Effective shear displacement on lateral adhesion force of a liquid bridge between separated plates

Adhesion force is among the most influencing factors in micro- and nano-mechanics. A liquid bridge between two bodies gives rise to the adhesion force, which usually acts as additional normal load. However, the adhesion force acts also in lateral. We measured the lateral adhesion force of a sheared liquid bridge between parallel plates. In addition, movement of contact lines is tracked by using an image processing technique, which allowed us to introduce an effective shear displacement. The lateral adhesion force has a linear relation with the effective shear displacement. It shows good agreement between experimental result and the analytical approach regarding changes of interfacial energy with simple rectangular shape of the liquid bridge. We further revealed that there is no contact line in pinned state even in the case with the very beginning of the sheared process. In this regard, however, the contact line on rougher surface is awkward in its movement. Therefore, the liquid bridge between two rougher surfaces has higher effective shear displacement, and it results in the higher lateral adhesion force

Review on the Modeling of Electrostatic MEMS

Electrostatic-driven microelectromechanical systems devices, in most cases, consist of couplings of such energy domains as electromechanics, optical electricity, thermoelectricity, and electromagnetism. Their nonlinear working state makes their analysis complex and complicated. This article introduces the physical model of pull-in voltage, dynamic characteristic analysis, air damping effect, reliability, numerical modeling method, and application of electrostatic-driven MEMS devices

Mechanical and geometric considerations for the airgapless motor

Indiana University-Purdue University Indianapolis (IUPUI)The purpose of this thesis is to perform modeling from different perspectives for an airgapless motor. The airgapless motor is a proposed type of electric machine whose purpose is to replace hydraulic machines in low speed high torque applications. Because of the nature of the movement for this device, modeling of this device is atypical to the modeling done with other electric machines. This thesis will present the operating principle of the airgapless motor and take an analytical approach to modeling the torque and total energy in the device. In addition, this thesis will present the power electronics necessary to drive this device and offer recommendations to maximize the torque and minimize the torque ripple. MATLAB simulations are used to verify that the conclusion of this thesis are consistent with observations made by previous publications and prototypes

Microturbopump Utilizing Microball Bearings

This dissertation presents the development of a microfabricated turbopump capable of delivering fuel with the flow rates and pressures required for portable power generation. The device is composed of a spiral-groove viscous pump that is driven by a radial in-flow air turbine and supported using a novel encapsulated microball bearing. First, the encapsulated microball bearing and methods to investigate the wear and friction behaviors were developed. Two primary raceway designs, point-contact and planar-contact designs, were developed with the key design factor being wearing of the raceway. A modification to the planar-contact design was made for the final turbopump that reduced both wear and debris generation. Second, two air turbine platforms were developed using the encapsulated microball bearings to characterize both the bearing and the turbine drive mechanism. A tangential air turbine platform was first developed and characterized using the point-contact bearing mechanism. Rotational speeds >37,000 rpm were demonstrated and long-term operation (>24 hours) using this platform, but with large driving pressures (tens of psi) and large raceway wear (tens of microns). Furthermore, the circumferential asymmetry of the turbine design led to difficulty in measuring pressure distribution and sealing for pump applications. Results from the tangential air turbine platform led to an axisymmetric radial in-flow air turbine platform using a planar-contact bearing design. Rotational speeds greater than 85,000 rpm with turbine pressure differentials in the range of 1 psi were demonstrated using this platform. The wear of the raceway was observed to be on the order of single microns (a 10x improvement). The radial in-flow air turbine platform allowed an empirical model to be developed relating the friction torque to the rotational speed and load for the planar-contact bearing. This enabled calculation of the power balance for pumping and a method to characterize future bearing designs and materials. Lastly, a microfabricated turbopump was demonstrated based on a spiral-groove viscous pump and the radial in-flow turbine platform using the planar-contact bearing. Pumping operation was demonstrated with a differential pressure up to +0.3 psi and flow rates ranging from 35 mL/hour to 70 mL/hour, within the range relevant to portable power generation

Design, Fabrication, and Characterization of a Rotary Variable-Capacitance Micromotor Supported on Microball Bearings

