Mechanocapillary Forming of Filamentary Materials.

The hierarchical structure and organization of filaments within natural materials determine their collective chemical and physical functionalities. Synthetic nanoscale filaments such as carbon nanotubes (CNTs) are known for their outstanding properties including high stiffness and strength at low density, and high electrical conductivity and current carrying capacity. Ordered assemblies of densely packed CNTs are therefore expected to enable the synthesis of new materials having outstanding multifunctional performance. However, current methods of CNT synthesis have inadequate control of quality, density and order. In pursuit of these needs, a new technique called capillary forming is used to manipulate vertically aligned (VA-) CNTs, and to enable their integration in applications ranging from microsystems to macroscale functional films. Capillary forming relies on shape-directed capillary rise during solvent condensation; followed by evaporation-induced shrinkage. Three-dimensional geometric transformations result from the heterogeneous strain distribution within the microstructures during the vapor-liquid-solid interface shrinkage. A portfolio of microscale CNT assemblies with highly ordered internal structure and freeform geometries including straight, bent, folded and helical profiles, are fabricated using this technique. The mechanical stiffness and electrical conductivity of capillary formed CNT micropillars are 5 GPa and 104 S/m respectively. These values are at least hundred-fold higher than as-grown CNT properties, and exceed the properties of typical microfabrication polymers. Responsive CNT-hydrogel composites are prototyped by combining isotropic moisture-induced swelling of the hydrogel with the anisotropic stiffness of CNTs to induce reversible self-directed shape changes of up to 30% stroke. Centimeter scale sheets are fabricated by mechanical rolling and capillary assisted joining of CNTs. The mechanical stiffness, strength and electrical conductivity of CNT sheets are comparable to those of continuous CNT microstructures; and can be tuned by engineering the morphology of the CNT joints. Finally, the applicability of mechanocapillary forming to other nanoscale filaments is demonstrated using silicon nanowires synthesized by metal assisted chemical etching. Further work using the methods developed in this dissertation could enable applications such as directional liquid transport, adhesives, and biosensors; toward an end goal of creating multifunctional surfaces having arbitrary structural, interfacial, and optical responsiveness.Ph.D.Mechanical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/91466/1/stawfick_1.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/91466/2/stawfick_2.pd

Frequency Response and Eddy Current Power Loss in Magneto-Mechanical Transmitters

Magneto-mechanical transmitters offer a compact and low-power solution for the generation of ultra-low frequency (ULF) magnetic signals for through-ground and through-seawater communications. Resonant arrays of smaller magneto-mechanical transmitters are particularly interesting in this context as the physical scaling laws allow for the increase of operating frequency and reduce the power requirements for ULF signal generation. In this work, we introduce a generalized model for accurate prediction of frequency and mode shape in generalized magneto-mechanical resonator arrays (MMRAs) that accounts for near-field magnetic interactions as well as magnetically induced nonlinearity. Using experiments, we demonstrate that our predictive capability is significantly improved compared against simplified dipole approximations. We additionally model the eddy current losses internal to the array and find that they are in agreement with experimental observations.Comment: 12 pages, 8 figures, 6 table

Ultra-tuning of nonlinear drumhead MEMS resonators by electro-thermoelastic buckling

Nonlinear micro-electro-mechanical systems (MEMS) resonators open new opportunities in sensing and signal manipulation compared to their linear counterparts by enabling frequency tuning and increased bandwidth. Here, we design, fabricate and study drumhead resonators exhibiting strongly nonlinear dynamics and develop a reduced order model (ROM) to capture their response accurately. The resonators undergo electrostatically-mediated thermoelastic buckling which tunes their natural frequency from 4.7 to 11.3 MHz, a factor of 2.4x tunability. Moreover, the imposed buckling switches the nonlinearity of the resonators between purely stiffening, purely softening, and even softening-to-stiffening. Accessing these exotic dynamics requires precise control of the temperature and the DC electrostatic forces near the resonator's critical-buckling point. To explain the observed tunability, we develop a one-dimensional physics-based ROM that predicts the linear and nonlinear response of the fundamental bending mode of these drumhead resonators. The ROM captures the dynamic effects of the internal stresses resulting from three sources: The residual stresses from the fabrication process, the mismatch in thermal expansion between the constituent layers, and lastly, the applied electrostatic forces. The ROM replicates the observed tunability of linear (within 5.5% error) and nonlinear responses even near the states of critical buckling. These remarkable nonlinear and large tunability of the natural frequency are valuable features for on-chip acoustic devices in broad applications such as signal manipulation, filtering, and MEMS waveguides

Scaling the Stiffness, Strength, and Toughness of Ceramic‐Coated Nanotube Foams into the Structural Regime

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/108652/1/adfm201400851-sup-0001-S1.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/108652/2/adfm201400851.pd

Magneto-Mechanical Transmitters for Ultra-Low Frequency Near-field Communication

Electromagnetic signals in the ultra-low frequency (ULF) range below 3 kHz are well suited for underwater and underground wireless communication thanks to low signal attenuation and high penetration depth. However, it is challenging to design ULF transmitters that are simultaneously compact and energy efficient using traditional approaches, e.g., using coils or dipole antennas. Recent works have considered magneto-mechanical alternatives, in which ULF magnetic fields are generated using the motion of permanent magnets, since they enable extremely compact ULF transmitters that can operate with low energy consumption and are suitable for human-portable applications. Here we explore the design and operating principles of resonant magneto-mechanical transmitters (MMT) that operate over frequencies spanning a few 10's of Hz up to 1 kHz. We experimentally demonstrate two types of MMT designs using both single-rotor and multi-rotor architectures. We study the nonlinear electro-mechanical dynamics of MMTs using point dipole approximation and magneto-static simulations. We further experimentally explore techniques to control the operation frequency and demonstrate amplitude modulation up to 10 bits-per-second.Comment: 10 pages, 9 figure

Fabrication and electrical integration of robust carbon nanotube micropillars by self-directed elastocapillary densification

Vertically-aligned carbon nanotube (CNT) "forest" microstructures fabricated by chemical vapor deposition (CVD) using patterned catalyst films typically have a low CNT density per unit area. As a result, CNT forests have poor bulk properties and are too fragile for integration with microfabrication processing. We introduce a new self-directed capillary densification method where a liquid is controllably condensed onto and evaporated from CNT forests. Compared to prior approaches, where the substrate with CNTs is immersed in a liquid, our condensation approach gives significantly more uniform structures and enables precise control of the CNT packing density and pillar cross-sectional shape. We present a set of design rules and parametric studies of CNT micropillar densification by this method, and show that self-directed capillary densification enhances the Young's modulus and electrical conductivity of CNT micropillars by more than three orders of magnitude. Owing to the outstanding properties of CNTs, this scalable process will be useful for the integration of CNTs as functional material in microfabricated devices for mechanical, electrical, thermal, and biomedical applications

Polymorphic Elastocapillarity: Kinetically Reconfigurable Self-Assembly of Hair Bundles by Varying the Drain Rate

We report various patterns formed by draining liquid from hair bundles. Hair-like fibers arranged in triangular bundles self-assemble into various cross sections when immersed in liquid then removed. The combinations of their length and the kinetics, represented by the drain rate, lead to various polymorphic self-assemblies: concave hexagonal, triangular, circular, or inverted triangular patterns. The equilibrium of these shapes is predicted by elastocapillarity, the balance between the bending strain energy of the hairs and the surface energy of the liquid. Shapes with a larger strain energy, such as the inverted triangular bundles, are obtained at the higher liquid drain rates. This polymorphic self-assembly is fully reversible by rewetting and draining and can have applications in multifunctional dynamic textures