edited by GEORGEL. GILBERT Denison University Granville, Ohio 43023
Fractal Structures Obtained by Electrodeposition of Silver at an Air-Water interface S u s ~ l m BY o
WoodfinV. Ligon, Jr. General Electrlc Company Corporate Research and Development Schenectady, NY 12301
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I t has recentlv been recoenized that manv natural nrocesses involving diffusion-limited aggregation of particles produce structures that may be described as fractals.' Examples include colloid particle aggregates and many eledrodeposited dendritic structures. C o m ~ u t e rsimulations of fractal growthlJ have been shown to produce structures that closely approximate the structures obtained from these natural ~ r o .. cesses.3,4 A recent cover photo for the iournal American Scientist5 is an example of sich a computer simulation. I was especially taken with this particular simulation because it immediately reminded me of some electrodeposited dendritic silver structures that I accidentally learned to generate 20 years ago while an undergraduate student at Longwood College in Farmville, Virginia. My procedure is quite simple compared withothers published to date3,"nd for this reason may he of interest to readers of this Journal for possible use as a lecture demonstration. In addition. the structures nroduced are quite attractive. A 1.5-in.-diameter silver dendrite grown by my method appears as the figure. This complex, highly branched treelike structure was produced electrochemically in a few seconds on the surface of a 0.1 M solution of ammoniacal silver nitrate. A solution of about 200 mL was used, and the experiment was conducted in a 250-mL beaker. The anode and cathode structures are not critical. Ordinary paper clips were used in this exam~le.The anode should extend below the surface of the soluiion and can be attached to the side of the beaker. I t is necessary that the negative electrode be supported by the surface tension of the solution near the center of the beaker. In this case voltaae was ~rovidedhv a 22119-V battery. A simple potentiomete;was used to cont;ol voltage and thereby to control growth rate and to some extent dendrite structure. The voltages required are not at all critical and are best determined experimentally for each dendrite. If a dark gray "blob" begins to grow beneath the surface because the cathode has penetrated the surface, then lifting the cathode sliehtlv " . will usuallv cause dendrites to oromgate on the surface. The dendrites can be lifted from the water surface usinp" filter paper in a manner exactly analogous to lifting Langmuir-Blodgett monolayers. If long-term storage is anticipated, the dendrites should he immediately redeposited on the surface of a beaker of distilled water and lifted off that surface using a second filter. This procedure yields a dendrite and filter relatively free of silver nitrate solution. In the A
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' Witten, T. A.: Sander, L. M. Phys. Rev. Len. 1981, 47, 1400.
Meakin, P. Phys. Rev. A 1983, 27, 5686. Brady, R. M.: Ball, R. C. Nature 1984, 309, 225. Matsushita, M.; Sano, M.; Hayakawa. Y.: Honjo, H.: Sawada, Y. Phys. Rev. Len. 1984,53,286. Meakin, P. Am. Sci. 1986, 74, Jan-Feb. cover.
A 1.bin:diametei fractal structure obtained inabout 10s by electrodeposition of silver at an air-water interface.
example shown the Alter and dendrite were next allowed to dry, a i d then the dendrite was covered with a clear adhesive tape. T o improve contrast. the filter naner was saturated from the back with black stamp-pad i i k . ' ~ salways ammoniacal silver nitrate solutions must never be allowed to evaporate to dryness and must be disposed of with care.
Musical Molecular Weights Revisited S U B M ~ EBY D
Frederick B. Augustine Chabot College Hayward, CA 94545
CHECKED BY
Herberl Slotnlck Central Connecticut State College New Britain, CT 06050
This note describes a simple way of comparing the propagation of sound in two different gases. A more quantitative study of this and other physical properties of gases has been reported by Davenport, Howe-Grant, and Srinivasan'. They arrived a t "musical molecular weights" by use of an organ pipe and balloons of various gases. A tape recorder was
' Davenport, D. A.: Howe-Grant, M.; Srinivasan. V. J. Chem. Edoc. 1979, 56, 523.
Volume 64
Number 12
December 1987
1053
recommended to retain one pitch t o compare with another. By use of a T-tube and a pair of pinch clamps, two balloons can be connected to the same sounding device (see Figure). Perhaps the simplest source of a whistle is to cut off the mouthpiece of a toy plastic recorder or flute, available a t most toy stores for less than $2.00.
Apparatus fcw comparing "musical" molecular weights
1054
Journal of Chemical Education
In operation, rapid alternation between the two gas sources allows discrimination of subtle differences in nitch. Nitrogen and oxygen are easily distinguished. With hklium oaired with carbon dioxide. an acceotable (unacceotahle?) wolf whistle results. With still more T-tub& and ba~~oonk (and hands) the possibilities are manifold.
