  We present thermophoretic measurements in aqueous suspensions of three
different polystyrene (PS) particles of varying negative charge, size and
surface coating. Our measurement technique is based on the observation of the
colloidal steady-state distribution using conventional bright-field microscopy,
which avoids undesirable effects such as laser-induced convection or local
heating. We find that the colloids with the weakest zeta potential exhibit the
strongest thermophoretic effect, suggesting that surface functionality leads to
a more intricate dependence of the Soret coefficient on hydrodynamic boundary
conditions than predicted by existing theoretical approaches. We also study the
relaxation of the colloids to steady-state and propose a model to quantify the
relaxation speed, based on the time evolution of the colloidal center of mass.
Our observations are well described by this model and show that the relaxation
speed tends to increase with the magnitude of the thermophoretic force.
