Viscoelastic effects on electrokinetic particle focusing in a constricted microchannel

Abstract

Focusing suspended particles in a fluid into a single file is often necessary prior to continuous-flow detection, analysis, and separation. Electrokinetic particle focusing has been demonstrated in constricted microchannels by the use of the constrictioninduced dielectrophoresis. However, previous studies on this subject have been limited to Newtonian fluids only. We report in this paper an experimental investigation of the viscoelastic effects on electrokinetic particle focusing in non-Newtonian polyethylene oxide solutions through a constricted microchannel. The width of the focused particle stream is found NOT to decrease with the increase in DC electric field, which is different from that in Newtonian fluids. Moreover, particle aggregations are observed at relatively high electric fields to first form inside the constriction. They can then either move forward and exit the constriction in an explosive mode or roll back to the constriction entrance for further accumulations. These unexpected phenomena are distinct from the findings in our earlier paper [Lu et al., Biomicrofluidics 8, 021802 (2014)], where particles are observed to oscillate inside the constriction and not to pass through until a chain of sufficient length is formed. They are speculated to be a consequence of the fluid viscoelasticity effects.

Electroosmotic flow of non‐Newtonian fluids in a constriction microchannel

“…However, the few particles that escaped from the gel could still pass through the constriction. The formation of such gelled rafts in PAA solution under high electric fields has also been reported previously . Under the electric fields of less than 200 V/cm, the tracing particles before the constriction appear to move a little slower than after it.…”

“… reported an unexpected oscillation of polystyrene microparticles that travel along with the electroosmotic flow of polyethylene oxide (PEO) solutions through a constriction microchannel. They found in a later paper that particles traveling against the electroosmotic flow experience no oscillations in the constriction and can be focused toward the channel center. More recently, Pimenta and Alves investigated the electroosmotic flow of PAA solutions through a cross‐shaped microchannel.…”

Recent advances in direct current electrokinetic manipulation of particles for microfluidic applications

“…However, the majority of the studies on electrokinetic particle motion are purely theoretical , and experimental investigations are desperately lacking. Recent experiments from the author's group have reported some astonishing results (with respect to those in Newtonian fluids), such as the oscillation of particle electrophoresis in the DC electroosmotic flow of viscoelastic fluids through a constriction microchannel and the wall‐directed particle migration in the DC electroosmotic flow of shear thinning fluids through straight rectangular microchannels . Future work in this direction may cover the study of how the fluid rheological properties affect the electrokinetic motion of particles in varying microchannels, and as well the development of numerical models for understanding such motions with efficient algorithms and appropriate constitutive equations. Joule heating and induced charge effects have been usually deemed detrimental to precise electrokinetic transport and placement of particles in microchannels because of the induced electrothermal and electroosmotic flows in high‐ and low‐conductivity fluids, respectively.…”

Manipulation of micro‐ and nanoparticles in viscoelastic fluid flows within microfluid systems

“…They observed an oscillatory particle motion in the constriction region in the VEF flow without arising electrokinetic focusing of particles due to the interaction between electrokinetic and viscoelastic forces (Figure b; Lu et al, ). It was also demonstrated that in low concentration of the PEO (50 ppm), the particle behavior in VEFs is similar to Newtonian fluids (Lu et al, ). In contrast, in higher concentrations (100–500 ppm), the particles showed a different behavior and were not focused along the channel desirably.…”