Development of aperture total internal reflection (A-TIR) for droplet characterization with 3-D ray tracing and modified Fresnel equation
This study proposes and demonstrates a new approach using aperture total internal reflection (A-TIR) by means of various apertures in front of a detector to characterize micro/macro droplet. Pure liquid is used to make micro and macro scale droplets. An aperture in front of a detector in TIR configuration generates unique reflectance curve by filtering the amount of aberrated beams from the top curved profiles of droplets. A scheme of three-dimensional (3-D) ray tracing is developed for the reflected beam profile from the curved surface of the droplet with the modified Fresnel modeling to show a good agreement with the measurement. The modified Fresnel modeling is proposed to consider the morphological features of the droplet such as the thickness, the diameter, the surface coverage fraction, the effective flatness ratio, and the quantum phenomenon of Goos-Hänchen (G-H) shift effect. Various sizes of apertures are employed to demonstrate the A-TIR reflectance dependence on the aperture sizes for the macro and micro-sized droplets with a good agreement between the experiment and the simulation. and 3-D ray tracing with the modified Fresnel equation shows good agreement. Furthermore, it is demonstrated that one of the morphological features of the droplet, the thickness can be successfully determined with a reasonable agreement with the measurement. This outcome can be used to determine the morphological features of droplets such as the thickness, the diameter, and the coverage fraction and to characterize uneven surface features like human fingerprinting.