Microfluidic particle trapping and separation using combined hydrodynamic and electrokinetic effects

This thesis describes a new methodology for particle trapping and fractionation
using flow-induced electrokinetic trapping (FIET). FIET is a particle-trapping
mechanism that relies on bidirectional, recirculating flow profiles generated by
opposition of pressure-driven (PF) and electroosmotic (EOF) flows in straight
channels that expand at both ends. Micrometer-sized particles are captured in
the closed recirculating streamlines and fractionated according to differences in
surface charge (zeta potential) and size.
First, the behavior of trapped particles inside the straight, narrow channel was
characterized under conditions of constant pressure and varying applied voltage.
For this, we propose a Gaussian model that accurately describes the spatial
distribution of particles along the trapping channel length as a function of the
applied voltage. This model provides valuable information about the trapping
process, such as the range of applied voltage within which particles of a particular
size and charge are caught inside the channel and the specific voltage at which a
maximum number of particles experience trapping. FIET enrichment of polymer
particle suspensions is also evaluated using the optimal trapping parameters
determined experimentally.
Second, we evaluate the implementation of this distribution model for
quantitative fractionation of binary mixtures of polymer microparticles in FIET
microfluidic channels. For this, particle distributions of beads having different
size or charge were registered as a function of applied voltage. A comparison of
the fractionation capacity for each mixture was subsequently conducted at
different applied pressures based on the acquired distribution curves. Particles
having different sizes exhibited better separation rates (clearer collected fractions
for both particle types) at lower applied pressures, whilst particles having
different zeta potentials could be fractionated at higher pressures. This fact
evidenced a clear distinction between two well-defined mechanisms
(hydrodynamic and electrokinetic) co-existing in the FIET process.
Lastly, the applicability of this model is further extended to the fractionation of
ternary mixtures of particles having different size and zeta potential. The
synergistic exploitation of the hydrodynamic and electrokinetic mechanisms
described above was accomplished by a stepwise increasing voltage program
applied at two different pressures. Two separation dimensions were clearly
revealed, with one based on size, and the other based on charge. The
simultaneous occurrence of these two mechanisms in the same FIET microfluidic
device leads to a unique orthogonality, described here for the first time in the
realm of microfluidic particle separations.