(8b) Surfactant Induced Electroosmotic Flow in Microfluidic Capillaries

Control of electroosmotic flow (EOF) in microfluidic devices is essential in applications such as protein/DNA sizing and high throughput drug screening. With the growing popularity of poly(methyl methacrylate) (PMMA) as the substrate for polymeric based microfludics, it is important to understand the effect of surfactants on EOF in these devices. In this article we present an extensive investigation exploring changes in EOF rate induced by sodium dodecyl sulfate (SDS), polyoxyethylene lauryl ether (Brij35) and cetyltrimethylammonium bromide (CTAB) in PMMA microfluidic capillaries. In a standard protein buffer (Tris-Glycine), microfluidic capillaries exhibited a cathodic EOF with measured mobility of 1.54 ± 0.1( ×10^-4 cm² / V.s). In the presence of surfactant, below a critical concentration, EOF was independent of surfactant concentration. At high concentrations of surfactants, the electroosmotic mobility was found to linearly increase/decrease as logarithm of concentration before reaching a constant value. While with SDS the EOF increased by 257%(compared to buffer), it was decreased by 238% with CTAB. In case of Brij35, the electroosmotic mobility was reduced by 70%. Possible mechanisms are proposed explaining the observed changes in EOF and zeta potential values. In a binary surfactant system of SDS/CTAB and SDS/Brij35, addition of oppositely charged CTAB reduced the SDS induced EOF more effectively compared to non-ionic Brij35. Using neutral polymer coatings in combination with SDS resulted in 50% reduction in the electroosmotic mobility with  0.1% Hydroxypropyl Methyl Cellulose(HPMC) while including 2% Poly (N,N-dimethylacrylamide)(PDMA) had no effect. These results will potentially contribute to the development of PMMA based microfluidic devices.