  The unique physical and optical properties of nanoparticles make their
reliable and rapid assembly an important goal for nanotechnology. To this end,
many competing driving forces have been targeted to control the self-assembly,
though issues with reliability and efficiency indicate that interfaces on the
nanoscale are not yet fully understood. In this letter we report for the first
time that the flow of water media from the interface between chemically
nanopatterned hydrophobic surfaces and adjacent hydrophilic regions (a Janus
interface) can effectively be employed for rapid nanoparticle assembly with
single nanoparticle resolution. We find that the dewetting of water molecules
from this Janus interface drives the assembly of nanoparticles from
electrostatically stabilised nanoparticle colloids onto the hydrophobic areas,
expedited by removing the resistance to adhesion between wetted interfaces
exerted by any hydration barrier, such that nanoparticle assembly is achieved
in under a minute. We call this method of controlled nanoparticle assembly
Janus Interface Nanoparticle Assembly - JINA. We then demonstrate control of
this mechanism through pH switching and prove its effectiveness for fabrication
by rapidly creating single-nanoparticle sites and single particle wide lines
whilst simultaneously densely coating macro-scale features. Not only does this
assembly method have far-reaching implications in nanomanufacturing, the
unanticipated nature of this mechanism also challenges many commonly held
assumptions about the interfacial interactions of nanoparticle self-assembly.
