In order to increase the heat transfer capability within the
air-compressor/expander, surface areas for heat transfer are
added by the use of heat exchanger structures and the
injection of tiny water droplets. Proper surface treatment
of the walls of the compressor/expander and of the heat
The research team led by Professor Eric Loth and Dr. Adam Steele at the University of Virginia
is investigating durable and cost-effective nano-texturing
techniques for achieving these goals.
- improve heat transfer effectiveness
- reduce mechanical losses due to liquid and air flow
Introduction and Achievements
The story of superhydrophobic materials starts in nature and
has been appropriately named the ‘lotus effect’ after the
lotus leaf (Nelumbo nucifera) as shown in the figure below.
Its characteristic micro- and nano-surface morphology
combined with low surface energy chemical functionality
allows for water droplets to roll and bounce freely on the
This addition of surface texture can greatly reduce the
solid-liquid surface contact area and leads to an apparent
contact angle, θ*, for a droplet on the surface, which can
be explained by two independently developed models: the
Wenzel model and the Cassie-Baxter model.
Surfaces with an apparent contact angle between 150° and
180° are termed superhydrophobic. In addition to high
contact angle, superhydrophobic surfaces also have very low
water contact angle hysteresis below 10°, which is the
difference between the advancing and receding contact
angles. Artificial superhydrophobic surfaces as shown
in the figures below have utilized this composite interface
and low hysteresis to generate extreme non-wettability and
low flow resistance.
These superhydrophobic nanocomposite coatings can be applied
to internal heat transferring components in this project's
liquid piston system to prevent liquid films from degrading
performance. Although key milestones such as coating
mechanical durability have already been achieved,
resistance to saturation remains the most important hurdle
to address as demonstrated in the figure below.
A laser illuminance experiment (shown below) has been built
to quantify this saturation effect in detail in order to
understand and design more resistant coatings.
Finally, active methods for prolonging superhydrophobic
performance and even reversing saturation are being
investigated. For example, conductive superhydrophobic
coatings have been created where an electrolytic reaction
can be induced to regenerate the air layer in a submerged
sample as shown below. Initial experiments have demonstrated
a 2x improvement in prolonging the superhydrophobic effect
on submerged samples.
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