Hamzeh Bardaweel

Continuous Droplet Removal upon Dropwise Condensation of Humid Air on a Hydrophobic Micropatterned Surface

Combination of two physical phenomena, capillary pressure gradient and wettability gradient, allows a simple two-step fabrication process that yields a reliable hydrophobic self-cleaning condenser surface. The surface is fabricated with specific microscopic topography and further treatment with a chemically inert low-surface-energy material. This process does not require growth of nanofeatures (nanotubes) or hydrophilic–hydrophobic patterning of the surface. Trapezoidal geometry of the microfeatures facilitates droplet transfer from the Wenzel to the Cassie state and reduces droplet critical diameter. The geometry of the micropatterns enhances local coalescence and directional movement for droplets with diameter much smaller than the radial length of the micropatterns. The hydrophobic self-cleaning micropatterned condenser surface prevents liquid film formation and promotes continuous dropwise condensation cycle. Upon dropwise condensation, droplets follow a designed wettability gradient created with micropatterns from the most hydrophobic to the least hydrophobic end of the surface. The surface has higher condensation efficiency, due to its directional self-cleaning property, than a plain hydrophobic surface. We explain the self-actuated droplet collection mechanism on the condenser surface and demonstrate experimentally the creation of an effective wettability gradient over a 6 mm radial distance. In spite of its fabrication simplicity, the fabricated surface demonstrates self-cleaning property, enhanced condensation performance, and reliability over time. Our work enables creation of a hydrophobic condenser surface with the directional self-cleaning property that can be used for collection of biological (chemical, environmental) aerosol samples or for condensation enhancement.

Cyclic operation of a MEMS-based resonant micro heat engine: Experiment and model

The operation of a MEMS-based micro heat engine at resonant and off-resonant conditions is presented. Both model and experiment are used to investigate resonant and off-resonant operation of the engine. In this work, we look at the pressure-volume (PV) diagrams of an engine operated at resonance and off-resonance. Model predictions of the PV diagram are in favorable agreement with measured data. The results show that resonant operation is beneficial. At resonance, the pressure and volume in the engine cavity are decoupled and more mechanical work is observed. The PV diagram describes an elliptical shape. However, for an off-resonant operation the pressure and volume become more coupled and less mechanical work is observed. The PV diagram is described by a sigmoidal shape.

Optimization of the dynamic and thermal performance of a resonant micro heat engine

The dynamic behavior of a flexing membrane micro heat engine is presented. The micro heat engine consists of a cavity filled with a saturated, two-phase working fluid bounded on the top by a flexible expander membrane and on the bottom by a stiff evaporator membrane. A lumped parameter model is developed to simulate the dynamic behavior of the micro heat engine. First, the model is validated against experimental data. Then, the model is used to investigate the effect of the duration of the heat addition process, the mass of the expander membrane and the thermal storage or thermal inertia associated with the engine cavity on the dynamic behavior of the micro engine. The results show the optimal duration for the heat addition process to be less than 10% of the engine cycle period. Increasing the mass of the flexible expander membrane is shown to reduce the resonant frequency of the engine to 130 Hz. Operating the engine at resonance leads to increased power output. The thermal storage or thermal inertia associated with the engine cavity is shown to have a strong effect on engine performance.

Design enhancement and non-dimensional analysis of magnetically-levitated nonlinear vibration energy harvesters

Over the past decade, there has been special interest in developing nonlinear energy harvesters capable of operating over a wideband frequency spectrum. Chief among the nonlinear energy harvesting techniques is magnetic levitation–based energy harvesting. Nonetheless, current nonlinear magnetic levitation–based energy harvesting approaches encapsulate design challenges. This work investigates some of the design issues and limitations faced by traditional magnetic levitation–based energy harvesters such as damping schemes and stiffness nonlinearities. Both experiment and model are used to quantify and evaluate damping regimes and stiffness nonlinearities present in magnetic levitation–based energy harvesters. Results show that dry friction, mostly ignored in magnetic levitation–based energy harvesting literature, contributes to the overall energy dissipation. Measured and modeled magnetic forces–displacement curves suggest that stiffness nonlinearities are weak over moderate distances. An enhanced design utilizing a combination of mechanical and magnetic springs is introduced to overcome some of these limitations. A non-dimensional model of the proposed design is developed and used to investigate the enhanced architecture. The unique potential energy profile suggests that the proposed nonlinear energy harvester outperforms the linear version by steepening the displacement response and shifting the resonance frequency, resulting in a larger bandwidth for which power can be harvested.

