Electroosmotic Flow in Microchannel with Black Silicon Nanostructures

Abstract

Although electroosmotic flow (EOF) has been applied to drive fluid flow in microfluidic chips, some of the phenomena associated with it can adversely affect the performance of certain applications such as electrophoresis and ion preconcentration. To minimize the undesirable effects, EOF can be suppressed by polymer coatings or introduction of nanostructures. In this work, we presented a novel technique that employs the Dry Etching, Electroplating and Molding (DEEMO) process along with reactive ion etching (RIE), to fabricate microchannel with black silicon nanostructures (prolate hemispheroid-like structures). The effect of black silicon nanostructures on EOF was examined experimentally by current monitoring method, and numerically by finite element simulations. The experimental results showed that the EOF velocity was reduced by 13 ± 7%, which is reasonably close to the simulation results that predict a reduction of approximately 8%. EOF reduction is caused by the distortion of local electric field at the nanostructured surface. Numerical simulations show that the EOF velocity decreases with increasing nanostructure height or decreasing diameter. This reveals the potential of tuning the etching process parameters to generate nanostructures for better EOF suppression. The outcome of this investigation enhances the fundamental understanding of EOF behavior, with implications on the precise EOF control in devices utilizing nanostructured surfaces for chemical and biological analyses.

Keywords: current monitoring method; electroosmotic flow; finite element method; injection molding; micro-/nanofabrication; reactive ion etching.

Figure 1

(a) 3-D exploded view diagram of micro-/nanofluidic device. (b) Schematic of microchannel designs with/without black silicon nanostructures. (c) Atomic force microscope (AFM) image of black silicon nanostructures on the bottom wall of the microchannel.

Figure 2

Schematics of Dry Etching, Electroplating and Molding (DEEMO) fabrication process for microchannel with large-area of black silicon nanostructures and smooth microchannel.

Figure 3

(a) Experimental setup for current monitoring technique. (b) Current-time curve for 0.95 mM NaHCO₃ displaced 1 mM NaHCO₃ in smooth microchannel.

Figure 4

(a) Fluid segment sliced from microchannel for simulation. (b) 3-D simulation domain.

Figure 5

(a) Displacement times, (b) electroosmotic flow (EOF) velocities and magnitude of effective zeta potentials for 1 mM, 5 mM and 10 mM of NaHCO₃ for microchannel with black silicon nanostructures, in comparison to smooth microchannel.

Figure 6

Simulated electric field lines for (a) smooth microchannel and (b) microchannel with black silicon nanostructures, for 1 mM of NaHCO₃.

Figure 7

Simulated EOF velocity profile for (a) smooth microchannel and (b) microchannel with black silicon nanostructures, for 1 mM of NaHCO₃.

Figure 8

Experimental and numerical average EOF velocities of 1 mM NaHCO₃ for microchannel with black silicon nanostructures, in comparison to smooth microchannel.

Figure 9

Variation of numerical average EOF velocity for 1 mM NaHCO₃ with (a) height h where diameter d = 270 nm and spatial distance s = 350 nm, and (b) d where h = 175 nm and s = 350 nm.