  We present a particle separation mechanism which induces motion of particles
of different sizes in opposite directions. The mechanism is based on the
combined action of a driving force and an entropic rectification of the
Brownian fluctuations caused by the asymmetric form of the channel along which
particles proceed. The entropic splitting effect shown could be controlled upon
variation of the geometrical parameters of the channel and could be implemented
in narrow channels and microfluidic devices.
