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CLASSIFICATION OF THE MODES OF EHD SPRAYING
Abstracts
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standard LMIS theory. However, discussions of the LMIS make use of the concepts of a &&viscous-drag-free source'' (for which the cone base pressure is zero), and &&viscous-drag-limited'' sources (for which the cone base pressure is non-zero). Liquid}metal ion source operation has been modelled, both analytically and numerically. Progress in modelling the steady-state-#ow mode has been substantial and successful. Attempts to model time-dependent EHD behaviour have run into fundamental computational di$culties.
REF ER E NCE S Forbes, R. G. (1997) Understanding how the liquid}metal ion source works.
CLASSIFICATION OF THE MODES OF EHD SPRAYING A. Jaworek and A. Krupa Institute of Fluid Flow Machinery, Polish Academy of Sciences, P.O. Box 621, 80-952 Gdansk, Poland Di!erent modes of electrohydrodynamic (EHD) spraying and their classi"cation according to geometric criteria, based on the forms of meniscus and jet, are discussed. The reason for interest in this classi"cation is that the drop sizes and geometrical forms of the aerosol, particularly important from a practical point of view, are di!erent for di!erent modes of spraying. It is well known that the jet can disintegrate into droplets, while issuing from a capillary maintained at high potential, in many di!erent ways. Several attempts, based on di!erent criteria, have been undertaken to classify the modes of EHD spraying, however, without referring to a clear de"nition. To classify the modes of EHD spraying the following de"nition of the modes of spraying is proposed in the paper: The mode of electrohydrodynamic spraying is the way the liquid is dispersed into droplets, and is characterized by two criteria: (1) The geometrical form of the liquid at the outlet of the capillary (drop, spindle, jet), (2) The mechanism of the disintegration of the jet into droplets (type of instability). In general, the spraying modes can be divided into two groups. The "rst group comprises the dripping, microdripping, spindle, multi-spindle and rami"ed-meniscus modes, i.e. the modes in which only fragments of liquid are ejected from the capillary. The second group includes: the cone-jet, precession, oscillating-jet, multijet and rami"ed-jet modes, which are characteristic in that the liquid issues a capillary in the form of a long continuous jet which disintegrates into droplets after some distance, usually a few mm, from the outlet of the capillary. The jet, and the meniscus, can be stable, can vibrate, rotate spirally around the capillary axis or whip irregularly. Usually, the spraying at negative polarity di!ers from that at positive for the same voltage value. At positive polarity the droplets are usually "ner. It is also easier to establish the regular modes of spraying when positive excitation voltage is applied to the capillary. The characteristics of some of the modes based on the experimental studies are as follows: (1) In the microdripping mode the generated droplets are usually monodisperse and have the diameters ranging from a few hundred of micrometers down to a fraction of micrometers. The stream of droplets is linear, and they #ow along the capillary axis. (2) In the spindle mode only elongated fragments of liquid are ejected from the capillary instead of regular drops. (3) In the multi-spindle mode two or more spindles are ejected periodically at the circumference of the capillary. Next, the spindles can disintegrate into a few smaller droplets. The droplets are polydisperse and are smaller than 100 mm in diameter. (4) The oscillating-jet mode has been de"ned as the one in which the cone and the jet oscillate in the plane of the capillary axis. This mode allows to generate polydisperse aerosol of the droplets smaller than a few hundreds of mm in diameter. The aerosol is sprayed in a plane or in an oblate cone of a #at ellipsoidal base. (5) In the precession mode the jet rotates around the capillary axis. The generated droplets are of a few tens of mm in diameter. The droplets are sprayed in a regular cone of circular base. This mode seems to be very promising in practical applications. (6) In the cone-jet mode the jet is straight linear, and the droplets are generated due to varicose or kink instabilities. The droplets are a few tens of mm or smaller. (7) The multijet mode is generated when two or more jets "ner than a few tens of micrometers in diameter issue simultaneously from the capillary at its circumference. The aerosol is "ne and is sprayed in distinct streams of droplets.
ELECTROSPRAYING OF GLASSES*PREPARATION OF GLASS COATINGS ON GLASS S. Rosenbaum and R. Clasen Universitaet des Saarlandes, Lehrstuhl fuer Pulvertechnologie von Glas und Keramik, Postfach 151150, 66041 Saarbruecken Glass coatings on ceramic or metallic substrates are prepared by two di!erent methods. In a two-step process the glass powder is deposited on the glass substrate by methods such as airless spraying of enamel or glaze suspensions, dipping in enamel suspensions, electrospraying or electrophoretic deposition of enamel or glaze suspensions, and electrostatic powder spraying. The particle size of the glass powders which are generally prepared
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standard LMIS theory. However, discussions of the LMIS make use of the concepts of a &&viscous-drag-free source'' (for which the cone base pressure is zero), and &&viscous-drag-limited'' sources (for which the cone base pressure is non-zero). Liquid}metal ion source operation has been modelled, both analytically and numerically. Progress in modelling the steady-state-#ow mode has been substantial and successful. Attempts to model time-dependent EHD behaviour have run into fundamental computational di$culties.
