UC San Diego

An understanding of transport phenomena in microscale and nanoscale channels is critical for a variety of practical applications, such as microfluidic devices for analytical chemical methods or heat dissipation. At such scales, the driving pressure requirements for fluid flow increase substantially and surface roughness effects on transport processes become more pronounced. The use of superhydrophobic (SH) surfaces, created by adding micro- or nano-scale patterns to a hydrophobic surface, can trap air pockets and create decreased resistance to flow. This dissertation considers the effects of such surfaces on fluid dynamics, heat transport and electrokinetics. In particular, the effects of a novel durable polymeric SH surface created through a new scalable roll-coating process on the velocity profile and pressure drop are experimentally and analytically investigated. It is shown that roughness at high air fractions led to a reduced pressure drop and higher velocities. In addition to the modified flow kinematics, the SH surfaces were also used to modulate heat transfer. For example, the air pockets create enhanced flow velocities at the interface that increase heat convection as well as reduce the overall heat conductivity of the substrate. An analytical methodology for characterizing the effects of heat transport in internal laminar flows over ridged patterns was developed using an effective medium approach to model the lowered thermal conductivity. The proposed analytical solutions were verified through comparison with numerical simulations. It was shown that while the convective heat transport was increased, the decrease in the thermal conductivity of the substrate played a larger role in determining the overall heat transfer in the channel. Finally, the electrokinetic phenomenon in microchannels, arising from the charge that forms on a surface in contact with an electrolyte solution, was explored. In electrokinetic flow, the charged fluid and velocity coupling occurs close to the wall, so considerable streaming potential increases could be obtained using SH surfaces. However, employing polymeric SH surfaces with microscale roughness resulted in lowered streaming potentials, showing the importance of the reduced charge of the gas-liquid interface.