Probing the role of interfacial rheology in the relaxation behaviour between deformable oil droplets using force spectroscopy

An experimental method is presented for investigating the effect of the nature of the interface on the relaxation behaviour accompanying hydrodynamic drainage occurring between oil droplets driven together in aqueous solution. This method is based upon force spectroscopy of droplet–droplet interactions. An atomic force microscope is used to drive two droplets together to a pre-defined force and then monitor relaxation of the force between the droplets. It is suggested that the observed relaxation is controlled by the hydrodynamic drainage of the interlamellar fluid separating the droplets. Data is presented for both ionic (sodium dodecyl sulphate) and non-ionic surfactants (Tween-20), uncoated oil droplets and droplets coated with the proteins, β-casein and β-lactoglobulin. Uncoated droplets, droplets coated with surfactants and droplets coated with the protein β-casein all exhibited fast relaxation, whereas droplets coated with β-lactoglobulin exhibited markedly slower relaxation and more complex behaviour.