of magnitude lower than previously recorded. It is thought the number of modes used to measure γ plays a large role on the value obtained. Using the relaxation method only one mode is used and therefore a value of γ close to its equilibrium value can be obtained.

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