Determining micro droplet profiles using reflection interference fringe (RIF) technique*

We show that the reflection interference fringe (RIF) technique can successfully determine the micro/macro droplet profiles such as the contact angle, the height, and the dual-profiles by simply measuring the reflected interference fringe formed on the screen away from the sessile droplet. The geometric modeling and the ray-tracing are conducted to show that the interference fringes are formed on the screen The measured droplets have low contact angles from 1 to 15 degrees with the height less than 20 microns and the droplet radius less than 200 microns. The ray-tracing simulation shows that the formed interference is caused by the interaction of the reflected laser beams from the convex spherical profile in the center and the concave hyperbolic profile in the contact line. RIF technique can effectively verify the existence of a dual profile of the microdroplets in a simple setup, not requiring a microscope. It shows a good agreement between the experiments and the ray-tracing simulation in the interference fringe profiles, such as the fringes numbers, the fringe location, the fringe radius (FR). The simulation shows that most droplets have the inflection points close to the edge within 5 % of the droplet radius and the precursor film thickness distribution from a few to ~100 nm. The optical path difference (OPD) is determined analytically as a function of the fringe numbers and the incidence angle, like the OPD in Fizeau interference. The droplet height is determined by counting the fringe numbers in the OPD relation, showing a linear dependency on the fringe numbers and an excellent agreement with the experimental measurements. An analytic expression is obtained between the droplet apparent contact angle and the interference fringe radius to show a good agreement. This research is expected to provide a new insight to understand the microdroplet dynamics and evaporation-induced liquid film formation phenomenon.