Engineering and modeling of microfluidic flow using press -fit microdevices

This thesis describes an approach that is particularly useful for prototyping new devices and systems for studies of fundamentals of microfluidics. By press-fitting microfibers with an elastomer, polydimethylsiloxane (PDMS), microchannels with various features can be rapidly constructed on top of a substrate. Proof of concept was first provided by glass fibers to produce microchannels that allow static mixing and protein separation through capillary flow. Other areas explored include the studies of microfluidic transport, manipulation of particles, and the influence of surface chemistry when devices are scaled down to microscopic level. The use of surface wettability of a hydrophilic glass microfiber and hydrophobicity of surrounding microchannel surfaces provides a means where a narrow band of liquid can be directed to flow next to the fiber. The discovery of this phenomenon not only yields fundamental insights but also enables many applications. A framework based on careful theoretical evaluation, modeling, simulation, and experimental observation is constructed to explain this phenomena. It is discovered that these intriguing surface wetting effects are due to the close proximity of hydrophilic and hydrophobic surfaces that enable a surface tension gradient to be formed which resulted in a stable boundary layer of moving fluid. The rapid nature of press-fit techniques also allows one to prototype microfluidic biosensors for the detection of pathogenic bacterial cells. The resulting hydrophobic microchannel enables fluid containing particles to be directed to flow in a 20 micron or less wide boundary layer next to the fiber. When illuminated with a photon tube, the particles are sufficiently illuminated to be counted. This device has been used to differentiate pathogenic E. coli from non-pathogenic ones. However, application is not limited to bacteria detection. There are wide varieties of microfibers available with different geometry, chemistry, and conductivity that not only create channels but can also be utilized as stationary objects for reaction, extraction, separation, actuation or sensing