  The controlled transport of colloids in dead-end structures is a key
capability that can enable a wide range of applications, such as bio-chemical
analysis, drug delivery and underground oil recovery. This letter presents a
new trapping mechanism that allows the fast (i.e., within a few minutes) and
reversible accumulation of sub-micron particles within dead-end micro-grooves
by means of parallel streams with different salinity level. For the first time,
particle focusing in dead-end structures is achieved under steady-state
gradients. Confocal microscopy analysis and numerical investigations show that
the particles are trapped at a flow recirculation region within the grooves due
to a combination of diffusiophoresis transport and hydrodynamic effects.
Counterintuitively, the particle velocity at the focusing point is not
vanishing and, hence, the particles are continuously transported in and out of
the focusing point. The accumulation process is also reversible and one can
cyclically trap and release the colloids by controlling the salt concentration
of the streams via a flow switching valve.
