STAGE-THREE: MELTING of A FROZEN DROP

A-- REPORT: CONF. on ATMOSPHERIC ELECTRICITY

1,   MICROSCOPICAL OBSERVATION:

Photomicroscopical Investigation of natural ice pellets collected in a thunderstorm reveal that electrically charged and particle embedded micro-droplets are being ejected by bursting of air bubbles released from the melting ice pellets. The observation of the physical appearance of more than 500 natural ice pellets (Figs. 1, 2) show that a large number of ice pellets have a spicule on their surface, in some cases multiple spicules. Ice pellets with a crack on the surface or semi-spherical pellets also can be found in the sample. There is a strong indication that extremely high pressure exists inside the ice shell of a water drop during the rapid freezing process. Unfrozen water in the form of microdroplets is suggested to be forced out from such spicules, or cracks, of the freezing water drops (Cheng, 1970).

Two types of air bubbles are observed in the collected ice pellets:

 (1)   Spherical or cylindrical air bubbles which are formed by the release of dissolved air in the water drop at the ice-water interface, due to different solubility of air in water and ice, during the freezing process as observed by Cheng (1973a).

 (2)   Numerous micros-size air bubbles are also observed in some cases in the form of milk- like opaque areas in the ice pellets (Fig. 3).

 It is suggested that these are formed by the rapid release of interior pressure by excreted unfrozen water from the ice shell of a freezing water drop.

2. EJECTION OF MICRODROPLETS:

An ice pellet placed in a warmer environment with a temperature above its melting point will first become covered with a melted film of liquid water. As the outer layer of ice melts, entrapped air bubbles can be observed moving toward the surface of the melting ice pellet. They remain at the ice surface for a few seconds, but eventually separate from the ice surface as melting continues. The air bubbles then travel radically with extremely high velocity in the liquid water surrounding the melting ice pellet . On close microscopically examination, numerous micro-droplets are observed being ejected from the melting ice pellet by bursting of the released air bubbles at the water-air interface. These ejected micro-droplets are formed from the shattered bubble film and by breakdown of a jet from the collapsed bubble cavity. (Blanchard, 1963). Three groups of micro-droplets with different masses being ejected from the water surface by the bursting of a stream of air bubbles produced by a hypodermic needle are shown in

Atmospheric particulates, embedded in ice pellets, tend to become attached to the surface of released air bubbles in the melting process and are then subsequently ejected into the surrounding environment with the micro-droplets by bursting these air bubbles

3. ELECTRICAL PROPERTIES:

Ejected micro-droplets are collected on glass slides coated with gelatin and attached to a pair of electrodes.A high voltage (1000 V DC) is applied to the electrodes in order to deflect charged droplets to the respective electrodes. A grounded conducting plate is placed between the melting ice pellet and the electrodes and a small opening in the center of the conducting plate works as a pathway for the ejected droplets to reach the collecting slides. In this way, possible induction by an electric field to the melting ice pellet and the ejecting droplets can be eliminated. Study of the electrical property of ejected droplets, simply by counting the number of collected droplets from both electrodes, indicates that 70%—75% of the ejected droplets carry a negative electric charge. The electrical conductivity of water from the melted ice pellet is l2Ox10-6 ohm/cm.

Experiments of melting artificial ice spheres, made from water with various electrical conductivity by dissolving salt into it, reveal interesting results as shown in Fig. 9, and strongly suggest that electrical properties of ejected micro-droplets are closely associated with the salt concentration in water from melting ice pellets.                                                      

The fragmentation and electrification of hydrometeors during phase transition play an important role in generation and separation of electricity in the atmosphere. It is widely accepted that the charge generation and separation processes in a thundercloud are closely associated with the development of precipitation and the main charge centers that appear above the freezing level. It is reported, from observations made inside thunderclouds that lightning flashes coincide with the appearance of solid precipitation in the form of ice pellets or soft hail. Ejection of electrically charged microdroplete by freezing a super-cooled water drop (Cheng, 1970) and sublimational breakup of electrically charged secondary ice fragments from a growing ice particle (Schaefer & Cheng,1971, Cheng, 1973b) were previously reported. Electrification of melting ice particles has been studied by Magono and Kikuchi, 1963; Dinger, 1965; and Drake, 1968. The observation of the ejection of electrically charged micro-droplets by bursting of air bubbles released from the melting ice pellet represents a third stage of microscopical process of fragmentation and electrification of hydrometeors, below freezing level, in the atmosphere.

