Experimental investigation of the water vapour concentration near phase boundaries with evaporation

We report on the development of a prototype diode laser absorption spectrometer suitable for measurements of water vapour concentration with high spatial and temporal resolution. The theoretical and experimental factors influencing the design of the instrument are discussed. A spatial resolution of 0.2 mm perpendicular to the line of sight suits the instrument particularly for measuring one-dimensional concentration profiles in the vicinity of planar liquid-vapour phase boundaries. As an example, the instrument was applied to the determination of the diffusive evaporation flux at a horizontal water-air interface. For mass transfer at a curved surface, as in a water droplet, the output of the spectrometer represents a convolution of the concentration distribution with the radiation intensity distribution integrated along the laser beam. The applicability of the spectrometer to droplet evaporation is demonstrated with humidity profiles measured near phase boundaries of acoustically levitated water droplets using an approximate mathematical approach to derive the parameters of the concentration field at the phase boundary. The rapid response of the instrument allows such measurements at approximately constant droplet radius. The measurements demonstrate the potential for detailed investigations of diffusion and evaporation phenomena.