Direct Optical Measurement of Evaporative Flux of Volatile Liquid Droplets*

An evaporating single-component liquid droplet leaves a coffee-ring stain after completely dried, which is due to the non-uniform evaporation rate along the surface of a droplet. So far, the evaporative flux is rarely measured while based on analytical models, indirect measurements, or model-based experiments, the flux profile is estimated. To understand the evaporation mechanism of the droplet is often important to control deposit patterns demanded by various industries including inkjet printing, biochemical applications. In this study, we utilized an optical interferometric measurement technique (Mach-Zehnder interferometer) to detect the optical distortion near the surface of an evaporating droplet. The optical signal represents variation of refractive indices and media density. Using Fourier transform profilometry based fringe analysis, we investigate the time-dependent evaporative flux profile of different volatile liquid droplets in the vicinity of the liquid-air interface. Furthermore, we explore effects of gravity and molar mass of the volatile liquid on the evaporative flux profile.