Wettability-gradient-driven micropump for transporting discrete liquid drops

In this paper, we report our efforts toward building a microelectromechanical system-based micropump. The micropump is driven by a wettability gradient and used to transport discrete drops. The gradient in wettability is distributed axisymmetrically, with hydrophobicity of the micropump surface decreasing radially toward the center. Both physical and chemical properties of the surface are altered to obtain the wettability gradient needed for driving the drops. The surface of the micropump is, first, patterned with pre-designed micro-features that define the roughness of the surface and, then, coated with a low-energy interface film. Results show that drops deposited on the surface of the micropump move, in a directional way, along the wettability gradient. The average velocity of the deposited drops is 5 mm s−1. Measured contact angles decrease gradually from 157.0° to 124.2° toward the center of the micropump surface. Maximum driving force exerted by the solid surface on the drops is 12.82 µN. The average size of the drops transported on the surface of the micropump is 2 µL.

Displacement transmissibility evaluation of vibration isolation system employing nonlinear-damping and nonlinear-stiffness elements:

Undesired oscillations commonly encountered in engineering practice can be harmful to structures and machinery. Vibration isolation systems are used to attenuate undesired oscillations. Recently, there has been growing interest in nonlinear approaches towards vibration isolation systems design. This work is focused on investigating the effect of nonlinear cubic viscous damping in a vibration isolation system consisting of a magnetic spring with a positive nonlinear stiffness, and a mechanical oblique spring with geometric nonlinear negative stiffness. Dynamic model of the vibration isolation system is obtained and the harmonic balance method (HBM) is used to solve the governing dynamic equation. Additionally, fourth order Runge–Kutta numerical simulation is used to obtain displacement transmissibility of the system under investigation. Results obtained from numerical simulation are in good agreement with those obtained using HBM. Results show that introducing nonlinear damping improves the performance of the vibration isolation system. Nonlinear damping purposefully introduced into the described vibration isolation system appears to eliminate undesired frequency jump phenomena traditionally encountered in quasi-zero-stiffness vibration isolation systems. Compared to its rival linear vibration isolation system, the described nonlinear system transmits less vibrations around resonant peak. At lower frequencies, both nonlinear and linear isolation systems show comparable transmissibility characteristics.

Broadband magnetic levitation-based nonlinear energy harvester

In this work, development of a broadband nonlinear electromagnetic energy harvester is described. The energy harvester consists of a casing housing stationary magnets, a levitated magnet, oblique mechanical springs, and a coil. Magnetic and oblique springs introduce nonlinear behavior into the energy harvester. A mathematical model of the proposed device is developed and validated. The results show good agreement between model and experiment. The significance of adding oblique mechanical springs to the energy harvester design is investigated using the model simulation. The results from the model suggest that adding oblique springs to the energy harvester will improve the performance and increase the frequency bandwidth and amplitude response of the energy harvester.

Retraction: Wettability-gradient-driven micropump for transporting discrete liquid drops

This is a Retraction for the article 2013 J. Micromech. Microeng. 23 035036. The science reported in this article is not incorrect. This article does not include all co-authors who contributed to the work. The article incorrectly attributes work performed at the University of California to the University of Jordan, and fails to acknowledge contributions from Georgia Institute of Technology. This article does not acknowledge the sources of funding for the work and the reference list is incomplete. This article was submitted by Hamzeh K Bardaweel without the knowledge of the other authors.

Building biomarker libraries with novel chemical sensors: correlating differential mobility spectrometer signal outputs with mass spectrometry data

Gas chromatography/mass spectrometry (GC/MS) is a widely used analytic tool for qualitative and quantitative analysis of volatile and semi-volatile compounds. However, GC/MS use is limited by its large size, lack of portability, high cost and inherent complexity. Smaller instruments capable of high-throughput analysis of volatile compounds have the potential of combining MS-like sensitivity with portability. The micromachined differential mobility spectrometer (DMS) is a miniature sensor capable of registering volatile compounds in sub-parts-per-million (ppm) concentrations. It is small, portable, and can be coupled with multiple other compound separation methods. Here we describe paired volatile sample analyses using both GC/MS and GC/DMS which show that the DMS is capable of registering known compounds as verified by MS. Furthermore, we show that MS can be used to help build a library for our unique DMS sensor outputs and detect compounds in chemically complex backgrounds.

Development of Scaling Model for a MEMS-Based Micro Heat Engine

In this work we investigate issues related to scaling of a MEMS-based resonant heat engine. The engine is an external combustion engine made of a cavity encapsulated between two thin membranes. The cavity is filled with saturated liquid-vapor mixture working fluid. We use both model and experiment to investigate scaling of the MEMS-based resonant heat engine. The results suggest that the performance of the engine is determined by three major factors: geometry of the engine, speed of operation, and thermal physical properties of engine components. Larger engine volumes, working fluids with higher latent heat of evaporation, slower engine speeds, and compliant expander structures are shown to be desirable.© 2010 ASME