Development of aperture total internal reflection (A-TIR) for micro droplets and fingerprint patterns characterization

This study proposes and demonstrates a new approach using aperture total internal reflection (A-TIR) by means an aperture in front of a detector to characterize micro droplet and fingerprint patterns on different surface coatings. A microcontact printing using PDMS stamp is employed to prepare micro droplet patterns on differently coated glass surfaces with synthetic sebum liquids. An aperture in front of a detector in TIR configuration generates unique reflectance curve by blocking aberrated beams from the top curved profiles of droplets and liquid–air interfaces due to the quantum phenomenon of Goos–Hänchen (G–H) shift. The Fresnel equation is modified with a modeling to consider geometric features of droplets and G–H shift. The measurement by A-TIR is compared with the simulation based on the modified Fresnel equation to show a good agreement. A-TIR reflectance curves show that the area fraction covered by droplets is linearly proportional to the measured reflectance and the oleophobicities of the coating surfaces. For practical application, A-TIR is employed to characterize human fingerprint patterns consisting of hundreds of micro droplets to show a good agreement between the measurement and the simulation. G–H shift is experimentally verified to cause the ray aberration at the liquid–air interface of micro droplets in fingerprint patterns, resulting in the reflectance decrease and the curved profile near the critical angles. This result can be used to characterize uneven surface features for surface coating condition monitoring.