Fluid Elasticity-Enhanced Insulator-Based Dielectrophoresis for Sheath-Free Particle Focusing in Very Dilute Polymer Solutions

Abstract

Focusing particles into a narrow stream is usually a necessary step in microfluidic flow cytometry and particle sorting. We demonstrate that the addition of a small amount of poly­(ethylene oxide) (PEO) polymer into a buffer solution can reduce by almost 1 order of magnitude the threshold DC electric field for single-line dielectrophoretic focusing of particles in a constricted microchannel. The particle focusing effectiveness of this fluid elasticity-enhanced insulator-based dielectrophoresis (E-iDEP) in very dilute PEO solutions gets enhanced with the increase of the PEO molecular weight and particle size. These two trends are consistent with a theoretical analysis that accounts for the fluid elasticity effects on the electrokinetic and dielectrophoretic particle motions. Surprisingly, the particle-focusing effectiveness of E-iDEP is observed to first increase and then decrease with an increase in the PEO concentration.

Particle Size-Dependent Electrophoresis in Polymer Solutions

“…In comparison to 1000 ppm PEO in Figure b, we see a decrease in both V eo and V ek with the increase of PEO concentration. This trend is consistent with our recent observation, which, as noted above, should arise from the increased fluid viscosity and elasticity effect (see Table ). Moreover, we observe an intensified dependence of V ek on the particle size as the PEO concentration increases from 500 ppm (Figure a) to 1000 ppm (Figure b) and 2000 ppm (Figure b).…”

“…As viewed from the timelapses on top of the images, the electrokinetic motion of particles slows down in the PEO solution (Figure b) compared to that in the Newtonian solution (Figure a). This observation is consistent with our previous studies, ,, which can be attributed to the polymer contribution to viscosity and polymer-induced elasticity . The electrokinetic velocity remains nearly independent of the particle size in the Newtonian solution because the lines tracking the positions of different particles exhibit a parallel alignment in Figure a.…”

“…To put together the results for the effects of polymer concentration and length into one plot for a unified understanding, we employ the so-called elasticity number, El, to characterize the fluid elasticity effect. This dimensionless number is defined as the ratio of the Weissenberg number to the Reynolds number and is independent of the fluid kinematics, as given by ,, E l = λ η 0 ( w + h ) ρ w 2 h where the fluid relaxation time, λ, and viscosity, η 0 , are both given in Table for our prepared PEO solutions, ρ is the fluid density, and w and h are the width and height of the microchannel, respectively. As viewed from Table , increasing the concentration or molecular weight of the PEO polymer leads to a greater value of El.…”

“…The studies reviewed above are all concerned with particle electrophoresis in Newtonian fluids. Recent investigations have highlighted the significant impacts of fluid rheological properties on various electrokinetic phenomena. − In particular, a couple of theoretical papers predict the onset of particle size-dependent electrophoresis in non-Newtonian fluids even under the thin-Debye-layer limit. Khair et al developed a general framework to calculate the electrophoretic velocity of a uniformly charged colloidal particle immersed in a complex fluid with a shear-rate-dependent viscosity.…”

Magnetophoresis-Enhanced Elasto-Inertial Migration of Microparticles and Cells in Microfluidics

“…Generally, microfluidic techniques for cell manipulation can be categorized as active and passive techniques based on the source of the manipulating force. Active techniques rely on external physical fields such as electrical, − magnetic, − acoustic, − optical, , and thermal fields, , while passive techniques take advantage of inherent channel geometry, − fluid rheology, − and fluid dynamics. − Each manipulating technique has its pros and cons. Active methods provide the precise control of cells through modulating the external energy fields in real time but offer a relatively low throughput due to the requirement of a long residual time of the manipulating forces on the target cells.…”