  We reveal an unreported physical mechanism that enables the
pre-concentration, sorting and characterization of charged polystyrene
nanobeads and liposomes dispersed in a continuous flow within a straight
micron-sized channel. Initially, a single $\Psi$-junction microfluidic chip is
used to generate a steady-state salt concentration gradient in the direction
perpendicular to the flow. As a result, fluorescent nanobeas dispersed in the
electrolyte solutions accumulate into symmetric regions of the channel,
appearing as two distinct symmetric stripes when the channel is observed from
top via epi-fluorescence microscopy. Depending on the electrolyte flow
configuration and, thus, the direction of the salt gradient field, the
fluorescent stripes get closer to or apart from each other as the distance from
the inlet increases. Our numerical and experimental analysis shows that,
although diffusiophoresis and hydrodynamic effects are involved in the
accumulation process, diffusioosmosis plays a crucial role in the observed
particles dynamics. In addition, we developed a proof-of-concept double
$\Psi$-junction microfluidic device which exploits this new accumulation
mechanism for the size-based separation and size detection of nanobeads as well
as for the measurement of zeta potential and charged lipid composition of
liposomes under continuous flow settings. This device is also used to
investigate the effect of fluid-like or gel-like states of the lipid membranes
on the liposome diffusiophoretic response. The proposed strategies for
solute-driven manipulation of colloids have great potential for microfluidic
bio-analytical testing applications, including bioparticle pre-concentration,
sorting, sensing and analysis.
