Current measurement in an electrospray system

Pergamon

Vol. 28, Suppl. I, pp. $535-$536, 1997 ©1997 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0021-8502/97 $17.00+0.00

J. A e r o s o l S c i .

PII:SO021-8502(97)O0316-9

Current Measurement in an Eleetrospray System M. Drrtboom, H. Fichtner and H. Fissan Process- and Aerosol Measurement Technology, Gerhard-Mercator-University Duisburg, 47048 Duisburg, FRG KEYWORDS electrospray, current measurement, particle charge, functioning modes INTRODUCTION An electrospray system provides the production of droplets in a wide size range with good monodispersity. The functioning mode of an electrospray system (e.g. cone-jet mode) can be determined with a light microscope. Measurements from Chen etal. (1995) showed that the functioning modes are given by the current versus voltage curve. The characteristic can easily be measured for open systems operating in a point to plate configuration where the droplets are deposited on a plate. In order to measure the current caused by the transport of the charged particles in a closed system the capillary must be shielded to suppress a leak current. For the calculation of the droplet charge the leak current has an important influence because it can be much stronger than the droplet current itself. THEORETICAL BACKGROUND The droplet current Io depends on the applied voltage U between the capillary and the plate as well as on the functioning mode of the spray system. Chen et al. showed that the current rises with increasing voltage. However, in the cone-jet mode the current is independent of the voltage. In electrospray systems working as aerosol generator an insulator (i.e. PVC, Teflon .... ) between the plate and the capillary is needed. A leak current IL caused by the surface conductivity of the insulator will arise even for very high resistance materials. The conductivity of the insulator surface depends on humidity and temperature. Dirt and even finger prints will also decrease the resistance and increase the leak current. Figure 2 shows the system used by Chen et al. Figure 3 shows U the new developed system. Here the capillary is shielded and only the Fig. 1: typical I-U curve droplet current ID is measured.

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$535

$536

Abstracts of the 1997 European Aerosol Conference

In order to test the proposed measurement system two experiments were performed. In the first experiment the capillary were electrically connected with the shield. The current from the voltage source to the capillary is measured ( I = ID + IL). In the second setup an ammeter was used instead of the connection between the capillary and the shield. FIRST RESULTS In the experiments ethanol was sprayed with two different flow rates. As can be seen from Figure 4 the current measured without shielding is a magnitude higher than with shielding, hence the leak current is the dominating current for both flow rates. It can be observed that with increasing flow rates the measured droplet current (with shielding) decreases. One reason for this surprising result can be the fact, that the flow rate of 6.8ml/h was to high for cone-jet mode. These measurements can be a measure to find whether the system operates correct or not.

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U [kV] Fig. 4: I-U curve for different flow rates REFERENCES D. R. CHEN, D. Y. H. PUI AND S. L. KAUFMAN(1995) Electrospraying of Conducting Liquids for Monodisperse Aerosol Generation in the 4nm to 1.8ktm Diameter Range, J. Aerosol Sci., 26, No. 6, pp. 963-977 J. FERNANDEZDE LA MORA AND A. GOMEZ (1995) Electrospray Atomization: Fundamentals and Applications, AAAR "95-Tutorial, pp. 1-33 A. M. GAIq,~N-CALVO,J. D,~VILAAND A. BARRERO (1997) Current and Droplet Size in the Electrospraying of Liquids, J. Aerosol Sci., 28, No. 2, pp. 249-275 J. M. GRACE AND J. C. M. MARIJNISSEN(1994) A Review of Liquid Atomization by Electrical Means, J. Aerosol Sci., 25, No. 6, pp. 1005-1019 ACKNOWLEDGEMENT This work was performed as part of the special research program (SFB 209) at the University of Duisburg supported by the German National Science Foundation (DFG).