Monitoring the evaporation of a sessile water droplet by means of integrated photonic resonator

We have investigated the behavior of the optical transduced signal of a photonic integrated polymer micro resonator as an evaporating water droplet is positioned upon it. The photonic chip is fabricated by means of deep-UV photolithography, and the circuits are made of polymer UV210. The device is then arranged in an optical bench so as to perform relevant measurements thanks to a broadband laser for the excitation of the resonators and an optical spectrum analyzer remotely controlled by a Matlab software for the acquisition and treatment of data. By dynamically tracking the free spectral range (FSR) of the optical mode, we come up with a signature of the thermodynamic evaporation process. In order to correlate this signature to the evaporation speed of the water droplet, a lateral camera is disposed to measure the evolution of the geometrical characteristics of the droplet, while weight measurements are performed thanks to a precision balance. These measurements provide a numerical link between the temporal variation of the FSR and the variation of the distance between the apex of the droplet and the substrate, correlating clearly the variation of the FSR with the mass change of the droplet. The measurements have been performed with 20 µL droplets for three different substrate temperatures: 21, 24 and 27°C. Then, theoretical investigations have been also performed in order to interpret the dynamics of the FSR in terms of guiding theory.