  The non-equilibrium distribution of colloids in a polymer solution under a
temperature gradient is studied experimentally. A slight increase of local
temperature by a focused laser drives the colloids towards the hot region,
resulting in the trapping of the colloids irrespective of their own
thermophoretic properties. An amplification of the trapped colloid density with
the polymer concentration is measured, and is quantitatively explained by
hydrodynamic theory. The origin of the attraction is a migration of colloids
driven by a non-uniform polymer distribution sustained by the polymer's
thermophoresis. These results show how to control thermophoretic properties of
colloids.
