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 threedimensional (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.
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 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 depending on the aperture sizes for the macro and micro-sized droplets with a good agreement between the experiment and the simulation. Furthermore, it is demonstrated that important morphological features of the droplet such as the thickness (micrometer-range) and the ultra small contact angle less than 6 degree can be successfully determined with a reasonable agreement with the measurement. This outcome can be used to measure the morphological features of droplets and to characterize uneven surface features like human fingerprinting.
Recently, aperture total internal reflection (A-TIR) was proposed to characterize the microdroplet patterns, such as the coverage fraction of the droplet, by placing the aperture just in front of the detector in classical total internal reflection (TIR). However, the reflection from the curved liquid-air interface was simulated using simple two-dimensional modeling, causing inaccuracy in A-TIR measurement. In addition, the reflectance dependency on the aperture size and the working distance of the aperture was not investigated, hindering its applications. In this study, the simulation based on three-dimensional (3-D) ray tracing with Fresnel equation modeling was successfully developed and verified to explain the internal reflection from the curved droplet liquid-air interface. With this developed 3-D modeling, A-TIR characteristics were explored using the parameters of the aperture size and the working distance of the aperture as well as the droplet surface coverage fraction, which shows a good agreement between the experiment and the simulation. Furthermore, it was for the first time demonstrated that the droplet contact angle can be effectively determined by obtaining the droplet thickness from the analytic quadratic solution by subtracting the measured reflectance at the two different sized apertures and using the spherical profile relation. Low contact angles in the range of 1∼ 15° were determined experimentally for the micro- and macro-sized droplets with a droplet diameter of 70 ∼ 7000 µm by the measured thickness of 1 ∼ 450 µm using A-TIR and compared with Fizeau interferometry and side-view imaging to show a good agreement. The simulation shows that A-TIR can be a new optical diagnostic tool to measure the contact angles 0 ∼ 90° regardless of the droplet diameter by adjusting the aperture size and the working distance. In addition, A-TIR can effectively determine the small contact angles less than 5°, even ultrasmall contact angles less than 1° for the submicron thickness, not requiring the complicated microscope setup. Thus, we can observe a sessile droplet's drastic contact angle change during wetting phenomena from 90° to 0° on the same A-TIR setup. Additionally, A-TIR can be used for a single or an array of micro or nanodroplets with a microscope objective by reducing the laser beam size and scanning methodology.
We introduce an optical diagnostics to determine the dual-surface profile of liquid droplet using internal reflection interferometry. A coherent laser beam is internally reflected on the air/liquid interface of a sessile droplet placed on a prism-based substrate to produce an interference fringe on a screen far from the substrate. The reflected laser rays consist of the reflection from the center spherical droplet profile and the one from the lower hyperbola-like droplet profile. The reflected rays are interfered each other to form the interference fringes. Ray tracing simulation is conducted using a custom-designed computer program. The simulation shows that the interfering rays reflected near the inflection point produce the outer-most fringes of the concentric interference pattern on the screen, and the reflected rays from the apex of the spherical profile and the contact line of the lower hyperbola-like profile construct the fringes at the center of the interference patterns. The simulated results are compared with the experimental observation to show a good agreement in the number and the location of the fringes and the radius of the outer-most-fringe where the number of the fringes is dependent on the droplet thickness and the radius of the fringe depends on the contact angle of the droplet. This result provides a new measurement technique to determine the morphological features of very small microdroplet such as the thickness (< a few micron thickness), the contact angle (< a few degree), and the dualsurface profile.
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