CLASSIFICATION OF THE MODES OF EHD SPRAYING

Abstract

Different modes of electrohydrodynamic (EHD) spraying and their classification according to geometric criteria, based on the forms of meniscus and jet, are discussed. The reason for interest in this classification is that the drop sizes and geometrical forms of the aerosol, particularly important from a practical point of view, are different for different 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 different ways. Several attempts, based on different criteria, have been undertaken to classify the modes of EHD spraying, however, without referring to a clear definition. To classify the modes of EHD spraying the following definition 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 first group comprises the dripping, microdripping, spindle, multi-spindle and ramified-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 ramified-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 differs from that at positive for the same voltage value. At positive polarity the droplets are usually finer. 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 flow 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 defined 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 flat 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 finer than a few tens of micrometers in diameter issue simultaneously from the capillary at its circumference. The aerosol is fine and is sprayed in distinct streams of droplets.