Droplet evaporation is a seemingly simple heat and mass transfer process, while of which high-fidelity characterization of the vapor field remains a long-standing challenge. In this brief communication, we report the first experimental observation of the local vapor concentration of the sessile water droplet with unprecedently high spatiotemporal resolution. The experimental measurement was enabled by a tailored quartz-enhanced photoacoustic spectroscopy (QPEAS) system. In this system, a near-infrared, continuous-wave, distributed feedback laser at 1392 nm was utilized to excite the photoacoustic signal of water vapor above the sessile droplet. The photoacoustic signal between the prongs of a tiny quartz tuning fork (QTF) was detected and further interpreted as the vapor concentration information. The feasibility of QEPAS for droplet evaporation characterization was carefully verified. As a demonstration, we measured the time histories of water vapor concentration above the evaporating droplet in both open and closed chambers at the spatial resolution of ~30 micrometers and temporal resolution of 0.1 ms. Interestingly, we found the vapor concentration above the evaporating droplet in open ambient featured distinct fluctuations indiscernible to the conventional photography method. Such fluctuation has never been reported before and further corroborated the insufficiency of quasi-steady approximation for droplet evaporation. This work not only extends the application of QEPAS to more complicated physical processes, but also advances the state of the art in the experimental characterization of sessile water droplet evaporation, which opens up a new possibility to unravel the intricate mechanism of the vapor-mediated droplet motions.
On the measurement of local vapor concentration around sessile water droplet with high spatiotemporal resolution
Wang Z;
2023
Abstract
Droplet evaporation is a seemingly simple heat and mass transfer process, while of which high-fidelity characterization of the vapor field remains a long-standing challenge. In this brief communication, we report the first experimental observation of the local vapor concentration of the sessile water droplet with unprecedently high spatiotemporal resolution. The experimental measurement was enabled by a tailored quartz-enhanced photoacoustic spectroscopy (QPEAS) system. In this system, a near-infrared, continuous-wave, distributed feedback laser at 1392 nm was utilized to excite the photoacoustic signal of water vapor above the sessile droplet. The photoacoustic signal between the prongs of a tiny quartz tuning fork (QTF) was detected and further interpreted as the vapor concentration information. The feasibility of QEPAS for droplet evaporation characterization was carefully verified. As a demonstration, we measured the time histories of water vapor concentration above the evaporating droplet in both open and closed chambers at the spatial resolution of ~30 micrometers and temporal resolution of 0.1 ms. Interestingly, we found the vapor concentration above the evaporating droplet in open ambient featured distinct fluctuations indiscernible to the conventional photography method. Such fluctuation has never been reported before and further corroborated the insufficiency of quasi-steady approximation for droplet evaporation. This work not only extends the application of QEPAS to more complicated physical processes, but also advances the state of the art in the experimental characterization of sessile water droplet evaporation, which opens up a new possibility to unravel the intricate mechanism of the vapor-mediated droplet motions.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
