Interfacial rheology of fluid-fluid interfaces at the micro to nanoscale with AFM

© 2019 Matthew Dominic BivianoThis thesis introduces and benchmarks 2 novel AFM based techniques to characterise the interfacial rheology of polymeric fluid-fluid interfaces. We develop and benchmark a time-dependent extension of capsule compression to model oscillatory indentations, determining the interfacial rheology of capsules from thin shells to thick shells. This is first tested and benchmarked using the food-based emulsifier beta-lactoglobulin at the MCT-Oil interface. This system is then crosslinked and the behavioural changes in the capsule oscillatory rheology, bicone shear rheology and pendant drop dilational rheology are tested. This investigation has shown that there is a significant frequency dependent response of beta-lactoglobulin capsules to an oscillatory indentation in both the native and the crosslinked state. We also show that there is a large indentation depth dependence on the time dependent dwell behaviour. When comparing these behaviours to the bicone shear rheology and pendant drop dilational rheology, the magnitude of the moduli at similar frequencies differs strongly between all techniques. Comparatively, the tan(delta) between the bicone and capsule measurements exhibits similar magnitudes for both the native and crosslinked cases. An extension of the capsule technique was made to include thicker interfaces, where the thickness of the interface is more than 5% of the overall capsule radius. This model was tested with the food hydrocolloid chitosan, which in acetate buffer concentrations above 0.3M makes particulates and becomes surface active, forming a thick film at the interface. We determine the properties of this film by assessing the film mechanically and visually. To mechanically characterise the film, we utilize bicone interfacial shear rheology, and both linear and oscillatory capsule compression. The film exhibits characteristic particle like behaviours and rapidly heals when broken in shear. Similar self-healing characteristics are then observed in the linear capsule compression, and the oscillatory capsule compression shows similar moduli and behaviour to that observed by the bicone. The visual characterisation was done with in-situ aqueous imaging and dry Langmuir-Blodgett imaging of a transferred film, where a thick, continuous, particulate film is observed. We continue to push the lengthscale of interfacial rheology by developing a novel method of observing the interfacial rheology at a fluid/fluid interface. This method consists of placing a cantilever tip at the interface between two fluids, laterally translating the tip and observing the torsional force on the cantilever. This force can then be transformed into an interfacial viscosity with the knowledge of the diameter of the tip at the interface. We apply this technique to two types of interfaces, linear polymeric interfaces and a globular protein interface. At the air/water interface, we test the linear polymers PEO and PSS, where we observe that shorter chain length PSS shows a reduction in the interfacial viscosity. beta-lactoglobulin is observed over time at the decane/water interface, where the interfacial viscosity increases over time, and non-linear strain dependent behaviour is observed