Mixtures of liquids that phase separate or partially de-mix are ubiquitous. Examples include mixtures that completely de-mix, such as oil and water, as well as partially de-mixed systems, in particular mixtures of colloids and polymers that form liquid-liquid interfaces of ultra-low surface tension. The boundary or interface that separates the de-mixed phases plays an important role in determining droplet size, emulsions stabilisation and flow-driven deformation of the droplets. We are working to develop OT as a technique to measure ultra-low surface tensions that cannot be measured easily by other conventional methods. We are also using OT to measure the micro-rheology at and near liquid interfaces.