Measurements of the ultralow interfacial tension and surfactant film bending rigidity for micron-sized heptane droplets in AOT-NaCl aqueous solutions were performed in a microfluidic device through the analysis of thermally-driven droplet interface fluctuations. The Fourier spectrum of the stochastic droplet interface displacement was measured through bright-field video microscopy and contour analysis technique. The droplet interfacial tension together with the surfactant film bending rigidity were obtained by fitting the experimental results to the prediction of a capillary wave model. Compared to existing methods for ultralow interfacial tension measurements, this contactless non-destructive all-optical approach has several advantages, such as fast measuring, easy implementation, cost-effectiveness, reduced amount of liquids and integration into lab-on-a-chip devices.