  Thermophoresis moves molecules along temperature gradients, typically from
hot to cold. We superpose fluid flow with thermophoretic molecule flow under
well defined microfluidic conditions, imaged by fluorescence microscopy. DNA is
trapped and accumulated 16-fold in regions where both flows move in opposite
directions. Strong 800-fold accumulation is expected, however with slow
trapping kinetics. The experiment is equally described by a three-dimensional
and one-dimensional analytical model. As an application, we show how a radially
converging temperature field confines short DNA into a 10 um small spot.
