J. Darabi

University of South Carolina, Columbia, SC, United States

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Publications (29)28.38 Total impact

  • A.K. Sen, J. Darabi, D.R. Knapp
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    ABSTRACT: This work presents simulation of aerosol formation in electrospray ionization (ESI) using a carbon fiber (CF) emitter. The model predicts droplet fission and trajectories of the droplets in a steady medium, within a monodisperse EHD spray in a Lagrangian framework. The droplet fission is simulated based on the principle of minimum free energy and droplet trajectories are predicted using Lagrangian single-droplet tracking method. The numerical model is validated by comparing model predictions with experimental results. The aerosol formation process using the CF emitter is simulated and results are presented and discussed.
    IEEE Sensors Journal 01/2011; 11(10):2335-2341. · 1.48 Impact Factor
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    ABSTRACT: A microfabricated fluidic interconnection system for polymer-based microfluidic nebulizer chips is presented and discussed. The new interconnection mechanism can be used to make fluidic connection between external capillary and the polymer microfluidic chip. The connector mechanism was fabricated using a combination of mechanical milling and laser micromachining. Preliminary leakage tests were performed to demonstrate that the interconnection system is leak-free and pressure tests were performed to evaluate the burst pressure (maximum working pressure). The interconnection system has several advantages over commercially available Nanoport™ interconnection system. The new fluidic interconnection system implemented onto a microfluidic nebulizer chip was successfully tested for desorption electrospray ionization mass spectrometry applications. The performance of the chip using the new connector mechanism was excellent demonstrating the usability of the new connector mechanism.
    Microsystem Technologies 04/2010; 16(4):617-623. · 0.83 Impact Factor
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    A K Sen, J Darabi, D R Knapp
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    ABSTRACT: This paper presents design, microfabrication, and test of a microfluidic nebulizer chip for desorption electrospray ionization mass spectrometry (DESI-MS) in proteomic analysis. The microfluidic chip is fabricated using cyclic olefin copolymer (COC) substrates. The fluidic channels are thermally embossed onto a base substrate using a nickel master and then a top substrate is thermally bonded to seal the channels. Carbon ink embossed into the top COC substrate is used to established electrical connection between the external power supply and the liquid in the channel. The microfluidic chip to external capillary connection is fabricated using Nanoport() interconnection system. Preliminary leakage test was performed to demonstrate the interconnection system is leak-free and pressure test was performed to evaluate the burst pressure. Finally, the nebulizer chip was used to perform DESI-MS for analyzing peptides (BSA and bradykinin) and reserpine on the nanoporous alumina surface. DESI-MS performance of the microfluidic nebulizer chip is compared with that obtained using a conventional DESI nebulizer.
    Sensors and Actuators B Chemical 04/2009; 137(2):789-796. · 3.84 Impact Factor
  • A K Sen, R Nayak, J Darabi, D R Knapp
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    ABSTRACT: 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.
    Biomedical Microdevices 09/2008; 10(4):531-8. · 2.72 Impact Factor
  • A.K. Sen, J. Darabi
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    ABSTRACT: 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.
    IEEE Sensors Journal 05/2008; · 1.48 Impact Factor
  • A K Sen, J Darabi
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    ABSTRACT: 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.
    Journal of Micromechanics and Microengineering 06/2007; 17(8):1420. · 1.79 Impact Factor
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    A. K. Sen, J. Darabi, D. R. Knapp
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    ABSTRACT: In this work, we propose a novel carbon nanofiber (CNF) emitter for electrospray ionization (ESI)–mass spectrometry (MS) applications. The proposed emitter comprises an array of CNFs around the orifice of a microscale capillary. The electrospray ionization process is simulated using a CFD code based on Taylor–Melcher leaky-dielectric formulations for solving the electrohydrodynamics and volume-of-fluid (VOF) method for tracking the interface. The code is validated for a conventional multiple electrospray emitter and then applied to simulate the CNF emitter model. The modeling results show that under steady state condition, individual cone-jets are established around each of the CNFs resulting in an array of electrosprays. The approach being taken to fabricate the CNF emitter is briefly discussed. Effects of geometrical parameters including aspect ratio of CNFs, total number of CNFs and distribution pattern of the CNFs on the electrospray performance are studied. The influence of operating parameters such as flow rate, potential difference and physical properties of the solvent on the electrospray behavior is thoroughly investigated. The spray current, ‘onset’ potential and jet diameter are correlated with total number and distribution of CNFs and physical properties of the liquid. The correlation results are compared with the available results in the literature. Higher spray current and lower jet diameter indicate that the device can perform equivalent to nanospray emitters while using a micro-scale orifice. This allows higher sample throughput and eliminates potential clogging problem inherent in nano-capillaries.
