1. Alternating Zeta-Potential Pattern to Eliminate Electro-Osmotic Flow
Caspar Schreuer, T. Brans, S. Vandewiele, F. Strubbe, K. Neyts and Filip Beunis
Department of Electronics and Information Systems, Ghent University
Center for Nano- and Biophotonics (NB-Photonics), Ghent University
Introduction
The electrophoretic mobility is a measure for the mobility and the stability of particles in a colloidal suspension. It is defined as the ratio of the particle velocity and the applied electric field:
𝜇 𝑒 = 𝑣 𝐸
In polar liquids, measurements of the electrophoretic mobility are complicated by a fluid flow, which superimposes the flow velocity on the particle velocity, effectively limiting the measurement accuracy. This flow is called electro-osmotic flow.
This poster discusses a novel method that aims to eliminate electro-osmosis in the bulk of a microfluidic channel.
1. Electro-osmosis
A glass substrate has a finite surface charge at the glass-liquid interface. This surface charge attracts an excess of mobile ions of opposite polarity near the surface. When an electric field is applied parallel to this interface, the Coulomb force induces the ions to move. This effect causes a parabolic fluid flow across the channel.
Surface charge
Mobile ions E
2. Patterned Microfluidic Channel
In order to eliminate the electro-osmotic flow, a pattern can be printed onto the surface of the channel. This pattern consists of parallel stripes that are transversal to the electric field. The material of the imprinted stripes is chosen to have a zeta-potential with the opposite polarity of the glass substrate.
Each stripe will build up a mobile ion layer with the opposite polarity with respect to its neighbors. In the presence of an electric field, these mobile ion layers will flow against each other. This gives rise to vortex-like flow near the boundaries, but when the channel is sufficiently thick (thickness > 10 x pattern period), no net flow is coupled to the bulk of the channel.
Electro-osmosis in a closed channel
3. Simulation results
A vertical cross section of the flow in the channel reveals that the patterned channel reduces the velocity by two orders of magnitude and has a uniform profile in the center. The channel has been optimized by varying the relative thicknesses of the stripes.
200µm
10-1
10-2
10-3
10-4
10-5
200µm
Flow (mm/s)
Glass, ζ = V
Distance from the bottom substrate (µm)
10µm 11µm
Simulation of the channel, applied DC voltage 10V,
(black) regular channel, (dotted) patterned channel
Al2O3, ζ = 0.06V
Patterned microfluidic channel, proportions are not to scale
Conclusions and prospects
The simulation reveals the potential of patterning the walls of a microfluidic channel: The electro-osmotic flow can be reduced by two orders of magnitude.
Measurements in a patterned channel will be performed with the aim to confirm these results.
11th International Symposium on Electrokinetic Phenomena
Ghent, Belgium – May 22, 2014
Research supported by IWT