 

• 5, REFERENCES: Blanchard, D.C. (1963). The electrification of the, atmosphere by particles from bubbles in the sea.    Prog.Oceanog., 1, 71-202. Cheng, R .J. (1970). Water drop freezing: ejection of micro-droplets. Science, 170, 3965, 1393—1396.   Cheng, R.J. (l973a). Photomicroscopical investigation of fragmentation of hydrometeors in the laboratory. The Microscope, 21, 3, 149—160.   Cheng, R.J. (1973b). The mechanism of multiplication process of glaciations in the atmosphere. Proceedings of 8th Intl. Conf. on Nucleation, Leningrad, U.S.S.R., 358—361.   Dinger, J.E. (1965). Electrification associated with the melting of snow and ice. J. Atmos. Sçi., 22, 162—166.   Drake, J.E. (1968). Electrification accompanying the melting of ice particles. Q. J. Roy. Met. Soc 94, 176—191. Msgono, C. and Kikuchi, K. (1963) On the positive electrification of snow crystals in the process of their melting.J. Met. Soc. Japan, 41, 270—277.

• AIR BUBBLE FORMATION on ICE SURFACE: Formation of air bubbles, which were observed at the INTERFACE of ICE and WATER on the surface of the FREEZING DROP(1mm). The air bubbles were formed as a result of the decrease of solubility of air in water when the temperature of the drop increased upon freezing. AIR has LESS SOLUBILITY in ICE than WATER
  SECTION of HAILSTONE: OBSERVED by POLARIZED MICROSCOPE, REVIEWED MANY AIR BUBBLES ( BACK POCKETS) AGAINST COLORED ICE. THE BUBBLES WERE FORMED DURING FREEZING of WATER ON ICE SURFACE. THE RINGS OF BUBBLES ZONES INSIDE THE HAILSTONE INDICATED THIS HAIL HAD TRAVELLED ( UP & DOWN) MANY TIMES INSIDE A THUNDERCLOUD.
  AIR BUBBLES being RELEASED from A MELTING ICE PELLET( FROZEN DROP): More than 300 air bubbles (average diameter of 50 um) are being released and accelerated radially from a half melted ice pellet-- (1 mm). which is following by ejection of micro-droplets by busting of these air bubbles from a melting ice pellet. .	please note those air bubbles keep distance with each other because they each carried same sign of electric charge(-).
  ANOTHER EXAMPLE of AIR BUBBLES FORMATION: DURING MELTING of A HOLLOW SEA SALT PARTICLE. MICRO-SIZE of MARINE AEROSOL ( CCN with SULFATE ) WILL BE GERERATED by BUSTING of AIR BUBBLES. THIS ACTION INDICATED THE TRANSFORMATION of SULFATE from OCEAN to the ATMOSPHERE. 1. "The Formation of Hollow Sea-Salt Particles from the Evaporation of Drops of Seawater" ATMOSPHERIC RESEARCH, Vol. 22, No. 1. June 1988 (Cheng, Blanchard & Cipriano). 2. "The Generation of Secondary Marine Aerosols: The Crystallization of Seawater Droplets" A. LECTURE NOTES IN PHYSICS, 309, Atmospheric Aerosols and Nucleation, Ed: P.E Wagner and G. Vail, Springer-Verlag. 1988. B. International Conference on Atmospheric Aerosols and Nucleation, Vienna, Austria. August 22-27, 1988 3. "Sulfate Aerosols Generation in the Marine Atmosphere: The Evaporation of SeawaterDroplets" International Conference on Global and Regional Environmental Atmospheric Chemistry, Beijing, China. May 1989.

  ents by Sublimation and Electr