    Microfluidics and Nanofluidics 05/2007; 3(3):283-298. · 3.22 Impact Factor
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    ABSTRACT: As researchers work to develop the “Lab-on-a-Chip” system, dielectrophoresis (DEP) is being examined as a mechanism for the micro-fluidic transport and separation of small biological samples such as cells, proteins, and DNA. Interdigitated electrodes are commonly used within such devices to generate the non-uniform electric fields that induce movement. Among other parameters, the magnitude of the DEP force depends upon the gradient of the square of the electric field that is generated by such electrodes. By understanding the effect that the dimensions of the electrodes have on this quantity, micro-fluidic devices can be designed to produce the most effective dielectrophoretic effect on the biological samples. This article examines the relationship between the geometry of the interdigitated electrodes and the magnitude of the DEP force. This is done by obtaining and analyzing an equation for the gradient of the square of the electric field. The equation is generated by fitting the results of extensive numerical simulations. Although the complete equation introduced in this work is based upon theoretical results, many fundamental portions of the present work agree with accepted experimental results. These verifications are also addressed.
    Sensors and Actuators B-chemical - SENSOR ACTUATOR B-CHEM. 01/2007; 125(2):672-679.
  • A. K. Sen, J. Darabi
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    ABSTRACT: This paper presents numerical investigations of jet breakup in electrospray ionization using a carbon fiber emitter. The emitter has a pointed carbon fiber located coaxial with a fused silica capillary of 360μm OD and 75μm ID, with its tip extending 30 micron beyond the capillary exit. The model employs leaky-dielectric formulations to solve electrodynamics and volume-of-fluid technique for tracking the interface. The existing leaky-dielectric model is modified to consider presence of free charges both in the bulk of the liquid as well as on the interface. A velocity perturbation is used at the capillary inlet to emulate the naturally occurring disturbance necessary for the jet breakup. The model is first validated by comparing the model predictions with experimental results for a conventional emitter. Then, it is used to simulate electrospray performance of the carbon fiber emitter, including the Taylor cone and jet breakup processes. The influence of emitter geometry and operating conditions are thoroughly investigated. The droplet diameter and velocity are correlated with flow rate and the correlation results are compared with that reported in literature.
    ASME 2007 International Mechanical Engineering Congress and Exposition; 01/2007
  • M. Ghajar, J. Darabi
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    ABSTRACT: A number of analytical and numerical models have been developed by various researchers to predict the behavior of loop heat pipes (LHP). However, none of those models use the thin-film evaporation principles in the capillary structures to evaluate the local evaporative heat transfer coefficients. In this work, principles of the thin film evaporation are applied in a submodel and combined with our previously developed loop solver model to more accurately simulate the performance of a flat micro loop heat pipe. The resulting code predicts the heat removal capability, surface temperature, and local and average heat transfer coefficients at various applied heat loads. The results indicate that extremely high cross-sectionally averaged evaporative heat transfer coefficients can be achieved. The modeling results are verified by experimental data.
    ASME 2006 International Mechanical Engineering Congress and Exposition; 01/2006
  • A. K. Sen, J. Darabi, D. R Knapp
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    ABSTRACT: This paper presents the concept and simulation of a novel multiple electrospray emitters for electrospray ionization mass spectrometric (ESI-MS) applications. The proposed emitter is based on an array of carbon nanofibers (CNF) vertically grown around the orifice of a microscale thermoplastic capillary. The electrospray ionization process is simulated using a CFD code that utilizes Taylor-Melcher leaky-dielectric formulations for the electrohydrodynamics and volume-of-fluid (VOF) method for tracking the interface. The modeling results predict that under steady state conditions, individual cone-jets are established around each of the CNFs resulting in an array of electrosprays. Effects of several design and operational parameters on the electrospray performance are thoroughly investigated. The results of the present study will facilitate design, fabrication and experiments using the CNF emitter. Higher spray current and lower jet diameter indicate that the proposed emitter can perform equivalent to nanospray emitters exhibiting improved MS sensitivity while using a microscale orifice. Use of microscale orifice benefits in terms of higher sample throughput and eliminates potential clogging problem inherent in nanoscale capillaries. Overall, the proposed emitter is believed to be a suitable candidate for ESI-MS applications.
    ASME 2006 International Mechanical Engineering Congress and Exposition; 01/2006
  • Source
    A K Sen, J Darabi, D R Knapp, J Liu
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    ABSTRACT: 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 µm OD 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.
    Journal of Micromechanics and Microengineering 01/2006; 16:620-630. · 1.79 Impact Factor
