  Water molecules, confined in a carbon nanotube, were monitored using
molecular dynamics simulation. Spontaneous directional transportation during a
long timescale was observed in the symmetrical nanochannel by a ratchet-like
mechanism. This ratchet-like system was without any asymmetrical structure or
external field, while the asymmetric ratchet-like potential solely resulted
from the transported water molecules that formed hydrogen-bonded chains.
Remarkably, the resulting net water fluxes reached the level of the biological
channel and the average duration for spontaneous directional transportation
reached the timescale of many biomolecular functions. This is the first report
that heat energy from the surroundings can be used to drive molecules
uni-directionally during a long timescale in a nanochannel system. This effect
is ascribed to the unique structure of the water molecule.
