  The manipulation of micro- and nano-objects is of great technological
significance to construct new materials, manipulate tiny amounts of liquids in
fluidic systems, or detect minute concentrations of analytes. It is commonly
approached by the generation of potential energy landscapes, for example, with
optical fields. Here we show that strong hydrodynamic boundary flows enable the
trapping and manipulation of nano-objects near surfaces. These thermo-osmotic
flows are induced by modulating the van der Waals interaction at a solid-liquid
interface with optically induced temperature fields. We use a thin gold film on
a glass substrate to provide localized but reconfigurable point-like optical
heating. Convergent boundary flows with velocities of tens of micrometres per
second are observed and substantiated by a quantitative physical model. The
hydrodynamic forces acting on suspended nanoparticles and attractive van der
Waals or depletion induced forces to enable precise positioning and guiding of
the nanoparticles. Fast multiplexing of flow fields further provides the means
for parallel manipulation of many nano-objects. Our findings have direct
consequences for the field of plasmonic nano-tweezers and other
thermo-plasmonic trapping schemes and pave the way for a general scheme of
nanoscopic manipulation with boundary flows.
