  We quantitatively study the transport of E. coli near the walls of confined
microfluidic channels, and in more detail along the edges formed by the
interception of two perpendicular walls. Our experiments establish the
connection between bacteria motion at the flat surface and at the edges and
demonstrate the robustness of the upstream motion at the edges. Upstream
migration of E. coli at the edges is possible at much larger flow rates
compared to motion at the flat surfaces. Interestingly, the bacteria speed at
the edges mainly results from collisions between bacteria moving along this
single line. We show that upstream motion not only takes place at the edge but
also in an "edge boundary layer" whose size varies with the applied flow rate.
We quantify the bacteria fluxes along the bottom walls and the edges and show
that they result from both the transport velocity of bacteria and the decrease
of surface concentration with increasing flow rate due to erosion processes. We
rationalize our findings as a function of the local variations of the shear
rate in the rectangular channels and hydrodynamic attractive forces between
bacteria and walls.
