In-situ measurement of contact angles for evaporating droplets using transmission interference fringe (TIF) technique

Recently we developed the transmission interference fringe (TIF) technique and verified it by experiment and simulation to show a good agreement. In this research, TIF technique is employed for the evaporating droplets on the plain glass substrate to show a good match compared with the contact angle by the sideview imaging technique. The contact angle is measured by measuring the fringe radius from the center to the edge ray from the dispersed beams on the screen away from the sample. The red laser (632.8 nm) is illuminated from the sample bottom at an incidence angle of 50 degrees with the transparent substrate. A total of three different volumes of droplets are used: 1, 2, and 3 ul, and the droplet sizes are close to the capillary length (~ 3 um). It shows that all droplets maintain the spherical profile with constant contact radius (CCR) mode. In TIF, contact angles can be easily detected in a simple configuration, not requiring a sophisticated microscope setup. The volume change rate is shown to match reasonably with the diffusion-limited model. TIF can effectively measure contact angles less than 1 degree in real-time, which is substantially important in ultra-small contact angle measurements in evaporation, condensation, and liquid film dynamics. However, the existing techniques are not easy. Furthermore, the top reflection interference fringe (TRIF) technique is formulated to verify that top reflection has the advantage of measuring the contact angles of droplets on the non-transparent substrates. With TRIF, it is expected that we can employ simultaneous measurement of contact angle or thin film thickness measurement from the top reflection interference fringe (TRIF) and the bottom reflection interference fringe (BRIF) or surface plasmon resonance (SPR) imaging technique for the ultra-small droplets or thin film with a few nanometer scales.