The design, fabrication, and characterization of a rotary micromotor supported on microball bearings are reported in this dissertation. This is the first demonstration of a rotary micromachine with a robust mechanical support provided by microball-bearing technology. One key challenge in the realization of a reliable micromachine, which is successfully addressed in this work, is the development of a bearing that would result in high stability, low friction, and high resistance to wear. A six-phase, rotary, bottom-drive, variable-capacitance micromotor is designed and simulated using the finite element method. The geometry of the micromotor is optimized based on the simulation results. The development of the rotary machine is based on studies of fabrication and testing of linear micromotors. The stator and rotor are fabricated separately on silicon substrates and assembled with the stainless steel microballs. Three layers of low-k benzocyclobutene (BCB) polymer, two layers of gold, and a silicon microball housing are fabricated on the stator. The BCB dielectric film, compared to conventional silicon dioxide insulating films, reduces the parasitic capacitance between electrodes and the stator substrate. The microball housing and salient structures (poles) are etched in the rotor and are coated with a silicon carbide film to reduce friction. A characterization methodology is developed to measure and extract the angular displacement, velocity, acceleration, torque, mechanical power, coefficient of friction, and frictional force through non-contact techniques. A top angular velocity of 517 rpm corresponding to the linear tip velocity of 324 mm/s is measured. This is 44 times higher than the velocity achieved for linear micromotors supported on microball bearings. Measurement of the transient response of the rotor indicated that the torque is 5.620.5 micro N-m which is comparable to finite element simulation results predicting 6.75 micro N-m. Such a robust rotary micromotor can be used in developing micropumps which are highly demanded microsystems for fuel delivery, drug delivery, cooling, and vacuum applications. Micromotors can also be employed in micro scale surgery, assembly, propulsion, and actuation

Symmetry engineering via angular control of layered van der Waals heterostructures

Crystal symmetry and elemental composition play a critical role in determining the physical properties of materials. In layered van der Waals (vdW) heterostructures, a two-dimensional (2D) material layer can be influenced by interactions between adjacent layers, dictating that the measured properties of the combined system will be in part derived from the geometric structure within the active layers. This thesis examines active crystal symmetry tuning in composite heterostructures of two-dimensional (2D) materials, engineered via nanomechanically assisted twist angle control, and designed by careful consideration of lowest energy stacking configurations. The material systems, devices, and experimental setups described in this thesis constitute a platform featuring highly programmable properties that are on-demand and reversible. Two prototypical systems are discussed in detail. The first is graphene encapsulated between boron nitride (BN) crystals, wherein the alignment state between the three layers is controlled. The second is the same system, but with no graphene between the encapsulating BN layers. In both systems, a long-wavelength geometric interference pattern, also known as a moiré pattern, forms between the adjacent crystals as a consequence of lattice-constant mismatch and twist angle. The moiré pattern caries its own symmetry properties that are also demonstrated to be tunable, and can be thought of as an artificially constructed superlattice of periodic potential with wavelength much greater than the lattice constants of the constituent layers. In the BN-encapsulated graphene system we show drastic tunability of band gaps at primary and secondary Dirac points (PDP and SDPs) indicating reversible on-demand inversion symmetry breaking, as well as evidence of dual coexisting moiré superlattices and additional higher-order interference patterns that form between them. The all-BN system shows substantial enhancement and suppression of second harmonic generation (SHG) response from the vdW interface between the BN crystals when the quadrupole component of the SHG response is engineered to be minimal, by controlling for total layer number and layer number parity. Changes in the physical properties of each composite system are measured with a combination of electronic transport measurements, and optical measurements (Raman and SHG), as well as piezo-force microscopy (PFM) measurements that give direct imaging of the moiré pattern. A number of invented and adapted fabrication and actuation techniques for controlling the twist angle of a bulk vdW crystal are discussed, and in the latter portion of this thesis these techniques are extended to include actuation of monolayer flakes of 2D crystals. In this discussion several case studies are discussed, including twist angle control for a single sample monolayer tungsten diselenide on monolayer molybdenum diselenide, as well as twist angle control for twisted bilayer graphene and graphene on BN. Additionally, a novel in-plane bending mode for graphene on BN is demonstrated using similar techniques. Further discussion of actuation via traditional electrostatic MEMS techniques is also included, illustrating complete on-chip control for on-demand nanomechanical actuation of 2D materials in vdW heterostructures