am7b05651_si_010.mpg (936.92 kB)
Dynamic Defrosting on Scalable Superhydrophobic Surfaces
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posted on 2017-06-27, 00:00 authored by Kevin R. Murphy, William T. McClintic, Kevin C. Lester, C. Patrick Collier, Jonathan B. BoreykoRecent
studies have shown that frost can grow in a suspended Cassie state
on nanostructured superhydrophobic surfaces. During defrosting, the
melting sheet of Cassie frost spontaneously dewets into quasi-spherical
slush droplets that are highly mobile. Promoting Cassie frost would
therefore seem advantageous from a defrosting standpoint; however,
nobody has systematically compared the efficiency of defrosting Cassie
ice versus defrosting conventional surfaces. Here, we characterize
the defrosting of an aluminum plate, one-half of which exhibits a
superhydrophobic nanostructure while the other half is smooth and
hydrophobic. For thick frost sheets (>1 mm), the superhydrophobic
surface was able to dynamically shed the meltwater, even at very low
tilt angles. In contrast, the hydrophobic surface was unable to shed
any appreciable meltwater even at a 90° tilt angle. For thin
frost layers (≲1 mm), not even the superhydrophobic surface
could mobilize the meltwater. We attribute this to the large apparent
contact angle of the meltwater, which for small amounts of frost serves
to minimize coalescence events and prevent droplets from approaching
the capillary length. Finally, we demonstrate a new mode of dynamic
defrosting using an upside-down surface orientation, where the melting
frost was able to uniformly detach from the superhydrophobic side
and subsequently pull the frost from the hydrophobic side in a chain
reaction. Treating surfaces to enable Cassie frost is therefore very
desirable for enabling rapid and low-energy thermal defrosting, but
only for frost sheets that are sufficiently thick.