Surface plasmon resonance imaging of drop coalescence at high-temporal resolution

Surface plasmon resonance imaging (SPRi) is an inexpensive label-free technique that is sensitive to the change of the refractive index at the solid–fluid interface. The mainstream application of SPRi in thermal and fluid science is associated with visualization of low-frequency interfacial phenomena. In this work, SPRi is employed as a means to visualize interfacial phenomena with high-frequency features of coalescence during dropwise condensation. The corresponding image processing method to detect materials on the solid surface is demonstrated. Two methods of increasing temporal resolutions are implemented to record the three steps of a coalescence evolution — bridge formation, composite peanut shape droplet formation, and the relaxation stage — at 10,000 frames per second (fps). The first method focuses on shortening the illumination path using a biconvex lens with a short focal length as a collimator. The second technique increases the optical power of an incident light by broadening a bandpass filter. The broadening increases the temporal resolution, but at the expense of reducing sensitivity in the SPR system. In our SPRi instrument, at 523 nm, broadening the bandpass filter from full width at half maximum (FWHM) of 10 nm to FWHM of 60 nm increases the temporal resolution by 233% but decreases the sensitivity by 14.5%.