Matthew McCarthy

Role of wickability on the critical heat flux of structured superhydrophilic surfaces.

While superhydrophilic coatings with enhanced wetting properties have been shown to increase the pool boiling critical heat flux (CHF), the role of nanostructures on its enhancement is not clear. Here, biological templates have been used to demonstrate that wickability is the single factor dictating CHF on structured superhydrophilic surfaces. The flexibility of biotemplating using the Tobacco mosaic virus has been leveraged to create surfaces with varying scales, morphologies, and roughness factors. Their wickabilities have been quantified via the wicked volume flux, a phenomenological parameter analogous to the contact angle, and the role of wickability on CHF has been demonstrated using data from over three dozen individual surfaces. These results are repeatable and independent of the substrate material, surface fouling, structure material, morphology, and contact angle as well as the structure scale. An experimentally validated correlation for CHF has been reported on the basis of the dimensionless wickability. Additionally, the surfaces have achieved a CHF of 257 W/cm(2) for water, representing the highest reported value to date for superhydrophilic surfaces. While the role of wickability on CHF has often been cited anecdotally, this work provides a quantitative measure of the phenomena and provides a framework for designing and optimizing coatings for further enhancement.

Hierarchical Three-Dimensional Microbattery Electrodes Combining Bottom-Up Self-Assembly and Top-Down Micromachining

The realization of next-generation portable electronics and integrated microsystems is directly linked with the development of robust batteries with high energy and power density. Three-dimensional micro- and nanostructured electrodes enhance energy and power through higher surface area and thinner active materials, respectively. Here, we present a novel approach for the fabrication of hierarchical electrodes that combine benefits of both length scales. The electrodes consist of self-assembled, virus-templated nanostructures conformally coating three-dimensional micropillars. Active battery material (V(2)O(5)) is deposited using atomic layer deposition on the hierarchical micro/nanonetwork. Electrochemical characterization of these electrodes indicates a 3-fold increase in energy density compared to nanostructures alone, in agreement with the surface area increase, while maintaining the high power characteristics of nanomaterials. Investigation of capacity scaling for varying active material thickness reveals underlying limitations in nanostructured electrodes and highlights the importance of our method in controlling both energy and power density with structural hierarchy.

Nanostructured nickel electrodes using the Tobacco mosaic virus for microbattery applications

The development of nanostructured nickel–zinc microbatteries utilizing the Tobacco mosaic virus (TMV) is presented in this paper. The TMV is a high aspect ratio cylindrical plant virus which has been used to increase the active electrode area in MEMS-fabricated batteries. Genetically modifying the virus to display multiple metal binding sites allows for electroless nickel deposition and self-assembly of these nanostructures onto gold surfaces. This work focuses on integrating the TMV deposition and coating process into standard MEMS fabrication techniques as well as characterizing nickel–zinc microbatteries based on this technology. Using a microfluidic packaging scheme, devices with and without TMV structures have been characterized. The TMV modified devices demonstrated charge–discharge operation up to 30 cycles reaching a capacity of 4.45 µAh cm−2 and exhibited a six-fold increase in capacity during the initial cycle compared to planar electrode geometries. The effect of the electrode gap has been investigated, and a two-fold increase in capacity is observed for an approximately equivalent decrease in electrode spacing.

Biotemplated hierarchical surfaces and the role of dual length scales on the repellency of impacting droplets

We fabricated biomimetic hierarchical superhydrophobic surfaces using the Tobacco mosaic virus and investigated the role of each length scale during droplet impact by decomposing the micro and nanoscale components. We found that 10 μl water droplets rebounded at impact velocities greater than 4.3 m/s on the hierarchical surfaces, outperforming the nanostructured surfaces, which underwent an observable wetting transition at an impact velocity of 2.7 m/s. This finding demonstrates that each length scale plays a distinct, but complementary, role in maximizing water repellency during droplet impact and, thus, provides insight into the evolutionary development of highly water-repellant hierarchical plant leaves.

Tobacco mosaic virus: A biological building block for micro/nano/bio systems

Tobacco mosaic virus (TMV) has the potential to be an ideal candidate for a building block of the next-generation micro/nano/bio systems. The TMV virion is a high-aspect ratio rigid nanotube that is robust and compatible with some conventional microfabrication processes. TMV can be chemically and genetically modified to enhance its physical properties and tailor them to specific applications. This review covers the use of TMV nanostructures in a wide range of micro/nano/bio systems. TMV has been utilized in the production of nanowires, nanostructured thin films, biomimetic surfaces, novel sensors, high performance microbatteries, solid-state electronics, and engineered biosystems. The work highlighted here is meant to give a perspective of the entire breadth of the properties of these virions, from their synthesis and functionalization to assembly and patterning, as well as feature works that represent key milestones in the field of biofabrication and biomaterial integration. The advantages already demons...