REF ER E NCE S Forbes, R. G. (1997) Understanding how the liquid}metal ion source works.
CLASSIFICATION OF THE MODES OF EHD SPRAYING A. Jaworek and A. Krupa Institute of Fluid Flow Machinery, Polish Academy of Sciences, P.O. Box 621, 80-952 Gdansk, Poland Di!erent modes of electrohydrodynamic (EHD) spraying and their classi"cation according to geometric criteria, based on the forms of meniscus and jet, are discussed. The reason for interest in this classi"cation is that the drop sizes and geometrical forms of the aerosol, particularly important from a practical point of view, are di!erent for di!erent modes of spraying. It is well known that the jet can disintegrate into droplets, while issuing from a capillary maintained at high potential, in many di!erent ways. Several attempts, based on di!erent criteria, have been undertaken to classify the modes of EHD spraying, however, without referring to a clear de"nition. To classify the modes of EHD spraying the following de"nition of the modes of spraying is proposed in the paper: The mode of electrohydrodynamic spraying is the way the liquid is dispersed into droplets, and is characterized by two criteria: (1) The geometrical form of the liquid at the outlet of the capillary (drop, spindle, jet), (2) The mechanism of the disintegration of the jet into droplets (type of instability). In general, the spraying modes can be divided into two groups. The "rst group comprises the dripping, microdripping, spindle, multi-spindle and rami"ed-meniscus modes, i.e. the modes in which only fragments of liquid are ejected from the capillary. The second group includes: the cone-jet, precession, oscillating-jet, multijet and rami"ed-jet modes, which are characteristic in that the liquid issues a capillary in the form of a long continuous jet which disintegrates into droplets after some distance, usually a few mm, from the outlet of the capillary. The jet, and the meniscus, can be stable, can vibrate, rotate spirally around the capillary axis or whip irregularly. Usually, the spraying at negative polarity di!ers from that at positive for the same voltage value. At positive polarity the droplets are usually "ner. It is also easier to establish the regular modes of spraying when positive excitation voltage is applied to the capillary. The characteristics of some of the modes based on the experimental studies are as follows: (1) In the microdripping mode the generated droplets are usually monodisperse and have the diameters ranging from a few hundred of micrometers down to a fraction of micrometers. The stream of droplets is linear, and they #ow along the capillary axis. (2) In the spindle mode only elongated fragments of liquid are ejected from the capillary instead of regular drops. (3) In the multi-spindle mode two or more spindles are ejected periodically at the circumference of the capillary. Next, the spindles can disintegrate into a few smaller droplets. The droplets are polydisperse and are smaller than 100 mm in diameter. (4) The oscillating-jet mode has been de"ned as the one in which the cone and the jet oscillate in the plane of the capillary axis. This mode allows to generate polydisperse aerosol of the droplets smaller than a few hundreds of mm in diameter. The aerosol is sprayed in a plane or in an oblate cone of a #at ellipsoidal base. (5) In the precession mode the jet rotates around the capillary axis. The generated droplets are of a few tens of mm in diameter. The droplets are sprayed in a regular cone of circular base. This mode seems to be very promising in practical applications. (6) In the cone-jet mode the jet is straight linear, and the droplets are generated due to varicose or kink instabilities. The droplets are a few tens of mm or smaller. (7) The multijet mode is generated when two or more jets "ner than a few tens of micrometers in diameter issue simultaneously from the capillary at its circumference. The aerosol is "ne and is sprayed in distinct streams of droplets.
ELECTROSPRAYING OF GLASSES*PREPARATION OF GLASS COATINGS ON GLASS S. Rosenbaum and R. Clasen Universitaet des Saarlandes, Lehrstuhl fuer Pulvertechnologie von Glas und Keramik, Postfach 151150, 66041 Saarbruecken Glass coatings on ceramic or metallic substrates are prepared by two di!erent methods. In a two-step process the glass powder is deposited on the glass substrate by methods such as airless spraying of enamel or glaze suspensions, dipping in enamel suspensions, electrospraying or electrophoretic deposition of enamel or glaze suspensions, and electrostatic powder spraying. The particle size of the glass powders which are generally prepared