STAGE ONE-FREEZING of A WATER DROP

A--CITATION-NEW SCIENTIST and SCIENCE JOURNAL

  • RAW MATERIAL
  • for FORGING THUNDERBOLTS
  • NEW SCIENTIS
  • VOL. 49, NO.734 14 JANUARY 1971
  • SCIENCE
    WATERDROP FREEZING:

    EJECTION of MICRO-DROPLETS

    vol 170, p 1395, 1970

Recent observations by Roger J. Cheng of the State University of New York reveal a fascinating new microscopic process which almost certainly has an important bearing on how thunderclouds become charged to their high voltages

(Science. vol 170, p 1395, 1970).

About 10 years ago Dr B. J. Mason, Director of the Meteorological Office (then at Imperial College, London), advanced a novel theory in which the charge separation depended, at the atomic level, on the greater mobility of the protons in water compared with that of the negative OH ions. His experiments confirmed that protons do, indeed, travel more readily towards the colder end of a piece of dry ice supporting a temperature gradient. He established that when a stream of supercooled water drop froze on being blown past a hailstone in a wind tunnel it burst, shattering with the ejection of positively charged ice splinters, and leaving the hailstone negatively charged. Gravity acting on the 'hailstone, and up-currents on the splinters, could then give realistic charging rates for a typical thundercloud with a net charge of 1000 coulombs.

Cheng's discovery is that a freezing supercooled drop can perform its own charge separation in an even neater fashion. Starting with drops of one mm diameter he cooled them until they began to freeze at about - 15C. Using cinematography he observed that at this juncture, the drops began to spray out tiny droplets 1 to 20um in diameter, through the thin, forming ice shell. They fell on hit micro slide in large numbers for approximately 50 seconds until the drop was completely frozen.

With an electrometer probe, and also by electrostatic deflection methods, Cheng showed that the ejected droplets carry a net positive charge, while the frozen drop retains a net negative charge. However the droplets form-whether forced out through minute cracks in the ice shell in response to internal pressure, condensed from water sublimated from the drop, or as a result of small bursting air bubbles beneath the ice surface-the charge separation at the atomic level clearly occurs as in Mason's hypothesis. The outer surface of the drop is colder than its interior and, again, concentrates the positive charge.

 
 

 

 

 

   
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