  Motility is a basic feature of living microorganisms, and how it works is
often determined by environmental cues. Recent efforts have focused on develop-
ing artificial systems that can mimic microorganisms, and in particular their
self-propulsion. Here, we report on the design and characterization of syn-
thetic self-propelled particles that migrate upstream, known as positive rheo-
taxis. This phenomenon results from a purely physical mechanism involving the
interplay between the polarity of the particles and their alignment by a
viscous torque. We show quantitative agreement between experimental data and a
simple model of an overdamped Brownian pendulum. The model no- tably predicts
the existence of a stagnation point in a diverging flow. We take advantage of
this property to demonstrate that our active particles can sense and
predictably organize in an imposed flow. Our colloidal system represents an
important step towards the realization of biomimetic micro-systems withthe
ability to sense and respond to environmental changes
