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Research Papers: Multiphase Flows

Analysis of Droplet Generation in Electrospray Using a Carbon Fiber Based Microfluidic Emitter

[+] Author and Article Information
A. K. Sen1

 Indian Institute of Technology, Guwahati 781039, Indiaashisen@gmail.com

J. Darabi

 University of South Carolina, 300 Main Street, Columbia, SC 29208

D. R. Knapp

 Medical University of South Carolina, Charleston, SC 29425

1

Corresponding author.

J. Fluids Eng 133(7), 071301 (Jul 05, 2011) (8 pages) doi:10.1115/1.4004325 History: Received November 14, 2008; Accepted February 19, 2011; Published July 05, 2011; Online July 05, 2011

This work presents simulation of jet break up in electrospray ionization using a microfluidic emitter. The emitter comprises a pointed carbon fiber located coaxial with a fused silica capillary of 360 microns OD and 75 microns ID, with its sharp tip extending 30 microns beyond the capillary terminus. The numerical model employs leaky-dielectric formulations for solving the electrodynamics and volume-of-fluid method for tracking the liquid-air interface. The existing leaky-dielectric model is modified to account for the presence of free charges inside the bulk of the liquid as well as at the interface. A small velocity perturbation is used at the capillary inlet to emulate the natural disturbance necessary for the jet break up. First, the model is validated by comparing model predictions with experimental results for a conventional emitter reported in literature. Then, it is applied to simulate the electrospray performance of the Carbon Fiber (CF) emitter including the Taylor cone and jet break up processes. Model predictions for CF emitter are compared with experimental results in terms of jet-length and current-flow characteristics. The influence of emitter geometry, operating conditions and liquid properties on the electrospray performance are investigated. Droplet diameter is correlated with flow rate and liquid properties and the correlation results are compared with that reported in literature.

Copyright © 2011 by American Society of Mechanical Engineers
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References

Figures

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Figure 1

Schematic of a typical ESI-MS process

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Figure 2

Comparison of droplet diameter and velocity predicted by the model and obtained in experiments [26]

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Figure 3

A schematic of the CF emitter–MS interface

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Figure 4

Schematic of the CF emitter based ESI-model

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Figure 5

Schematic showing the boundaries of the computational domain

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Figure 6

Results of mesh-convergence study: number of cells versus maximum electric field and droplet diameter

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Figure 7

Influence of the amplitude of fluctuation on droplet diameter and velocity

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Figure 8

Influence of the period of fluctuation on jet length and droplet diameter

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Figure 9

Influence of the amplitude of fluctuation on droplet diameter and velocity for a reported emitter [26]

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Figure 10

Taylor cone and jet break up profiles with electric potential contours in the computational domain

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Figure 11

Electric field intensity contours in the computational domain

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Figure 12

Axial velocity contours in the computational domain

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Figure 13

A close-up image of the Taylor cone and jet break up profiles in ESI using the CF emitter

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Figure 14

Comparison of spray current-flow rate predicted by the model and measured in experiments

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Figure 15

Influence of capillary ID on droplet diameter for different CF diameters

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Figure 16

Influence of capillary ID on droplet velocity for different CF diameters

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Figure 17

Influence of extension length of the CF on cone-length at different applied voltage

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Figure 18

Effect of the separation distance between CF-tip and MS electrode on onset-voltage

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Figure 19

Influence of flow rate on droplet diameter and velocity

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Figure 20

Influence of applied voltage on the straight length of the jet

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Figure 21

Influence of surface tension on droplet diameter and velocity

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Figure 22

Influence of conductivity on droplet diameter and velocity

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Figure 23

Influence of viscosity on droplet diameter and velocity

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