IR laser caused droplet evaporation on the hydrophobic surface

In this work, the characteristics of the water droplet evaporation on the hydrophobic surface was visually studied, which was actuated by the photothermal effect of an infrared laser with the wavelength of 1550 nm. The interface temperature was measured by the infrared temperature measurement technique. The variations of the droplet geometrical parameters were captured, with which the evaporation rate was determined by the image process technique. Experimental results indicated that once the IR laser was applied, accompanying with the rapid increase of the interface temperature, the droplet evaporation was instantly induced. Generated vapor was condensed to form satellite droplets near the triple-phase contact line due to such a tiny local heating source that would not significantly change the surrounding temperature. After an increase in the contact radius and corresponding quick decrease of the droplet height, the original droplet gradually shrank. In this process, there existed a period with a relatively stable interface temperature and evaporation rate. In addition, the effects of the laser power and initial droplet volume were also explored. It was shown that both the interface temperature rising and average evaporation rates almost linearly increased with the laser power because more heat was generated by absorbing the light energy. Regarding the initial droplet volume, increasing the droplet volume firstly resulted in an increase in the interface temperature rising and average evaporation rates and then caused both of them to be reduced. The results obtained in this work are helpful for the future applications of droplet-based devices utilizing the photothermal effect.