J. Darabi

An electrohydrodynamic polarization micropump for electronic cooling

This paper presents the design, fabrication, and characterization of an innovative microcooling device for microelectronics applications. The device incorporates an active evaporative cooling surface, a polarization micropump, and temperature sensors into a single chip. The micropump provides the required pumping action to bring the working fluid to the evaporating surface, allowing the effective heat transfer coefficient through a thin-film evaporation/boiling process. The device is based on VLSI microfabrication technology, allowing the electrohydrodynamic (EHD) electrodes to be integrated directly onto the cooling surface. Since the EHD electrodes are fabricated using the same technology as the electronic systems themselves, the proposed microelectronic cooling system in the form of an integrated microchip is very suitable for mass production. The prototype devices demonstrated a maximum cooling capacity of 65 W/cm/sup 2/ with a corresponding pumping head of 250 Pa. The results of this investigation will assist in the development of future microcooling devices capable of operating at high power levels.

Development of an electrohydrodynamic injection micropump and its potential application in pumping fluids in cryogenic cooling systems

Cryogenic cooling has become a widely adopted technique to improve the performance of electronics and sensors. A potential application of an electrohydrodynamic (EHD) pumping system is its use in pumping fluids in cryogenic cooling systems. In this paper, we present the results of a theoretical/experimental investigation to study the feasibility of using an EHD injection micropump for pumping liquid nitrogen. First, the mechanisms of charge transport and ionization phenomenon in cryogenic liquids are discussed. Next, the design and fabrication of an EHD injection micropump that employs an array of interdigitated saw-tooth/plane electrodes are described. Finally, experimental results and observations are presented. An asymmetric saw-tooth/plane geometry was designed to achieve a strong inhomogeneous electric field. Each emitter electrode had a base width of 10 /spl mu/m. Each tooth on the emitter electrode had a base length of 10 /spl mu/m with a tip angle of 60/spl deg/. The collector electrode consisted of a planar strip with a width of 10 /spl mu/m. The gap between emitter and collector electrodes was 20 /spl mu/m. The distance between each neighboring stage (a pair of emitter and collector electrodes) was 40 /spl mu/m. The patterned area was 10 mm by 20 mm allowing approximately 200 stages to be fabricated along the length of the micropump. The maximum pressure head achieved by this micropump in the absence of a net flow was 550 and 205 Pa for 3Ms HFE-7100 thermal fluid and liquid nitrogen, respectively. Also, the maximum mass flow rate was 3.9 g/min at the generated pressure of 180 Pa during a closed loop test with HFE-7100.

Droplet ejection performance of a monolithic thermal inkjet print head

This paper presents a simulation study of the droplet ejection performance of a thermal inkjet print head. The geometry of the print head comprises a dome-shaped ink chamber, a nozzle guide and a ring-shaped heater integrated on each chamber. The design eliminates direct contact between the heater and the ink, thus minimizing heater burnout. The ink manifold, ink chamber and nozzle are aligned, thus facilitating higher nozzle density. The model simulates thermal bubble dynamics including nucleation and growth of thermal bubbles caused by a thermal pulse. The model was validated by comparing model predictions with experimental results for a previously reported print head design. Then, the model was used to simulate the droplet ejection performance of the proposed inkjet print head. Effects of print head geometry including nozzle diameter, nozzle length, chamber size, heater dimensions and location, thermal conductivity of the passivation layer, operating conditions including total thermal energy and pulse width, properties of the ink including density, viscosity and surface tension on the performance of the inkjet device are investigated. The influence of these parameters on the drop volume and velocity, threshold energy and tail length of the ejected droplets is studied.

Modeling and characterization of a carbon fiber emitter for electrospray ionization

A novel microscale emitter employing a pointed carbon fiber (CF) can be used for electrospraying in mass spectrometric (MS) analysis. The carbon fiber is located coaxial with a fused silica capillary tube of 360 µ mO D and 75 µm ID with its sharp tip extending 30 µm beyond the tube terminus. The electrospray ionization (ESI) process is simulated using a computational fluid dynamics (CFD) code based on the Taylor–Melcher leaky-dielectric fluid model for solving the electrohydrodynamics and the volume of fluid (VOF) approach for tracking the liquid–gas interface. The CFD code is first validated for a conventional geometry and then used to simulate the CF emitter based ESI model. The simulated current–flow and current–voltage results are in good agreement with experimental results for the CF emitter. The effects of emitter geometry, potential difference, flow rate and the physical properties of the liquid on the electrospray behavior of the CF emitter are thoroughly investigated. The spray current and jet diameter are correlated with the flow rate, potential difference and physical properties of the liquid and the correlation results are quantitatively compared with the results reported in the literature. (Some figures in this article are in colour only in the electronic version)

A hybrid CFD-mathematical model for simulation of a MEMS loop heat pipe for electronics cooling applications

A hybrid CFD-mathematical (HyCoM) model was developed to predict the performance of a micro loop heat pipe (MLHP) as a function of input heat rate. A micro loop heat pipe is a passive two-phase heat transport device, consisting of microevaporator, microcondenser, microcompensation chamber (CC) and liquid and vapor lines. A CFD model was incorporated into a loop solver code to identify heat leak to the CC. Two-phase pressure drop in the condenser was calculated by several two phase correlations and results were compared (Izenson and Crowley 1992 AIAA Paper A92-47847). Capillary tube correlations (Blevins 1984 Applied Fluid Dynamics Handbook (New York: Van Nostrand-Reinhold)) were used for pressure drop calculations in fluid lines. Effects of working fluid and change in geometry were studied. For a heat transport distance of 10 mm, the base model MLHP was 50 mm long, 16 mm wide and 1 mm thick. In the base model, widths of the grooves, liquid and vapor lines, evaporator and condenser were 55 µm, 200 µm, 750 µm, 2 mm and 4 mm, respectively.

