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Alternating zeta-potential pattern to eliminate electro-osmotic flow C. Schreuer1,2, T. Brans1,2, S. Vandewiele1, F. Strubbe1, K. Neyts1, F. Beunis1 1UGent, Ghent, Belgium 2IWT, Brussels, Belgium E-mail: caspar.schreuer@ugent.be Optical trapping electrophoresis is a technique that accurately determines the zeta-potential of a single colloidal particle. An IR optical laser traps a particle and an AC electric field induces oscillatory movement. The amplitude of the movement is measured with a quadrant photodi-ode. In nonpolar liquids, this technique can measure with unit charge resolution, revealing the charging dynamics of a particle [1]. However, in polar liquids, such as water, electro-osmosis strongly disturbs the measurement. Up to now, electro-osmosis has been reduced in two ways; decreasing the zeta-potential of the channel surface and increasing the frequency of the electric field. The first does lower the ve-locity of the flow, but the necessary capillary force would vanish under a perfectly zero zeta-potential. Increasing the electrical frequency yields a smaller amplitude of movement. A novel approach to counteract this flow, is to pattern the microfluidic with long strips of al-ternating polarity. Each strip tries to direct a flow in the opposite direction with respect to its neighbors. This system creates vortex-like flows near the channels surface, which has been applied for mixing purposes [2]. In a wide channel (>10 times pattern period), this flow does not couple to the bulk. Fig. 1 compares the flow profile of a cross section of a classical channel with a patterned channel. The flow velocity in the center of the patterned channel is uniform and is 150 times lower than in the classic channel. References [1] F. Beunis, F. Strubbe, K. Neyts, D. Petrov, Phys. Rev. Letters 2012, 108, 1 [2] A. Stroock, M. Weck, D. Chiu, W. Huck, P. Kenis, R. Ismagilov, G. Whitesides, Phys. Rev. Letters 2000, 84, 15 |
