Making the smallest heat engine

LG14179 - There is nothing that envisages the idea of classical physics more than a heat engine, but how can this stem from quantum mechanics. The time reversible nature of quantum mechanics suggests that the most simple ingredient of a heat engine, the system being in thermal equilibrium, is not possible.

We present a simple scheme of atoms confined to a double well, that demonstrates thermalization of the atoms and is simple enough to perform a heat engine cycle. By modifying the trapping confinement and well depths, one well acts as the heat engine and the other as the heat reservoir. Most notably, the heat engine, reservoir and hence the energy transfer is described fully quantum mechanically in this finite isolated system. The whole system could be as little as 10 microns in size, so if experimentally realization, it would be the smallest heat engine ever.

Smart surface structuring can help creating good electrical contacts

LH13420 - In light switches, in relays, on car batteries and in light bulk sockets
– many devices require a well conducting contact between two metals, at
the least possible cost and effort. Up until recently, it was common
practice to assume linearity between conductivity and normal force – in
other words, it was believed that one would have to press twice as hard
if one wished to channel the double amount of current. In our paper
though, we showed that the characteristic of the surface roughness has a
significant influence upon this behavior by altering the surface's
contact stiffness. With the help of computer simulations, we compared
surfaces which had a rather fuzzy look to more wavy ones and found out
that the later performed significantly better at low forces as can be
found in most technical applications.

A new, brief formula will help technicians and engineers estimating the
electrical resistance by taking into account the surface characteristic
from their manufacturing method. A better prediction may help them save
material or abandon a costly surface treatment.

Swirling airflow helps stabilize flapping-wing flyers

LG14064 - As the Wright brothers demonstrated one hundred years ago, the key challenge of flight is maintaining balance. Although insects took to the air 400 million years earlier, their flight stability remains a mystery because of the complex aerodynamics of flapping wings. We approach this problem by discovering the conditions needed to achieve stable hovering in mechanical flyers. Our system consists of pyramid-shaped bugs constructed from paper that hover when placed in an oscillating column of air, mimicking the effect of flapping wings. To our surprise, we find that top-heavy bugs hover stably: if the body tilts to the side, the swirls of fluid ejected from the wings automatically adjust to keep the bug upright. By providing this connection between wing shape and flow features, these findings offer a blueprint for achieving stability in highly maneuverable flapping-wing robots.