Modeling and Optimization of a Microscale Capacitive Humidity Sensor for HVAC Applications

This paper presents a comprehensive numerical study of the performance of a capacitive humidity sensor for heating, ventilation, and air conditioning (HVAC) applications. The proposed sensor comprises a sensing layer sandwiched between an array of top and bottom electrodes. A combination of both parallel plate and interdigitated electrode arrangements is considered to achieve their distinctive advantages. Polyimide is used as the humidity sensing material due to good sensing characteristics and aluminum is used as the electrode material because of the ease of fabrication. A layer of polyimide covers the top electrodes to provide protection from atmospheric contamination thus improving durability. The influence of relative humidity on the dielectric constant of the sensing layer is determined theoretically using the models of Looyenga and Shibata The model is validated by comparing model predictions with experimentally measured data for a previously reported capacitive humidity sensor. The model is then used to simulate and predict the performance of the proposed humidity sensor. The effects of design configuration, sensing layer thickness, electrode polarity, electrode width and thickness, and electrode gap are studied. The influence of operating conditions including relative humidity, temperature and voltage is investigated. Based on the simulation results, the optimum design configuration is identified.

Numerical modeling of evaporator surface temperature of a micro loop heat pipe at steady-state condition

Numerical investigations have been performed to simulate a novel micro loop heat pipe (MLHP) under steady-state conditions. For most electronics, the maximum working temperature is an important design factor; therefore an accurate prediction of this temperature is crucial. The model predicts the steady-state temperature distribution at the surface of the heat source as a function of applied heat loads. This code builds upon a previous code developed by the authors (Ghajar et al 2005 J. Micromech. Microeng. 15 313–21), and utilizes a hybridizing of an alternating direction implicit (ADI) computational fluid dynamics (CFD) code and relevant thermodynamic equations. Using this simulation tool, the minimum required compensation chamber cavity has been calculated and checked for various operating temperature ranges. Additionally, the design of the MLHP has been improved by evaluating the effects of the geometric feature variations. Considering the fabrication constraints, some of the optimized geometry dimensions were found to be a groove wall thickness of 2 µm, a groove width of 7 µm, a wicking structure length of 500 µm and a vapor line width of 2 mm.

On-chip magnetophoretic isolation of CD4 + T cells from blood

This paper presents the design, fabrication, and testing of a magnetophoretic bioseparation chip for the rapid isolation and concentration of CD4 + T cells from the peripheral blood. In a departure from conventional magnetic separation techniques, this microfluidic-based bioseperation device has several unique features, including locally engineered magnetic field gradients and a continuous flow with a buffer switching scheme to improve the performance of the separation process. Additionally, the chip is capable of processing significantly smaller sample volumes than conventional methods and sample losses are eliminated due to decreased handling. Furthermore, the possibility of sample-to-sample contamination is reduced with the disposable format. The overall dimensions of the device were 22 mm by 60 mm by 1 mm, approximately the size of a standard microscope slide. The results indicate a cell purity of greater than 95% at a sample flow rate of 50 ml/h and a cell recovery of 81% at a sample flow rate of 10 ml/h. The cell purity was found to increase with increasing the sample flow rate. However, the cell recovery decreases with an increase in the flow rate. A parametric study was also performed to investigate the effects of channel height, substrate thickness, magnetic bead size, and number of beads per cell on the cell separation performance.

Use of nanoporous alumina surface for desorption electrospray ionization mass spectrometry in proteomic analysis

This paper presents use of a nanoporous alumina surface for desorption electrospray ionization mass spectrometry (DESI MS). The DESI MS performance of the nanoporous alumina surface is compared with that of polymethylmethacrylate (PMMA), polytetrafluroethylene (PTFE) and glass, which are popular surfaces in DESI MS experiments. Optimized operating conditions were determined for each of these surfaces by studying the effects of flow rate, tip to surface and surface to MS capillary distance, and spray angle on the DESI MS performance. The analytes (reserpine and BSA tryptic digest) were analyzed on all the surfaces. The results show that the nanoporous alumina surface offers higher ion intensity and increased peptide detection as compared to the other surfaces. Additionally, comparison of ion intensities obtained from the nanoporus alumina and an alumina film confirms that improved performance is due to the inherent nature of the nanostructured surface. Limits of detection (LODs) were determined for the analytes on all the surfaces. It was observed that the nanoporous alumina surface offers improved limits of detection as compared to other surfaces. Another advantage of the nanoporous alumina surface is that it provides to faster analysis associated with rapid drying of liquid samples on the surface. Additionally, porous alumina surface can be used as a dual ionization platform for combined DESI/LDI analysis for further improved peptide detection in proteomic analysis.

Effect of Heating Boundary Conditions on Pool Boiling Experiments

The two widely used experimental methods for pool boiling experiments are evaluated. Boiling heat transfer data employing either the water heating or the electric heating method obtained by various researchers are compared and discussed. The electric heating method is a constant heat flux process, while the water heating method represents a pseudo-exponential temperature profile for the heating water along the tube. It is concluded that the temperature profile along the tube is the major cause of the difference between results from the two methods. The parameters that shape the temperature profile and cause differences are discussed.