Dynamic Friction and Wear in a Planar-Contact Encapsulated Microball Bearing Using an Integrated Microturbine

The demonstration and characterization of a novel planar-contact encapsulated microball bearing using a radial in-flow microturbine are presented. Stable operation of the air-driven silicon microturbine is shown for over 1 000 000 revolutions at speeds, pressure drops, and flow rates of up to 10 000 r/min, 0.45 lbf/in2, and 3.5 slm, respectively. Incorporation of a gas thrust plenum using a novel packaging scheme has enabled comprehensive spin-down friction characterization of the encapsulated microball bearing. An empirical power-law model for dynamic friction has been developed for speeds of 250-5000 r/min and loads of 10-50 mN, corresponding to torques of 0.0625-2.5 muNldrm and friction torque constants of 2.25-5.25 x 10-4 muNldrm/ r/min. The onset and effect of wear and wear debris have been studied, showing negligible wear in the load bearing surfaces for the operating conditions considered.

Increasing Boiling Heat Transfer using Low Conductivity Materials.

We report the counterintuitive mechanism of increasing boiling heat transfer by incorporating low-conductivity materials at the interface between the surface and fluid. By embedding an array of non-conductive lines into a high-conductivity substrate, in-plane variations in the local surface temperature are created. During boiling the surface temperature varies spatially across the substrate, alternating between high and low values, and promotes the organization of distinct liquid and vapor flows. By systematically tuning the peak-to-peak wavelength of this spatial temperature variation, a resonance-like effect is seen at a value equal to the capillary length of the fluid. Replacing ~18% of the surface with a non-conductive epoxy results in a greater than 5x increase in heat transfer rate at a given superheat temperature. This drastic and counterintuitive increase is shown to be due to optimized bubble dynamics, where ordered pathways allow for efficient removal of vapor and the return of replenishing liquid. The use of engineered thermal gradients represents a potentially disruptive approach to create high-efficiency and high-heat-flux boiling surfaces which are naturally insensitive to fouling and degradation as compared to other approaches.

Self-Organization of Microscale Condensate for Delayed Flooding of Nanostructured Superhydrophobic Surfaces

Superhydrophobic surfaces enhance condensation by inhibiting the formation of an insulating liquid layer. While this produces efficient heat transfer at low supersaturations, superhydrophobicity has been shown to break down at increased supersaturations. As heat transfer increases, the random distribution and high density of nucleation sites produces pinned droplets, which lead to uncontrollable flooding. In this work, engineered variations in wettability are used to promote the self-organization of microscale droplets, which is shown to effectively delay flooding. Virus-templated superhydrophobic surfaces are patterned with an array of superhydrophilic islands designed to minimize surface adhesion while promoting spatial order. By use of optical and electron microscopy, the surfaces are optimized and characterized during condensation. Mixed wettability imparts spatial order not only through preferential nucleation but more importantly through the self-organization of coalescing droplets at high supersaturations. The self-organization of microscale droplets (diameters of <25 μm) is shown to effectively delay flooding and govern the global wetting behavior of larger droplets (diameters of >1 mm) on the surface. As heat transfer increases, the surfaces transition from jumping-mode to shedding-mode removal with no flooding. This demonstrates the ability to engineer surfaces to resist flooding and can act as the basis for developing robust superhydrophobic surfaces for condensation applications.

Full-Field Dynamic Characterization of Superhydrophobic Condensation on Biotemplated Nanostructured Surfaces

While superhydrophobic nanostructured surfaces have been shown to promote condensation heat transfer, the successful implementation of these coatings relies on the development of scalable manufacturing strategies as well as continued research into the fundamental physical mechanisms of enhancement. This work demonstrates the fabrication and characterization of superhydrophobic coatings using a simple scalable nanofabrication technique based on self-assembly of the Tobacco mosaic virus (TMV) combined with initiated chemical vapor deposition. TMV biotemplating is compatible with a wide range of surface materials and applicable over large areas and complex geometries without the use of any power or heat. The virus-structured coatings fabricated here are macroscopically superhydrophobic (contact angle >170°) and have been characterized using environmental electron scanning microscopy showing sustained and robust coalescence-induced ejection of condensate droplets. Additionally, full-field dynamic characterization of these surfaces during condensation in the presence of noncondensable gases is reported. This technique uses optical microscopy combined with image processing algorithms to track the wetting and growth dynamics of 100s to 1000s of microscale condensate droplets simultaneously. Using this approach, over 3 million independent measurements of droplet size have been used to characterize global heat transfer performance as a function of nucleation site density, coalescence length, and the apparent wetted surface area during dynamic loading. Additionally, the history and behavior of individual nucleation sites, including coalescence events, has been characterized. This work elucidates the nature of superhydrophobic condensation and its enhancement, including the role of nucleation site density during transient operation.

A rotary microactuator supported on encapsulated microball bearings using an electro-pneumatic thrust balance

The development of a rotary microactuator supported on encapsulated microball bearings and driven by electro-pneumatic actuation is reported. The encapsulated bearing provides full support to an encased rotor, while an electro-pneumatic thrust balance is used to minimize normal load, and therefore bearing friction. Experimental results show excellent agreement with predictions, demonstrating the ability to operate at optimal normal loads up to 2000rpm. This is the first demonstration of a ball bearing supported electrostatic microactuator with a fully encased rotor, capable of being integrated within practical microsystems. The fully-supported rotor allows for direct mechanical attachment or reliable interaction with external media and is a necessity for useful implementation.