Waves – Drops - Bubbles
Some Processes involved in the production
Of An Atmospheric Aerosol
Copyright Dr. Colin Pounder 1978 -
Sand grains from desert storms, pulverised materials from erupting volcanoes, hail, meteoritic detritus and raindrops fall in an incessant rain into oceans, seas, lakes, rivers, streams and ponds on the surface of the planet. Breaking waves produce billions of bubbles. From each bubble that bursts and each fragment which falls in the incessant rain of material a column of water is ejected upwards from the bulk water. A column that breaks up into drops of diminishing size due to surface tension. The smallest of these drops or droplets drifts into the air and becomes a component part of the atmospheric aerosol. The larger droplets fall back, under gravity, and become part of the incessant rain. They send up columns that break into droplets contributing more components to the aerosol and more falling droplets of the incessant rain. In each instance the process continues until the falling droplets are of such a size they can no longer send up a jet or column.
In the 1960s Dr D.C. Blanchard of State University of New York at Albany found that the droplet component to the aerosol was electrically charged in the range of a few elementary charges and usually positive. Water evaporated from each droplet to leave behind a crystalline particle. Further investigation of bubbles rising through bulk water found they scavenged material such as bacteria and viruses. From which he demonstrated that an aerosol particle carries an electric charge, salts and what we can call pollutant solids, up into the atmospheric aerosol.
(An excellent account of this work along with some history and details of experiments – some very simple to conduct – will be found in `From Raindrops to Volcanoes – adventures with sea surface meteorology – by Duncan C. Blanchard, Anchor Books, Doubleday & Co). There are many scientific papers in various journals should you wish to follow this in greater depth.
Overall the average salinity of the oceans and seas is 3.5%. Salts are dissolved from land deposits and taken by the rivers into the sea. How is then that the sea salinity does not increase rapidly? Blanchard calculated that billions of tons of salt, in the form of particles from jet droplets, are carried away in the winds. Whilst some will fall back into the seas some will be deposited on land and in due time return to the seas in run off water. This is a salt cycle.
My Investigations.
The following is a small part of some investigations dating initially from 1978, immediately following from work on Leidenfrost drops and my discovery of charged particles. I will omit numbers and too much detail, again with the idea that you may wish to try some of this for yourself. The investigations were conducted as part of a M.O.D. research carried out as a Consultant and Research Assistant to U.M.I.S.T. Some has been published e.g. in Oceanic Whitecaps & their role in Air-Sea Exchange Processes, E.C. Monahan & G. Mac Niocaill, Oceanographic Sciences Library, D. Reidel Pub Co & Galway Univ. Press 1986: The Production & Dispersal of Marine Aerosol, Quarterly Journal Royal Meteorological Society 1984: but most is in classified reports and not generally available.
Drops and Bursting Bubbles.
Falling drops and bursting bubbles were investigated. Both produce the Worthington, sometimes called Rayleigh Jet, so descriptions of this jet apply to both means of jet production. Much of what follows is concerned with bubbles and how air is entrained into the sea.
Some of a drop meeting the surface, and forming a crater, is flung outwards as small drops. This was found by colouring the drop with Potassium Permanganate or ink. The walls of the container of bulk water were marked by coloured water from impacting droplets. These detached droplets also fall into the bulk and are revealed by downwards plumes of coloured water. A crater and detached droplets is shown in this photograph.
Shortly after the majority of the material of the falling drop has entered the bulk water, or a bubble has burst, a jet begins to rise. The onset of jet production is shown in the next photograph.
The jet develops as a tapered liquid column:-
A characteristic of drawn out liquid columns is that surface tension as it were pinches them into a shape similar in appearance to a string of sausages. As the jet rises the narrowed regions break. Surface tension then causes each pinched off section to become spherical. The jet has now become a vertical sequence of droplets with the largest in diameter nearest the base each becoming smaller up to the tip of the jet. In some instances the jet breaks up in such a way that sometimes a smaller droplet is produced between two larger droplets. This phenomenon was first recorded by Plateau and is known as Plateau’s spherule.
(The method of obtaining the photographs was by means of a Minolta SRT 303B and Flash bulbs. The technique adopted was to listen to drops splashing into a full beaker of water and operating the camera at what was judged to be the appropriate moment).
Water tinted with milk
Jet beginning to break up
Droplets produced by broken up jet
Large droplets from jet base falling back into bulk water where they will repeat the jet production process
