posted on 2016-04-27, 00:00authored byAditya Chandramohan, Susmita Dash, Justin A. Weibel, Xuemei Chen, Suresh V. Garimella
We quantitatively characterize the
flow field inside organic liquid
droplets evaporating on a nonwetting substrate. A mushroom-structured
surface yields the desired nonwetting behavior with methanol droplets,
while use of a cooled substrate (5–15 °C) slows the rate
of evaporation to allow quasi-static particle image velocimetry. Visualization
reveals a toroidal vortex within the droplet that is characteristic
of surface tension-driven flow; we demonstrate by means of a scaling
analysis that this recirculating flow is Marangoni convection. The
velocities in the droplet are on the order of 10–45 mm/s. Thus,
unlike in the case of evaporation on wetting substrates where Marangoni
convection can be ignored for the purpose of estimating the evaporation
rate, advection due to the surface tension-driven flow plays a dominant
role in the heat transfer within an evaporating droplet on a nonwetting
substrate because of the large height-to-radius aspect ratio of the
droplet. We formulate a reduced-order model that includes advective
transport within the droplet for prediction of organic liquid droplet
evaporation on a nonwetting substrate and confirm that the predicted
temperature differential across the height of the droplet matches
experiments.