This document summarizes research on using electrokinetic flows to separate particles in a bifurcating microchannel. The researchers fabricated microfluidic devices using soft lithography and tested different methods to generate electrokinetic flows. Their goals were to develop a reproducible method to create voltage-driven flows, study how flow ratios affect particle separation, and compare electrokinetic and pressure-driven bifurcation. Initial experiments had issues like bubble formation, non-reproducible results, and hydrophobic lids impeding flows. While electrokinetic effects were observed, reproducibility remained a challenge. The researchers concluded the first two methods tested were difficult to implement reproducibly, and the holder design was hindered by the lid properties.

1. Quantification of Particle Separation in Electrokinetically Driven
Flow through a Bifurcating Microchannel
Elaine Mau, Jeffrey Zahn, Rutgers University
Introduction
To separate various particles or cells within a
sample for diagnostic assay, a bifurcating
microchannel utilizes varying flow ratios within the
daughter channels to direct particles into a certain
channel (Figure 1).
Figure 1: Bifurcation Law using Pressure Driven Flows a) Greater pressure difference
and higher shear force causes particle to move towards higher flow branch b) Position
of centroid with respect to the critical stream line determines where the particle
flows.
The purpose of this work is to study the
properties of using electrokinetic flows to create
the different flow ratios.
Objectives
To design a means of generating electrokinetic
flow in a reproducible way
To demonstrate the effect of voltage/flow ratio
on particle recovery counts
To analyze the differences, if any, between
electrokinetic and pressure driven flows.
To determine the how to apply electrokinetic
flow for particle separation using data recorded
Methods
Fabrication of Device
Standard Soft Lithography Techniques
PDMS/Glass
Beads used:
15 m Polystyrene Divinylbenzene
(PS-DVB), green fluorescence
1 m, PS-DVB, green fluorescence
1 m, Melamine, red fluorescence
Well techniques
Poked holes using needle end
Needle as well, convenient
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Method 1: Needle
Drilled and poked holes
Used various ports and fittings
Epoxy
PDMS
Method 2: Ports and Fittings
Figure 2: Early methods for creating wells (a) Needle (b) Ports and Fitting.
Biopsy punch for wells in PDMS
Oxygen Plasma Treated
Polycarbonate lid with wells, easy
access for electrodes
Aluminum base with viewing window
Grease between lid and base, vigorous
tapping
Method 3: Housing
Figure 3: Housing (a) ProEngineering model (b) Actual Device
Polycarbonate Lid
Figure 4: To test the affect of Magnalube-G PTFE grease on lid, the lid was
placed on the table with and without grease.
Experimental Set-up
a) b)
Figure 5: To create the high and low flow rates using electrokinetic flow,
different voltages were applied at the end of each reservoir, such as the figure
above.
Figure 6: System used for experiment a) Schematic b) Actual.
a)
b)
Results
General Problems
Settling of Beads
Head pressure and back flow
Some clogging issues
No bubble trapped in channels
Method 1: Syringe
Bubble Formation
Method 2: Ports and Fittings
Inconvenient for electrodes
Affected by bubble formation
No reproducibility
Method 3: Housing
Convenient for electrodes
Difficult reproducibility
Lid’s hydrophobicity (Figure 8)
Electrophoretic effect seen (Figure 9)
Figure 7: Bubble formation at tip of the needle.
Figure 9: When electrokinetic flow worked, beads traveled in the opposite direction as
expected. (a) 1 µm beads polystyrene beads traveled from negative to positive terminal.
Most 15 µm beads followed the direction of flow for small particles; a few went the other
direction (b) 1 µm melamine beads were seen to travel from negative to positive terminal
and vise versa.
a) b)
a) b)
Figure 8: Results of test for polycarbonate lid. (a) Without grease, water would spread on
the table; however, when assembled, there was some leaking and electrokinetic flow did
not occur. (b) With grease, hydrophobic effect was observed. When assembled, vigorous
tapping occasionally allowed connection between water from the lid and and that from
the well.
a) b)
Conclusions
Acknowledgments
I would like to acknowledge Dr. Zahn’s graduate
students, Larry Sasso and Alex Fok.
I would like to thank Rutgers University and Aresty
Research Center for providing funding for this
research.
First two methods are difficult to implement and
produce results that are not reproducible
The holder, although easier to assemble, is
difficult to get working because of the lid’s
properties.