Modeling electroosmotic and pressure-driven flows in porous microfluidic devices: Zeta potential and porosity changes near the channel walls

This work presents analytical solutions for both pressure-driven and electroosmotic flows in microchannels incorporating porous media. Solutions are based on a volume-averaged flow model using a scaling of the Navier-Stokes equations for fluid flow. The general model allows analysis of fluid flow in channels with porous regions bordering open regions and includes viscous forces, permitting consideration of porosity and zeta potential variations near channel walls. To obtain analytical solutions problems are constrained to the linearized Poisson-Boltzmann equation and a variation of Brinkman's equation [Appl. Sci. Res., Sect. A 1, 27 (1947); 1, 81 (1947)]. Cases include one continuous porous medium, two adjacent regions of different porosities, or one open channel adjacent to a porous region, and the porous material may have a different zeta potential than that of the channel walls. Solutions are described for two geometries, including flow between two parallel plates or in a cylinder. The model illustrates the relative importance of porosity and zeta potential in different regions of each channel.

Effects of surface morphology on the anchoring and electrooptical dynamics of confined nanoscale liquid crystalline films

The orientation and dynamics of two 40-nm thick films of 4-n-pentyl-4'-cyanobiphenyl (5CB), a nematic liquid crystal, have been studied using step-scan Fourier transform infrared spectroscopy (FTIR). The films are confined in nanocavities bounded by an interdigitated electrode array (IDA) patterned on a zinc selenide (ZnSe) substrate. The effects of the ZnSe surface morphology (specifically, two variations of nanometer-scale corrugations obtained by mechanical polishing) on the initial ordering and reorientation dynamics of the electric-field-induced Freedericksz transition are presented here. The interaction of the 5CB with ZnSe surfaces bearing a spicular corrugation induces a homeotropic (surface normal) alignment of the film confined in the cavity. Alternately, when ZnSe is polished to generate fine grooves along the surface, a planar alignment is promoted in the liquid crystalline film. Time-resolved FTIR studies that enable the direct measurement of the rate constants for the electric-field-induced orientation and thermal relaxation reveal that the dynamic transitions of the two film structures are significantly different. These measurements quantitatively demonstrate the strong effects of surface morphology on the anchoring, order, and dynamics of liquid crystalline thin films.

Stabilization of liquid interface and control of two-phase confluence and separation in glass microchips by utilizing octadecylsilane modification of microchannels

We demonstrated a liquid/liquid and a gas/liquid two-phase crossing flow in glass microchips. A 250-microm-wide microchannel for aqueous-phase flow was fabricated on a top glass plate. Then, as a way to utilize the surface energy difference for stable phase confluence and separation, a 250-microm-wide microchannel for organic-phase (or gas-phase) flow was fabricated on a bottom glass plate and the wall was chemically modified by octadecylsilane (ODS) group. The top and bottom plates were sealed only by pressure. A microchannel pattern was designed so that the two phases made contact at the crossing point of the straight microchannels. The crossing point was observed with an optical microscope. Results showed that the ODS modification of the microchannel wall clearly improved stability of the interface between the two fluids. Pressure difference between fluids was measured and the interface of water and nitrobenzene was stable for the pressure difference from +300 Pa to -200 Pa. The pressure drop in a countercurrent flow configuration was also estimated, and the pressure difference required to realize the counter current flow was less than the allowable pressure range. Finally, we discussed the advantages of utilizing this approach.