Heat conduction important for droplet dynamics
Action is important in repelling water from windshields, airplane wings


Date:
January 6, 2022
Source:
Washington University in St. Louis
Summary:
Engineers have found that conduction of heat plays a larger role
than previously thought in the dynamics of droplets on smooth
surfaces that repel water.



FULL STORY ==========================================================================
When driving in the rain, it's preferable that the raindrops roll or
bounce off of the windshield instead of coating it or even freezing. A
team of engineers in the McKelvey School of Engineering at Washington University in St. Louis found that conduction of heat plays a larger role
than previously thought in the dynamics of droplets on smooth surfaces
that repel water.


========================================================================== Patricia Weisensee, assistant professor of mechanical engineering &
materials science, and Junhui Li, a doctoral student in her lab, made
the finding after using high-speed imaging methods to study a microscopic entrapped bubble that forms when droplets of water hit a heated, smooth, water-repellant surface.

Results of the research are published in the Experimental Thermal and
Fluid Science Jan. 1, 2022 print issue.

The bubble -- only a few hundred microns in size -- forms inside a water droplet from absorbing the air underneath it as it begins to lift off
from the surface.

"We're creating capillary waves on the droplet, because as the droplet
impacts, it compresses, and that sends a shockwave through the droplet
and creates a doughnut-shaped droplet with the air bubble entrapped in
the middle," Weisensee said.

In her lab, Weisensee and Li tested water droplets on three heated
surfaces: Teflon and two materials that have similar surface chemistry:
PDMS, a biocompatible material; and HTMS, a hydrophobic silane-based
monolayer coating.

Using synchronized high-speed optical and high-speed infrared imaging,
they found that the amount of heat transferred from the smooth surface
to the water droplet increases with increased spreading velocity. In
addition, they found that the bubble changed in size and shape as
the temperature of the surface increased. Interestingly, during the
retraction of the droplet, the total heat transfer was reduced by 5.6%
and 7.1% at surface temperatures of 50C and 65C, respectively, as the
bubble reduced the total liquid-solid interface area.

Overall, this entire process lasts only a few milliseconds, but can have
a profound influence on the cooling efficiency and droplet dynamics of
these systems.

"We found that thermal conduction was the most prominent form of
heat transfer during droplet impact over convection or evaporation,"
Weisensee said.

In addition, they tested the droplets on a rough surface. The droplets
showed a smaller spreading area due to enhanced friction, a smaller
heat transfer area, and consequently, a lower rate of heat transfer,
which would ultimately lower the efficiency of, for example, spray
cooling processes.

"Though we used heated surfaces for this particular study, our findings
also have implications for other systems where you have droplets impacting
a surface, such as a windshield, an airplane wing or a wind turbine
blade," she said. "For example, in cold conditions, you don't want the
droplets to stay there and freeze. Lifting off is important so that you
don't flood a surface or accumulate a lot of liquid on the surface. So
you need know the interplay of droplet dynamics and heat transfer." ========================================================================== Story Source: Materials provided
by Washington_University_in_St._Louis. Original written by Beth
Miller. Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. Junhui Li, Patricia B. Weisensee. Low Weber number droplet impact on
heated hydrophobic surfaces. Experimental Thermal and Fluid Science,
2022; 130: 110503 DOI: 10.1016/j.expthermflusci.2021.110503 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2022/01/220106143701.htm

--- up 4 weeks, 5 days, 7 hours, 13 minutes
* Origin: -=> Castle Rock BBS <=- Now Husky HPT Powered! (1:317/3)