An IR thermal imaging method to investigate spreading process of ethanol solution droplets on carbon fiber mats

Porous media are a class of basic engineering material, and the applications of a porous medium are significantly influenced by its surface and bulk wettabilities. Conventionally, the surface wettability and dynamics of a liquid droplet on a stationary material surface can be observed by performing a dyed droplet spreading test captured with a high-speed camera. Such a method, however, is not effective in the quantification of droplet dynamics within a porous medium. In this work, we propose an infrared (IR) thermal imaging method to investigate the spreading of colorless droplet beneath the surface of, and within, the porous network of the porous medium. The spreading rim of a colorless droplet is accurately detected by the locations of extreme points in its temperature gradient curve, as deduced from temperature distribution data collected by an IR camera imaging from the top. Compared with the images captured simultaneously by a high-speed camera from the side, the droplet spreading rim is revealed to be an inner precursor rim located inside the porous media. To evaluate this method, carbon fiber (CF) mats with different porosities are used as the porous media. The spreading velocities of droplets in CF mats are measured successfully and coincide exactly with conclusions obtained by the background subtraction method. This finding validates the effectiveness of IR thermal imaging method in a droplet spreading test. This work demonstrates that IR imaging holds great promise in the visualization of the inner precursor rim of droplets in porous media.