A team of researchers from US have created
evaporation-driven engines that can power common tasks like locomotion
and electricity generation. These engines start and run autonomously
when placed at air–water interfaces. They are made from biologically
inspired artificial muscles which respond to humidity fluctuations.
Doesn’t evaporation take too long for any reasonable device?
Evaporation is a ubiquitous phenomenon in
the natural environment and a dominant form of energy transfer in the
Earth’s climate. Engineered systems rarely use naturally occurring
evaporation as a source of energy, despite a vast number of examples in
the biological world. The potential of evaporation to power engineered
systems is largely neglected. Furthermore, normal evaporation time
scales (daily / weekly) are too slow to use for everyday devices, yet
the process carries a significant amount of energy.
The breakthrough here is that near the evaporating
surfaces, there exists spatial gradients in relative humidity which
provides a potential opportunity to exploit. By confining water to the
nanoscale, in specially designed hydroscopic materials, it’s possible to
convert energy from evaporation to mechanical work. The confinement
induces large pressures in response to changing relative humidity.
Scaling this phenomena up to macroscopic (real-world) devices has faced
many problems in the past, but the team have managed to overcome a
number of them here.
Their solution: Artificial muscles from bacteria
By using cleverly modified naturally occurring bacteria, the team of researchers created hygroscopy-driven artificial muscles (HYDRA’s) that
exhibit strong hydration-driven actuation. The HYDRA’s can be thought
of as muscle-like elastic bands that contract and expand under changes
in humidity.
The material is made from plastic tape coated with a micrometer-thick bacterial spore layer. This layer is formed with modified Bacillus subtilis
spores, missing most of their outer protein protective layers. These
films change curvature as a function of relative humidity, and dramatic
changes in the overall length of tapes occur in humid and dry
conditions. The tapes can lift weight against gravity in dry conditions,
which is conceptually a remarkable feat for just bacteria-covered tape!
Assembling several tapes as a stack (while leaving
air gaps) allows rapid moisture transport to and from the spores,
resulting in a material that can be scaled in two dimensions, without
compromising hydration/dehydration kinetics.
Engines from artificial muscles
They next thought is that if a small portion of the
power generated by the spores could be used to control the evaporation
rate or, alternatively, move the spores in and out of the high humidity
zone at the surface, the relative humidity experienced by the spores
would change rapidly in a cyclical fashion – analogous to a primitive
mechanical engine. This is exactly what the team did, and created two
types of macroscale evaporation-driven engines to prove the concept: an oscillatory and rotary engine. These engines start and run autonomously when placed at air–water interfaces.
When the water on the surface naturally evaporates,
the engine interior becomes slightly more humid, and the HYDRA’s expand.
By using many HYDRA’s in combination and connecting them to a small
electromagnetic generator, their movement is converted into mechanical
energy in which it could drives the engine. The engines are able to
power an electricity generator to light up LEDs and drive a miniature
car as the water evaporates.
Impressive Performance
When loaded with increasing weight, the range of
motion of the artificial muscles reduces, but remains significant even
at load weights 50x more than the strips. The estimated work density of
the entire strip is 17 J / kg, which is close to mammalian skeletal
muscles.
Evaporation-driven engines may find many
‘off-the-grid’ applications in powering things such as robotic systems,
sensors, devices and machinery that require function in the natural
environment. Furthermore, the researchers suggest the efficiency of the
material (which is currently only a few percent), can be drastically
improved with further work on the spore design. If achieved, it’s hard
to see why this technology won’t be heavily utilized, especially because
the material is cheap and the building blocks (spores) are naturally
occurring. Places where access to electricity is limited to non-exist,
would find this technology truly magnificent.
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