The peculiar structure of water becomes particularly noticeable on freshly formed water surfaces which occur in all water sprays. The outermost water surface consists of a dipole layer with the negative charges on the outside. When you spray water through a fine nozzle which produces very small water droplets, and let these droplets fall on a still water surface, you see many droplets roll on the water surface before finally coalescing with it. Furthermore, if you bring an electric field in the vicinity of the spray, the stream of water droplets will be deflected, and the water droplets will coalesce.
A nozzle can be made from an eye-dropper glass tube by holding the small end into a flame (best is gas flame, especially Bunsen burner), rotating it in the flame until the glass is soft, and then pulling, with a pair of tweezers or pliers, the softened tip quickly into a fine capillary. After the glass has cooled, break off the end as shown in Fig. 90. Connect the capillary with the water source and let a fine spray fall onto a water surface in a bowl. (If the bowl is made of plastic, it may be necessary to line its inside with aluminum foil, because plastic often acquires static electricity and thereby causes an electric field in its vicinity.) We should observe many droplets rolling on the water surface. Now, rub any plastic object (comb, fountain pen, etc.) on cloth to charge it, and slowly bring it near the bowl.
1. What happens to the rolling droplets? 2. What happens to the spray, when the charged object is brought near it? 3. What happens to the spray, when a charged object is brought very near the nozzle from which the spray emerges? Can you explain the behavior of the water spray in an electric field? Note: It will be necessary to dry and recharge the object repeatedly.
b) You can make a more permanent and portable demonstration model; this consists essentially of a stand to hold the water reservoir for the spray and the bowl with water in the center of which the nozzle is mounted to produce a fountain effect. The fountain bowl is hanging by its rim supported by pieces of wood so that the rubber hose and nozzle have space below the bowl. The water reservoir can be made of a tin can, but a large plastic bottle (from household detergents) is easiest to work with. We drill a hole through the screw cap and cement a piece of copper or glass tubing or the rubber hose directly into it. With a sharp knife the bottom is cut out of the bottle which is then fastened upside-down with a strip of metal from a tin can to an upright piece of wood 2 to 3 ft. long, 2" wide and 3/4" thick. This wood stick is fastened to a wooden base 1 x 12 x 12" with glue, nails, and an iron "L" or angle and screws. A hole, just big enough to receive the nozzle, is drilled or punched into the center of a bowl of aluminum or plastic (enamel is not suitable) of about 12" diameter, 2 to 5" deep. The nozzle is inserted from below and cemented to the bowl; also see Experiment 18 for a method for fastening the nozzle to the bowl bottom. At three or four places, the bowl is supported at its rim by pieces of wood of appropriate sizes nailed and glued to the base. The bowl is filled with water so that the water level remains below the tip of the spray nozzle.
The reservoir is filled with water; to get the fountain started, it may be necessary to squeeze the hose along its entire extent to get all the air out. Also make sure that the capillary of the nozzle is not clogged; if this is the case, the particle clogging the capillary is carefully pushed down with a fine bristle or hair-thin wire and rinsed out at the bottom of the nozzle. To stop the fountain, the hose can be clamped shut.
2 ft. plastic or rubber hose
Bowl with water and a plastic object (comb, etc.)
One quart plastic (or metal) bottle with screw cap (detergent bottle)
One aluminum or plastic bowl, 12" diameter, 3" deep.
Two eye droppers
30" rubber or plastic hose to fit eye droppers
Wood board 1 x 12 x 12"
Wood stick 3/4 x 2 x 30"
Three or four pieces of wood 1/2 x 2 x 4"
One strip of tin-can metal 2 x 12"
One iron "L", 1" wide, 3" length of the legs
Nails, glue, screws
A plastic comb or similar object and cloth, water.