Measuring the Nonuniform Evaporation Dynamics of Sprayed Sessile Microdroplets with
Quantitative Phase Imaging
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Abstract
We demonstrate real-time quantitative
phase imaging as a new optical
approach for measuring the evaporation dynamics of sessile microdroplets.
Quantitative phase images of various droplets were captured during
evaporation. The images enabled us to generate time-resolved three-dimensional
topographic profiles of droplet shape with nanometer accuracy and,
without any assumptions about droplet geometry, to directly measure
important physical parameters that characterize surface wetting processes.
Specifically, the time-dependent variation of the droplet height,
volume, contact radius, contact angle distribution along the droplet’s
perimeter, and mass flux density for two different surface preparations
are reported. The studies clearly demonstrate three phases of evaporation
reported previously: pinned, depinned, and drying modes; the studies
also reveal instances of partial pinning. Finally, the apparatus is
employed to investigate the cooperative evaporation of the sprayed
droplets. We observe and explain the neighbor-induced reduction in
evaporation rate, that is, as compared to predictions for isolated
droplets. In the future, the new experimental methods should stimulate
the exploration of colloidal particle dynamics on the gas–liquid–solid
interface