  We demonstrate a mechanochemical approach to fabrication of a bidirectional
surface on a doped silicon wafer. In the initial step of our fabrication
procedure, a high-density polyethylene (HDPE) stylus is pressed and drawn on
the Si surface. This rubbing action provides a mechanochemical treatment of the
Si surface, which unexpectedly alters the etching direction in the subsequent
metal-assisted chemical etching step. Hence, Si nanowires of two different
orientations can be patterned conveniently by a rub-write step. The mechanism
of bidirectional Si nanowire formation is investigated. This anisotropy is
exploited to propel water droplets on a fluidic track.
