  Spatial control of wettability is key to many applications of microfluidic
devices, ranging from double emulsion generation to localized cell adhesion. A
number of techniques, often based on masking, have been developed to produce
spatially-resolved wettability patterns at the surface of
poly(dimethylsiloxane) (PDMS) elastomers. A major impediment they face is the
natural hydrophobic recovery of PDMS: hydrophilized PDMS surfaces tend to
return to hydrophobicity with time, mainly because of diffusion of low
molecular weight silicone species to the surface. Instead of trying to avoid
this phenomenon, we propose in this work to take advantage of hydrophobic
recovery to modulate spatially the surface wettability of PDMS. Because
temperature speeds up the rate of hydrophobic recovery, we show that
space-resolved hydrophobic patterns can be produced by locally heating a
plasma-hydrophilized PDMS surface with microresistors. Importantly, local
wettability is quantified in microchannels using a fluorescent probe. This
"thermo-patterning" technique provides a simple route to in situ wettability
patterning in closed PDMS chips, without requiring further surface chemistry.
