Publications

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We have demonstrated a novel microfluidic technique for aqueous media, which uses super-hydrophobic materials to create microfluidic channels that are open to the atmosphere. We have demonstrated the ability to perform traditional electrokinetic operations such as ionic separations and electrophoresis using these devices. The rate of evaporation was studied and found to increase with decreasing channel size, which places a limitation on the minimum size of channel that could be used for such a device.

Recent advances in nanoparticle inks have enabled inkjet printing of metal traces and interconnects with very low (100-200°C) process temperatures. This has enabled integration of printable electronics such as antennas and radio frequency identification (RFID) tags with polyimide, teflon, PCBs, and other low temperature substrates. We discuss here printing of nanoparticle inks for three dimensional interconnects, and the apparent mechanism of nanoparticle ink conductivity development at these low process temperatures.

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We have designed and built electrostatically actuated microvalves compatible with integration into a PDMS based microfluidic system. The key innovation for electrostatic actuation was the incorporation of carbon nanotubes into the PDMS valve membrane, allowing for electrostatic charging of the PDMS layer and subsequent discharging, while still allowing for significant distention of the valveseat for low voltage control of the system. Nanoparticles were applied to semi-cured PDMS using a stamp transfer method, and then cured fully to make the valve seats. DC actuation in air of these valves yielded operational voltages as low as 15V, by using a supporting structure above the valve seat that allowed sufficient restoring forces to be applied while not enhancing actuation forces to raise the valve actuation potential. Both actuate to open and actuate to close valves have been demonstrated, and integrated into a microfluidic platform, and demonstrated fluidic control using electrostatic valves.

We have designed and built electrostatically actuated microvalves compatible with integration into a PDMS based microfluidic system. The key innovation for electrostatic actuation was the incorporation of carbon nanotubes into the PDMS valve membrane, allowing for electrostatic charging of the PDMS layer and subsequent discharging, while still allowing for significant distention of the valveseat for low voltage control of the system. Nanoparticles were applied to semi-cured PDMS using a stamp transfer method, and then cured fully to make the valve seats. DC actuation in air of these valves yielded operational voltages as low as 15V, by using a supporting structure above the valve seat that allowed sufficient restoring forces to be applied while not enhancing actuation forces to raise the valve actuation potential. Both actuate to open and actuate to close valves have been demonstrated, and integrated into a microfluidic platform, and demonstrated fluidic control using electrostatic valves.

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A gecko's extraordinary ability to suspend itself from walls and ceilings of varied surface roughness has interested humans for hundreds of years. Many theories and possible explanations describing this phenomenon have been proposed including sticky secretions, microsuckers, and electrostatic forces; however, today it is widely accepted that van der Waals forces play the most important role in this type of dry adhesion. Inarguably, the vital feature that allows a gecko's suspension is the presence of billions 3 of tiny hairs on the pad of its foot called spatula. These features are small enough to reach within van der Waals distances of any surface (spatula radius %7E100 nm); thus, the combined effect of billions of van der Waals interactions is more than sufficient to hold a gecko's weight to surfaces such as smooth ceilings or wet glass. Two lithographic approaches were used to make hierarchal structures with dimensions similar to the gecko foot dimensions noted above. One approach combined photo-lithography with soft lithography (micro-molding). In this fabrication scheme the fiber feature size, defined by the alumina micromold was 0.2 um in diameter and 60 um in height. The second approach followed more conventional photolithography-based patterning. Patterned features with dimensions %7E0.3 mm in diameter by 0.5 mm tall were produced. We used interfacial force microscopy employing a parabolic diamond tip with a diameter of 200 nm to measure the surface adhesion of these structures. The measured adhesive forces ranged from 0.3 uN - 0.6 uN, yielding an average bonding stress between 50 N/cm2 to 100 N/cm2. By comparison the reported literature value for the average stress of a Tokay gecko foot is 10 N/cm2. Acknowledgements This work was funded by Sandia National Laboratory's Laboratory Directed Research & Development program (LDRD). All coating processes were conducted in the cleanroom facility located at the University of New Mexico's Center for High Technology Materials (CHTM). SEM images were performed at UNM's Center for Micro-Engineering on equipment funded by a NSF New Mexico EPSCoR grant. 4

Low solid interfacial energy and fractally rough surface topography confer to Lotus plants superhydrophobic (SH) properties like high contact angles, rolling and bouncing of liquid droplets, and self-cleaning of particle contaminants. This project exploits the porous fractal structure of a novel, synthetic SH surface for aerosol collection, its self-cleaning properties for particle concentration, and its slippery nature 3 to enhance the performance of fluidic and MEMS devices. We propose to understand fundamentally the conditions needed to cause liquid droplets to roll rather than flow/slide on a surface and how this %22rolling transition%22 influences the boundary condition describing fluid flow in a pipe or micro-channel. Rolling of droplets is important for aerosol collection strategies because it allows trapped particles to be concentrated and transported in liquid droplets with no need for a pre-defined/micromachined fluidic architecture. The fluid/solid boundary condition is important because it governs flow resistance and rheology and establishes the fluid velocity profile. Although many research groups are exploring SH surfaces, our team is the first to unambiguously determine their effects on fluid flow and rheology. SH surfaces could impact all future SNL designs of collectors, fluidic devices, MEMS, and NEMS. Interfaced with inertial focusing aerosol collectors, SH surfaces would allow size-specific particle populations to be collected, concentrated, and transported to a fluidic interface without loss. In microfluidic systems, we expect to reduce the energy/power required to pump fluids and actuate MEMS. Plug-like (rather than parabolic) velocity profiles can greatly improve resolution of chip-based separations and enable unprecedented control of concentration profiles and residence times in fluidic-based micro-reactors. Patterned SH/hydrophilic channels could induce mixing in microchannels and enable development of microflow control elements. Acknowledgements This work was funded by Sandia National Laboratory's Laboratory Directed Research & Development program (LDRD). Some coating processes were conducted in the cleanroom facility located at the University of New Mexico's Center for High Technology Materials (CHTM). SEM images were performed at UNM's Center for Micro-Engineering on equipment funded by a NSF New Mexico EPSCoR grant. 4

In order to develop the next generation of high peak intensity lasers, new grating technology providing higher damage thresholds and large apertures is required. The current assumption is that this technical innovation will be multilayer dielectric gratings, wherein the uppermost layer of a thin film mirror is etched to create the desired binary phase grating. A variant of this is explored with the upper grating layer being a lower density gelatin-based volume phase grating in either sol-gel or dichromated gelatin. One key benefit is the elimination of the etching step.