  This letter reports on the femtosecond laser fabrication of
gradient-wettability micro/nano- patterns on Si surfaces. The dynamics of
directional droplet spreading on the surface tension gradients developed is
systematically investigated and discussed. It is shown that microdroplets on
the patterned surfaces spread at a maximum speed of 505 mm/sec, that is the
highest velocity demonstrated so far for liquid spreading on a surface tension
gradient in ambient conditions. The application of the proposed laser
patterning technique for the precise fabrication of surface tension gradients
for open microfluidic systems, liquid management in fuel cells and drug
delivery is envisaged.
