Interferometric scattering microscopy for studying dynamics of soft materials

Many soft materials show heterogeneous dynamics in three dimensions (3D), which are difficult to study with traditional microscopy techniques. Interferometric scattering microscopy (iSCAT) is a label-free technique with nanoscale sensitivity that can image systems in 3D with greater dynamic range than techniques like confocal laser scanning microscopy or atomic force microscopy. iSCAT has previously been used to make precise single-particle measurements in biophysical studies. We extend the capabilities of iSCAT to study the dynamics of films. We use a holographic reconstruction technique to extract the scattered field, and we translate phase differences to variations in film height. We show that the technique produces accurate topography and can image changes in topography over a wide range of timescales. We then show results from studies of how surfactant solutions break down phase-separated grease films, which give insight into the breakdown mechanisms.