in a liquid undergo constant irregular motion because of the thermal
motion of the molecules of the liquid.The observed motion is due to instantaneous imbalance of the net
force of the collisions of the liquid molecules on the particle. The net
force varies in magnitude and direction from moment to moment.Brownian motion was early proof of
kinetic theory.The motion is
proportional to the square-root of the absolute temperature and
proportional to the particle mass.
Brownian motors and ratchets are nanometer scale and molecular devices
that harness Brownian motion to generate directed motion (motion along a
fiber or rotation of a shaft in one direction).This at first seems to be a violation
of the second law of thermodynamics (a perpetual motion machine of the
second kind).However converting
the random Brownian motion to directed motion requires that energy be
expended to lock and unlock the ratcheting mechanism.
See the discussion and links in (http://www.Wikipedia.com) and in the
Brownian Ratchet applet site.