(185h) In Situ Synthesis of Nanostructures in Microfluidic Devices for Pressure-Driven Studies of Fluids in Confinement

Many fundamental properties and governing equations of fluid mechanics are invalid for fluids confined within geometrical constructs of less than ~12 nm in size. There is a great need to develop a new fundamental understanding of the transport and interfacial properties of fluids at this scale. However, experimental studies of fluids in confinement are extremely difficult to perform as appropriate nanoscale systems are difficult to create and characterize, and fluid properties on this scale are difficult to visualize and measure. Previous experimental and computational studies have been performed to address this issue, but their results have largely disagreed.

In this work, two distinct methods were used to create fluidic devices with nanoscale features. These included traditional MEMs fabrication techniques for creation of nanoscale channels from silicon wafers and low-cost techniques to create larger microscale channels that were then packed with materials consisting of nanoporous structures. Silicon wafers were used to create devices containing a large array of 10 nm deep channels. It was shown that many common MEMs techniques are not appropriate for devices of this scale as variations in channel depths can be significantly greater than 10 nm. Careful consideration of surface chemistry resulting from thin layer deposition of different materials and selective chemical etching was shown to successfully produce channels with consistent 10 nm depths, even after the bonding process to enclose the microfluidic system for experimental use.

Simple xerographic and soft lithographic techniques were employed to create larger, low-cost microchanels to serve as substrates for proof of successful in situ synthesis of the nanoporous silica material SBA-15 and colloidal silica crystals. These silica materials allowed for variation of pore sizes as well as the geometries of the resulting porous networks. In situ synthesis of these materials was determined to be essential for seamless synthesis and the elimination of post synthesis bonding which often leads to material and device failure. In situ synthesis of the SBA-15 material was completed following a sol-gel process with the additional, novel use of ozone gas treatment in place of the traditional calcination step to hollow out the silica pores and finalize the porous structure network. The calcination process was shown to be unsuccessful with in situ synthesis. The success of the ozone gas treatment and overall in situ SBA-15 synthesis was verified using TEM and FTIR analysis and the success of the in situ colloidal silica crystal synthesis was confirmed using SEM. The final structures of the porous network of the SBA-15 was ~7 nm in diameter and that of the colloidal crystals ranged from ~7 nm to ~10 nm in diameter depending on the choice of silica particle size.

Future research work includes expanding the 10 nm deep silicon wafer channel array device, filling the deeper channels with SBA-15 and colloidal silica crystals synthesized in situ, varying the pore sizes and geometries of the porous silica materials, extending the studies to analyze a broader set of , and visualizing experiments using inverted microscopy.