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Friction reduction in microchannel flows can help alleviate the inherently taxing pumping power requirements associated with the dimensions involved. One possible way of achieving friction reduction is through the introduction of surface microtexturing that can lead to a superhydrophobic Cassie-Baxter state. The Cassie-Baxter state is characterized by the presence of air pockets within the surface microtexturing believed to act as an effective "shear free" (or at least shear reduced) layer, decreasing the overall friction characteristics of the surface. Most work in this area has concentrated on optimizing the surface microtexturing geometry to maximize the friction reduction effects and overall stability of the Cassie-Baxter state. However, less attention has been paid to the effects of partially wetted conditions induced by pressure and the correlation between the liquid-gas interface location within the surface microtexturing and the microchannel flow characteristics. This is mainly attributed to the difficulty in tracking the interface shape and location within the microtexturing in the typical top-down view arrangements used in most studies. In this paper, a rectangular microchannel with regular microtexturing on the sidewalls is used to visualize and track the location of the air-water interface within the roughness elements. While visually tracking the wetting conditions in the microtextures, pressure drops versus flow rates for each microchannel are measured and analyzed in terms of the non-dimensional friction coefficient. The frictional behavior of the Poiseuille flow suggests that (1) the air-water interface more closely resembles a no-slip boundary rather than a shearfree one, (2) the friction is rather insensitive to the degree of microtexturing wetting, and (3) the fully wetted (Wenzel state) microtexturing provides lower friction than the non-wetted one (Cassie state), in corroboration with observations (1) and (2).

We present a cost-efficient and rapid prototyping technique for polymethylmethacrylate (PMMA) microfluidic devices using a polydimethylsiloxane (PDMS)-based hot embossing process. Compared to conventional hot embossing methods, this technique uses PDMS molds with enhanced thermo-mechanical properties. To improve the replication performance, increases in both PDMS stiffness and hardness were achieved through several processing and curing means. First, the amount of curing agent was increased from 1/10 to 1/5 with respect to the amount of prepolymer. Second, the cured PDMS was thermally aged either over three days at 85 • C or for 30 min at 250 • C. Those combined steps led to increases in stiffness and hardness of up to 150% and 32%, respectively, as compared to standard PDMS molds. Using these enhanced molds, structures with features of the order of 100 μm in PMMA are successfully embossed using a standard laboratory press at 150 • C. The PDMS molds and process produce identical structures through multiple embossing cycles (10) without any mold damage or deterioration. A Y-shaped microfluidic mixer was fabricated with this technique. The successful demonstration of this enhanced PDMS-based hot embossing technique introduces a new approach for the rapid prototyping of polymer-based microfluidic devices at low-cost.

This paper presents a unique optical technique that utilizes the reabsorption and emission of two fluorescent dyes to accurately measure film thickness while minimizing errors caused by variations in illumination intensity and surface reflectivity. Combinations of dyes are selected that exhibit a high degree of emission reabsorption and each dye concentration is adjusted to create an optically thick system where emission reabsorption is intrinsic to the fluorescence of the film being measured. Film thickness information as well as excitation and dye response characteristics are all imbedded in the emission intensities of the dyes. Errors normally associated with laser induced fluorescence based film thickness measurements, including those due to optical distortion, variations in surface reflectivity and excitation non-uniformities, are minimized by observing the ratio of the dye emissions. The principle and constitutive equations characterizing emission reabsorption laser induced fluorescence (ERLIF) film thickness measurement are presented. In addition, film thickness measurements from 5 to 400 µm with 1% accuracy are demonstrated.

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