(601d) Novel Strategies for Fluid Confinement and Experimental Effects of Pressure-Driven Flow

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, three distinct methods were used to create devices for fluid confinement and the resulting flow was compared to expectations from the continuum model. The methods included fabrication of nanoscale channels as well as fabrication of larger microscale channels either containing nanoporous materials or paired with selectively chosen high molecular weight polymer solutions.

Nanoscale channel devices containing a large array of 10 nm deep channels were fabricated from silicon wafers using MEMs techniques. It was shown that many of the 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 anodic bonding process to enclose the microfluidic system for experimental use. Pressure-driven flow experiments were made possible by strategic channel array design and the use of carefully selected high-pressure fittings, tubing, high-pressure/low flow rate pump equipment, and a custom machined stainless steel pressure housing to prevent device deflection and ensure successful sealing with fluidic tubing. Experiments were performed with a variety of different liquid types, both organic and aqueous, at low flow rates (~10nm/min). The pressure drop through the channel arrays were measured and compared to the results expected from the continuum model.

Single microscale channels were fabricated from silicon wafers following reactive ion etching, anodic bonding, and dicing. High-pressure epoxy was then used to attach and seal fluidic tubing. Channel dimensions were chosen to be large enough to utilize simpler, traditional MEMs techniques with good repeatability. High molecular weight polymer solutions were subsequently chosen for the channel dimensions. Confinement of polymer solutions was assumed when the polymer hydrodynamic radii were less than 10 times smaller than channel depth. Flow enhancement and shear thinning of the polymer solutions with and without confinement were calculated.

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 nanoporous structure as the calcination process was shown to be unsuccessful for in situ synthesis. The success of the in situ SBA-15 synthesis with ozone gas treatment was verified using TEM and FTIR analysis. The success of the in situ colloidal silica crystal synthesis was confirmed using SEM. The final SBA-15 structure contained ~4 nm diameter pores while the porous structure between particles 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 flow experiments of silicon-based microchannels containing SBA-15 and colloidal silica crystals synthesized in situ, varying the pore sizes and geometries of the porous silica materials, extending all studies to analyze a broader set of liquids, and visualizing flow experiments using inverted microscopy.