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    J. Darabi, C. Rhodes
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    ABSTRACT: A two-dimensional numerical simulation study has been performed to model an electrohydrodynamic (EHD) micropump. The micropump consists of an array of interdigitated electrodes along the top and bottom of the micropump channel. The focus of the simulations was to study the effects of electrode gap, stage gap, channel height, and applied voltage. The numerical results were first validated with experimental data and then applied to obtain optimum electrode design and pump characteristics curves for geometries of interest. Additionally, the simulation results provide details about the flow behavior of the micropump and clearly capture local flow induced by EHD forces.
    Sensors and Actuators A: Physical. 01/2006;
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    M Ghajar, J Darabi
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    ABSTRACT: 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.
    Journal of Micromechanics and Microengineering 09/2005; 15(10):1963. · 1.79 Impact Factor
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    J. Darabi, H. Wang
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    ABSTRACT: 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 $mu$ m. Each tooth on the emitter electrode had a base length of 10 $mu$ m with a tip angle of 60 $^circ$ . The collector electrode consisted of a planar strip with a width of 10 $mu$ m. The gap between emitter and collector electrodes was 20 $mu$ m. The distance between each neighboring stage (a pair of emitter and collector electrodes) was 40 $mu$ 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 3M's 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. hfillhbox[1063]
    Journal of Microelectromechanical Systems 09/2005; · 2.13 Impact Factor
  • M. Ghajar, J. Darabi
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    ABSTRACT: 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 estimation 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 [1], 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 limitations, some of the optimized geometry dimensions were found to be a groove wall thickness of 2um, a groove width of 7um, a wicking structure length of 500μm, and a vapor line width of 2mm.
    ASME 2005 Summer Heat Transfer Conference collocated with the ASME 2005 Pacific Rim Technical Conference and Exhibition on Integration and Packaging of MEMS, NEMS, and Electronic Systems; 01/2005
  • M Ghajar, J Darabi, N Crews Jr
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    ABSTRACT: 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.
    Journal of Micromechanics and Microengineering 11/2004; 15(2):313. · 1.79 Impact Factor
  • M. Ghajar, J. Darabi, N. Crews
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    ABSTRACT: 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, micro-compensation 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 [2]. Capillary tube correlations [3] 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 50mm long, 16mm wide and 1mm thick. In the base model, widths of the grooves, liquid and vapor lines, evaporator, and condenser were 55μm, 200μm, 750μm, 2mm, and 4mm respectively.
    ASME 2004 International Mechanical Engineering Congress and Exposition; 01/2004
  • J. Darabi
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    ABSTRACT: An analytical model has been developed to predict the heat transfer and flow characteristics of electrohydrodynamic (EHD)-enhanced microscale ultra thin-film evaporation. The model described in this paper is based on a previously published microcooling device [1] that incorporated an active evaporative cooling surface, an EHD micropump, and temperature sensors into a single chip. The device was fabricated using microelectromechanical systems fabrication technology, allowing the EHD electrodes and temperature sensors to be integrated directly onto the cooling surface. One end of the device was immersed in a pool of liquid. The film originated at the liquid–vapor interface and flowed upward under the influence of the electric field. The model predicts the film thickness, dryout location, local and average heat transfer coefficients, and velocity profile. The agreement between the model and the experimental data is satisfactory. Both the analytical model developed in this study and the experimental results reported previously will facilitate the design of new microcooling devices capable of operating at high power levels.
    Heat Transfer Engineering 01/2004; 25(6):14-22. · 0.69 Impact Factor
  • J. Darabi, H. Wang
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    ABSTRACT: 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 strong inhomogeneous electric field. Each saw-tooth had a base length of 10 μm with a tip angle of 60°. The gap between emitter and collector electrodes was 20 μm and the distance between each stage (a pair of emitter and collector electrodes) from neighboring stage was 40 μm. The dimensions of the patterned area were 10 mm by 20 mm allowing approximately 300 stages to be fabricated along the length of the micropump. The maximum pressure head achieved by this micropump was 550 Pa and 205 Pa for HFE-7100 and liquid nitrogen, respectively.
    ASME 2003 International Mechanical Engineering Congress and Exposition; 01/2003

Publication Stats

281 Citations
28.38 Total Impact Points

Institutions

  • 2002–2009
    • University of South Carolina
      • Department of Mechanical Engineering
      Columbia, SC, United States
  • 2001
    • University of Maryland, College Park
      • Department of Mechanical Engineering
      College Park, MD, United States