On the wetting behavior of surfaces in boiling
File(s)
Accepted version
Author(s)
Ardron, Keith
Giustini, Giovanni
Type
Journal Article
Abstract
Nucleate boiling heat transfer is strongly influenced by surface wettability as characterized by the Young's contact angle, 𝜃𝑌. The contact angle is usually obtained from measurements on sessile droplets on horizontal test surfaces, but in the case of water at high temperatures and pressures, 𝜃𝑌 values from droplet experiments appear to be typically 30°–50° higher than values needed to explain bubble departure sizes for similar surfaces and temperatures. We explain the differences between 𝜃𝑌 values for droplets and vapor bubbles by using the surface adsorption theory of Adamson. This theory suggests that in the case of bubble formation in high pressure boiling, as the non-wetted surface inside the bubble is in contact with a saturated vapor, it will be covered by an adsorbed liquid layer of nanoscale thickness. Droplet experiments, on the other hand, generally use autoclaves pressurized by permanent gases in which the vapor pressure is far below saturation: in these relatively dry gases, the adsorbed liquid nanolayer is expected to be absent. We suggest that the presence of the adsorbed layer in the case of vapor bubbles will increase the work of formation of a new wetted surface by an amount comparable to the liquid surface tension, resulting in a significant reduction in 𝜃𝑌. We show that by applying Adamson's model with plausible choices for unknown parameters, it is possible to explain the magnitude of the differences in 𝜃𝑌 in bubble and droplet experiments and to explain why 𝜃𝑌 appears much less sensitive to surface material conditions in the case of departing vapor bubbles than in the case of sessile droplets. We conclude that 𝜃𝑌 measurements for sessile droplets on heated surfaces in pressurized gas rather than saturated vapor environments may not be relevant to vapor bubbles and values should not be used directly in models of nucleate boiling.
Date Issued
2021-11-11
Date Acceptance
2021-10-21
Citation
Physics of Fluids, 2021, 33 (11), pp.1-10
ISSN
1070-6631
Publisher
American Institute of Physics
Start Page
1
End Page
10
Journal / Book Title
Physics of Fluids
Volume
33
Issue
11
Copyright Statement
© 2021 Author(s). This article may be downloaded for personal use only. Any other use requires prior permission of the author and the American Institute of Physics. The following article appeared in Phys. Fluids 33, 111302 (2021); https://doi.org/10.1063/5.0069686
Sponsor
Engineering & Physical Science Research Council (EPSRC)
Identifier
https://doi.org/10.1063/5.0069686
Grant Number
EP/T027061/1
Subjects
Fluids & Plasmas
01 Mathematical Sciences
02 Physical Sciences
09 Engineering
Publication Status
Published
Date Publish Online
2021-11-11