Nanoscale gas kinetics
Energy conversion in gas-filled channels with structured walls
A gas at atmospheric pressure confined in a gap of a width below 1 μm behaves in a manner different from its macroscopic dynamics. Since collisions of the gas molecules with the channel walls become equally probable as collisions with other molecules, novel physical effects show up. In particular, since gas molecules carry both energy and momentum between surfaces at different temperatures, it is possible to impart not only a net force normal to the wall (pressure), but also a tangential force, enabling conversion of thermal into mechanical energy.
The figure below depicts the model system we consider. Imagine a wheel (red) with a nanostructured surface enclosed by an annulus (blue). The nanostructures may consist of a surface topography, but also of a pattern of different material patches representing different boundary conditions for the wall reflection of gas molecules. The wheel and the annulus are at different temperatures. One key question in that context is: Will the collisions of the molecules with the surface of the wheel lead to a net rotation?
We have shown that if equipped with suitable nanostructures, the wheel will in fact rotate. This represents a novel scheme for converting thermal into mechanical energy. Furthermore, at Knudsen numbers of the order of one an arrangement of a flat and a nanostructured surface (as shown in the figure below) may serve as a thermally-actuated pump, enabling a net gas transport normal to an applied temperature gradient.
T. Baier, J. Dölger and S. Hardt, Energy harvesting through gas dynamics in the free molecular flow regime between structured surfaces at different temperatures, Phys. Rev. E 89 (2014), 053003.
A. A. Donkov, S. Tiwari, T. Liang, S. Hardt, A. Klar and W. Ye, Momentum and mass fluxes in a gas confined between periodically structured surfaces at different temperatures, Phys. Rev. E 84 (2011), 016304.
Thermally-induced migration and orientation of nanoscale Janus particles
A small particle, comparable in size to the mean free path of gas molecules, placed in a thermal gradient, experiences a force towards the colder side. This happens because gas molecules originating from the hotter side on average impart a larger momentum to the particle upon collision than molecules from the colder side. In clean rooms this is used for “repelling” dust particles from a heated wafer surface. Alternatively, this can be used for deposition of particles on a cooled surface or removing aerosol particles from a gas stream.
In some applications it is desirable to deposit particles in a particular orientation, for example when the particle has different functionalized groups on its surface. As a model system for this case we imagine a Janus sphere with different reflective properties on its “northern” and “southern” hemispheres. If put into a temperature gradient, the particle will orient itself such that its diffuse side points in the direction of the colder gas. However, both the thermophoretic translational motion of the particle and thermal fluctuations act against this thermal torque. As a result, the orientation of the particle can be described using a probability density function that can be computed analytically.
S. Shrestha, S. Tiwari, A. Klar and S. Hardt, Numerical simulation of a moving rigid body in a rarefied gas, J. Comp. Phys. 292 (2015), 239–252.