Hydrophobic
coatings with low thermal resistance promise a significant
enhancement in condensation heat transfer performance by promoting
dropwise condensation in applications including power generation,
water treatment, and thermal management of high-performance electronics.
However, after nearly a century of research, coatings with adequate
robustness remain elusive due to the extreme environments within many
condensers and strict design requirements needed to achieve enhancement.
In this work, we enable long-lasting condensation heat transfer enhancement via dropwise condensation by infusing a hydrophobic polymer,
Teflon AF, into a porous nanostructured surface. This polymer infused
porous surface (PIPS) uses the large surface area of the nanostructures
to enhance polymer adhesion, while the nanostructures form a percolated
network of high thermal conductivity material throughout the polymer
and drastically reduce the thermal resistance of the composite. We
demonstrate over 700% enhancement in the condensation of steam compared
to an uncoated surface. This performance enhancement was sustained
for more than 200 days without significant degradation. Furthermore,
we show that the surfaces are self-repairing upon raising the temperature
past the melting point of the polymer, allowing recovery of hydrophobicity
and offering a level of durability more appropriate for industrial
applications.
Bubble evolution plays a fundamental role in boiling and gas-evolving electrochemical systems. One key stage is bubble departure, which is traditionally considered to be buoyancy-driven. However, conventional understanding cannot provide the full physical picture, especially for departure events with small bubble sizes commonly observed in water splitting and high heat flux boiling experiments. Here, we report a new regime of bubble departure owing to the coalescence of two bubbles, where the departure diameter can be much smaller than the conventional buoyancy limit. We show the significant reduction of the bubble base area due to the dynamics of the three-phase contact line during coalescence, which promotes bubble departure. More importantly, combining buoyancy-driven and coalescence-induced bubble departure modes, we demonstrate a unified relationship between the departure diameter and nucleation site density. By elucidating how coalescing bubbles depart from a wall, our work provides design guidelines for energy systems which can largely benefit from efficient bubble departure.
Bubble
nucleation is ubiquitous in gas evolving reactions that are instrumental
for a variety of electrochemical systems. Fundamental understanding
of the nucleation process, which is critical to system optimization,
remains limited as prior works generally focused on the thermodynamics
and have not considered the coupling between surface geometries and
different forms of transport in the electrolytes. Here, we establish
a comprehensive transport-based model framework to identify the underlying
mechanism for bubble nucleation on gas evolving electrodes. We account
for the complex effects on the electrical field, ion migration, ion
diffusion, and gas diffusion arising from surface heterogeneities
and gas pockets initiated from surface crevices. As a result, we show
that neglecting these effects leads to significant underprediction
of the energy needed for nucleation. Our model provides a non-monotonic
relationship between the surface cavity size and the overpotential
required for nucleation, which is physically more consistent than
the monotonic relationship suggested by a traditional thermodynamics-based
model. We also identify the significance of the gas diffuse layer
thickness, a parameter controlled by external flow fields and overall
electrode geometries, which has been largely overlooked in previous
models. Our model framework offers guidelines for practical electrochemical
systems whereby, without changing the surface chemistry, nucleation
on electrodes can be tuned by engineering the cavity size and the
gas diffuse layer thickness.
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