Looking to nature for self-cleaning surfaces
Jiangtao Cheng
“In nature, lotus
leaves can achieve
self-cleaning by main-
taining dew conden-
sate in droplet form,
which can easily roll
off the leaf surface. In
industry, biomimetic
lotus-leaf-like surfaces
can not only allow for
easy droplet remov-
al at micrometric length scales
during condensation but also
promise to enhance heat transfer
performance if designed properly.
In this paper Dr. Cheng’s group
report energy-based analysis of
growth dynamics of dropwise con-
densates on biomimetic
surfaces with two-tier mi-
cro/nano-textures, which
are superior to solely
nanotextured surfaces in
controlling nucleation
density. To understand
the role of condensate
state transition, i.e., from
partially wetting state
(PW) to suspended Cas-
sie state (S), in enhancing
condensation heat transfer, they
considered adhesion energy, vis-
cous dissipation and contact line
dissipation as the main portion of
resistant energy that needs to be
overcome by the condensate drop-
lets formed in surface cavities. By
minimizing the energy barrier of
the state transition, they optimized
first tier roughness on the hierar-
chically textured surfaces allowing
condensates to grow preferentially
in the out-of-plane direction. The
nanoroughness of the second tier
plays an important role in abat-
ing the adhesion energy in the
cavities and contact line pinning.
From the perspective of molecular
kinetic theory (MKT), the hierar-
chically engineered surface is bene-
ficial to remarkably mitigating
contact line dissipation. This study
indicates that scaling down surface
roughness to submicron scale can
facilitate self-propelled condensate
removal.”
When Thermodynamics Meets Quantum Mechanics
Micha el von Spakovsky
It has long been known that
while mechanics is funda-
mental in its description,
thermodynamics is phenom-
enological. However, things
have changed dramatically in
that the gap between these
two disciplines has consis-
tently been bridged with the
help of quantum mechanics.
A widespread view that
thermodynamics is already
complete as evidenced by the 1974 remark: “[w]e
may be reasonably sure that a treatise on, say, ther-
modynamics, published in the year 2000 will not be
fundamentally different from one available today …”
has been proven wrong due to significant conceptual
breakthroughs in thermodynamics resulting in part
from the miniaturization of devices at the quantum
level. Differing from other quantum-level thermody-
namic methodologies, the “steepest-entropy-ascent
quantum thermodynamics” (SEAQT) framework,
which our Center for Energy Systems Research
has been helping develop and apply, has attracted
considerable attention asserting that the second law
of thermodynamics supplements rather than emerges
from quantum mechanics.
SEAQT has developed over decades, growing ever
more rapidly and addressing fundamental issues such
as a consistent description of heat and work as fun-
damental tools at the quantum level. In this research,
this issue and others have been and are continuing
to be addressed, making a consistent description of
quantum/nano-scale thermal machines possible and
shedding new light on various fundamental but yet
unresolved questions of thermodynamics. This in
turn has led to a unified understanding of multi-scale
phenomena.
From a practical standpoint, this methodology can
now be and has already been applied across all spatial
and temporal scales, i.e., from the atomistic to the
macroscopic, doing so with a single kinematic and
dynamic description and without the typical limiting
assumptions that come with traditional or classical
approaches. We have furthermore rigorously gener-
alized the equilibrium description of classical ther-
modynamics and the near-equilibrium realm to the
entire non-equilibrium region.