Interference Fringe (IF) Technique for Droplet Contact Angle and Thickness Measurements: nanoscale thickness and ultra-low contact angle measurement

Recently, reflection interference fringe (RIF) and transmission fringe (TIF) techniques have been introduced to investigate the origin of far-field interference fringe formation and to determine a droplet’s contact angle and thickness by measuring the fringe radius. In this study, characteristics of the interference fringe (IF) technique are analyzed based on the RIF and TIF, by varying the schematics, such as configuration (transmission/reflection), the droplet’s side (left-hand side/right-hand side), and the substrate types (flat/prism). The analysis also investigates the refraction effect at the droplet edge and the maximum incidence and contact angles. The schematic variation shows that the widest contact angle range can be measured in a transmission configuration with droplet’s right-hand side, and that the fringe radius decreases with incidence angles on a prism substrate, consistent with recent observation. Refraction at the droplet edge causes the fringe radius to increase or decrease depending on degree of refraction. Based on this characteristics study, it is revealed that the IF technique can determine nanometer-scale thicknesses below 100 nm on droplets, corresponding to ultra-small contact angles of less than 0.01°, with an extended working distance of 3,000 mm and an optimized incidence angle, assuming a spherical profile. This finding is significant, as it demonstrates that nanoscale thickness can be determined in-situ under ambient conditions using a simple optical configuration, without requiring a sophisticated setup such as a microscope. It is anticipated that the IF technique can be combined with other nanoscale thickness measurement techniques to enhance its measurement reliability.