| With a variety of practical applications and the promising commercial potential, microfluidics has become a fast growing MEMS technology. In a microfluidic system, regardless of open or closed, two-phase environment is rather common. A two-phase system may be essentially different from a single-phase system. Generally, the transport phenomena in a two-phase system are more complicate due to the interactions between dissimilar phases. Because the changes in properties resulting from the phase transition can be beneficial or detrimental to the system, this research will limit itself to demonstrating the positive utilization of interfacial properties to enhance system design and performance.; Interfacial force and wettability of surfaces are studied, as they play vital roles in analyzing nearly all two-phase systems. Their significance is even more obvious when we study two-phase microfluidic systems. Two microfluidic systems were designed specifically to explore the influence of the interfacial force and the wettability of surfaces in evaporative spray cooling and passive gas-liquid separation system. The first system used microstructured silicon surfaces fabricated by deep reactive ion etching (DRIE) as heat transfer surface in water spray evaporative cooling tests. It was designed to create extra interfaces to utilize the capillary force in helping the spreading, thinning and holding of a liquid film on a microfabricated silicon surface to enhance the evaporative heat transfer. Experimental results reveal that capillary effect is a determining factor in evaporative heat transfer on microstructured silicon surfaces. It also shows that a liquid—solid system with a more wettable surface is preferred in evaporative cooling applications.; In the second example, a surface-tension-assisted gas-liquid separation unit is designed to passively separate gas bubbles from a two-phase flow. The system is made of a silicon wafer with a number of etched-through holes and the silicon surface is selectively deposited with a thin layer of hydrophobic polymer to modify the wettability of the surface, and consequently facilitate the gas-liquid separation. The gas-liquid separation system works well. A relation derived from the well-known Young-Laplace equation is proposed to estimate the maximum working pressure of the gas separator. The prediction is reasonably compatible with the experimental values. |
