A NEW WAY TO DEMONSTRATE "WETTER" WATER E. A. HAUSER
AND
H. H. REYNOLDS*
Massachusetts Institute of Technology, Cambridge, Massachusetts
W
HEN thinking of a duck, everyone subconsciously pictures this bird floating on a smooth surface of water. The expression "floats like a duck" has become a layman's definition for bodies of different composition which for different physical reasons remain on the surface of a liquid, although we know of other birds, as for example, the swan or the seagull, which are equally capable of floating. Therefore it must be considered as one of the most ingenious advertising ideas ever used, when one of our large manufacturers of wetting agents demonstrated the extreme efficiency of his products by proving the inability of a duck to stay afloat in water which has been made "wetter" by the addition of a chemical compound. The first announcement1 created considerable sensation and even in laboratories not connected with the originators, requests for a few ounces of "wet" water came pouring in. At the meeting of the A. C. S. in Milwaukee in September, 1938, a colored moving picture featured the plight of a duck placed in a tub filled with "wet" water2 and quite recently a similar picture was even shown in a weekly newsreel. I t is a known fact that the duck's feathers are coated with a thin film of oil, thus preventing the water from wetting them, and increasing their weight. The polar molecules of the wetting agent adsorb with their hydrophobic group in the oil film, retaining their hydrophilic * Department of Chemical Engineering. HONE,H. E., Barren's, 18, No. 3, 3 (1938). ' CARYL,C. R. AND W. P. ERICKS,Ind. Eng. Chem., 31, 44 (1939).
ones in the water. Such a polar orientation in the interface enables the water to wet the duck's feathers. The weight of the duck being thus increased, its over-all buoyancy decreases and the bird will no longer be able to float as before, but will sink somewhat. Although the experiment is a very striking one, i t has very definite drawbacks. It is not always easy to pro-
AUDIENCE
EXPERIMENTER
T,Tank W , Water R, Rocks and sand B, Battleships with perforated metal or wire sieve bottoms G.Glass tube WA-SOL. Wetting agent solution RB, Rubber bulb A , AEroplane WA. Wetting agent powdered form
cure a duck; to make an effective demonstration a fairly large glass tank is necessary; if the duck's legs are
not tied together, the experiment is a rather "splashy" affair; but if tied, it is an unfair test and the sensitive members of the audience frequently object to it; also the experiment is not fully convincing, because the duck does not sink completely. In the following, we shall give a brief discussion of an experiment which we have found to be a very striking
will force the wetting agent to flow through the tube into the tank just below the water surface. If the experimenter has a special fancy for aviation, a simple affoplane model can be pulled over the tank by means of a wire stretched across the lecture table. I n this case, the dry wetting agent can be spread on the surface of the water in the tank by perforations in the bottom of the plane's cabin, so that it acts like a salt shaker when pulled along the wire. In both cases the surface tension of the water in the tank can be decreased rapidly. The degree of reduction naturally depends on the efficiencyof the chemical used. After having set up the equipment as described, a fleet of small battleships is placed on the surface of the water. These ships can be easily constructed out of tin, aluminum or copper foil, care only having to be exercised to leave any seams or folds open. Small holes are then made in the bottom. One can also shape the bottom of the ship out of fine wire gauze and then make the super-structure out of tin or aluminum foil (Figure 2). There are a few special precautions which should be observed. First, care must be taken not to touch the FIGURE 2.-MODEL OF A BATTLESHIP AS USED FOR THE EXPERIMENT. THE SUPERSTRUCTURE, bottomof the ships as the perspiration from the fingers MADEOF ALUMINUMFOIL.HAS BEENCUT AWAY will make the gauze more easily wetted; secondly, the ships must be placed in the water very carefully so that the water does not cover the gauze a t any point; and, thirdly, the water in the tank must initially contain no demonstration of "wetter" water, cheap, and easily soap or other wetting agent. The high surface tension of the water and the hydrocarried out, besides permitting the experimenter to use phobic properties of the perforated metal or gauze will his personal imagination f ~ e e l y . ~ A tank of appropriate size is filled with water. If prevent the water from penetrating the ship's body. desired, rocks, sand, waterplants, and a few small fish The ships will float, therefore, as they would, had they are placed therein to give the spectator a cross-sectional been built normally. If we now inject the solution of the wetting agent idea of the ocean. (See Rigure 1.) On the side of the tank facing the experimenter a narrow glass tube is bent under the water surface, or sprinkle the dry, readily over the wall of the container, and one of its openings soluble wetting agent onto the surface, the surface tenis allowed to dip just under the surface of the water, sion will immediately be reduced and the molecules of while the other one is connected with a wash bottle con- the wetting agent will adsorb in oriented fashion a t the taining the wetting agent in solution. The wash bottle metal-water interface. This results in a rapid wetting is also connected with a rubber bulb, which, if pressed, of the perforated metal or gauze; the water now readily penetrates the ship's body. The little models begin to The experiment was first demonstrated in public at the meet- wobble and sink in a most realistic fashion to the boting of the Northeastern Section of the A. C . S . in Cambridge, tom of the ocean. Mass., on February 10, 1939.
