A. K. Sen

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)

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.

Dual desorption electrospray ionization-laser desorption ionization mass spectrometry on a common nanoporous alumina platform for enhanced shotgun proteomic analysis.

A gold coated nanoporous alumina surface was used for dual ionization mode mass spectrometric analysis using desorption electrospray ionization (DESI) and laser desorption ionization (LDI). DESI and LDI mass spectrometry (MS) from the nanoporous alumina surface were compared with conventional electrospray ionization (ESI) mass spectrometry and matrix assisted laser desorption ionization (MALDI) for analysis of tryptic digests of proteins. Combined use of DESI and LDI offer greater peptide coverage than either method alone and comparable peptide coverage as with dual MALDI and ESI. This dual ionization technique using a common platform with same sample spot demonstrates a potential time and cost-effective tool for improved shotgun proteomic analysis.

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.

Aerosol Formation in Electrospray Ionization Using a Microfluidic Emitter

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.

Desorption Electrospray Ionization Using a Porous Alumina Surface

Desorption electrospray ionization (DESI) is a technique used for direct sampling of a sample or an analyte deposited on a surface under ambient conditions [1]. In DESI, ionized droplets of a spray are directed towards the sample causing desorption of ions due to exchange of charge and momentum. The resulting ions are carried into an ion trap mass spectrometer and analyzed. DESI was originally demonstrated by Takats et al. [1]. They described the new method and applied the same to analyze various compounds present on a variety of surfaces. Followed by this, several researchers [1–5] have investigated on DESI for a wide range of applications including analysis of pharmaceuticals, explosives detection, natural products discovery and in vivo clinical analysis. Recently, Kauppila et al [5] have introduced porous silicon (pSi) and ultra-thin layer chromatography (UTLC) plates for DESI-MS. Similar or improved sensitivities were obtained with pSi and UTLC surfaces as compared to PMMA and PTFE surfaces. This work presents use of a nanoporous alumina surface [6] for DESI – MS. The DESI – MS performance of nanoporous alumina surface is compared with that of PMMA, which is a popular surface in previous DESI-MS experiments. Optimized operating conditions were determined for the surfaces using BSA tryptic digest as the sample. The results show that the nanoporous alumina surface offers significantly higher ion intensity as compared to the other surfaces.Copyright

Numerical Investigations of the Jet Breakup Regime of Electrospray Ionization Using a Carbon Fiber Emitter

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.Copyright

Modeling of a Novel Multi-Jet Emitter for ESI-MS Applications

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.© 2006 ASME