Chemical Generation and Visualization of hydrodynamic Instability An Extremely Simple Demonstration of Self-organization Peter G. Bowers and Leonard J. Soltzberg Simmons College, Boston, MA 021 15 BBnard convection is perhaps the most frequently cited example of self-organization in a far-from-equilibrium system and takes the form of parallel convectionrolls or hexagonal convection cells in ashallow liquid layer whichis heated from below (I). If the laver is sandwiched between two confining plates, the fluid motion, called Rayleigh-BBnard convection is due to the coupling of buoyancy, heat diffusion, and viscous forces, while if the upper liquid surface is free (BBnard-Maranaoni convection). surface tension chanaes (rather than density changes) also become important.'~n each case, the onset of pattern formation occurs at a sharply defined temperature difference between the upper and lower liquid surfaces. The value of this critical temperature difference deoends on the deoth of the liouid laver and the geometry of the confining vessel as well as upon those properties of the liquid mentioned above. Methods used to visualize and study such convection phenomena have included suspending fine metal particles in the liquid (often a silicone oil) and employing special point source illumination. Development of these patterns is slow, usuallv on the time scale of hours. Buoyancy instabilities can also be produced and visualized in a thin liquid layer by initiating a chemical reaction a t the upper surface. This was first observed relatively recently bv Mockel (2).who reoorted the aooearance of concentratfon patterns Ghile irradiating aqueous KIfstarch solutions. Visualization in this case was due to the formation of the colored starch-iodine complex formed by air photooxidation of iodide ion. Kagan and Avnir and their co-workers (3-5) have subsequently shown that patterns can be produced from a wide variety of chemical processes that can be carried out at a surface, either photochemically or by surface absorption of a reactant, and that give a colored product. The chemically visualized structures are transient and usually have lifetimes on the scale of minutes. While it is generally aereed that what is beine observed is indeed convective i o t i o n , there has been le& consensus on whether such motion is generated by the chemical reaction itself (which could cause, for example, changes in density, diffusion rates, or surface tension), or whether the color changes simply visualize pre-existing convective motions due to evaporative coolinp. I t now appears that in individual systems either of these factors may dominate (6). Perhaps the simplest of all the chemical convective systems, the one we describe here and that appears on the cover of this issue of the Journal, is the pattern formation seen when an acidic vaoor comes into contact with the surface of an aqueous solution containing indicator. The general mcthod for ~roducinethe effect. which mas he varied within wide limits,~consistsuofexposing the surface of an aqueous and 'The spontaneous pattern formation also makes a remarkable overhead projector demonstration. For best results, the projector should be allowed to heat up somewhat. 210
Jouinal of Chemical Education
slightly hasic layer (10-3-10-4 M in OH- and 5-8 mm deep) to HCl vapor. The solution'is contained in a deep Petri dish and contains sufficient indicator (almost any indicator) to make even the shallow layer significantly colored. The HCI vapor originates from a filter paper disc soaked in 6 M HCI, which is of sufficient size to cover the vessel; the paper is allowed to rest on the top edge of the dish until pattern formation begins, typically within 20 t o 40 s depending on the svstem beina emdoved. specific condrtion; fo; the pattern development shown in the cover photographs are given in the Appendix below. T o facilirate photography, our solution was placed on a light table', and there is little doubt that it developed a thermal gradient from the light source, perhaps enhanced by surface evaporation. The thermal effect acts in concert with the density gradient produced by HCI absorption. In any case, it is evident that a thin surface layer develops that has a higher densitv and substantiallv hiaher aciditv than the liouid beneath;such a ~ o n f i ~ u r a t i bkni ,t h the denser liquid Ln top, is not a t eauilihrium. Eventuallv. .. the svstem is driven far enough &om equilibrium to undergo-the self-orpaniring transition shown in the photographs. The sequence clearly shows how a roll develops, and how liquid in adjacent rolls rotates in opposite directions; the initial descending vertical sheets of acidic solution (yellow) formed at the start of the convection are seen in the first two pictures. When a yellow sheet reaches the bottom of rhe dish, it flows outwards horizontally along the dish bottom (third photograph) and begins to rise again when it collides with the front boundary of a neighboring sheet, forming a rising vertical wall common to both rolls (as in the last i)hotomaoh). On the averare. the distance between rolls is equal to the depth of the layer, as predicted by convection theory (I). - BYvarying the starting conditions, it is possible to observe a great diversity of patterns. For example, if our procedure is repeated without background heat, the patterns take much longer to form (up to 10 min) and then appear as circular, approximately close-packed cells. Again, if thiosulfate is present (10-3 M NazSzOs), the acidic regions of the pattern form a temolate for *orecioitation of sulfur and can be seen without indicator. Chemically visualized convective self-oreanization is indemonstration because it is quick, easy, and teresting as ; economical. Clearly the chemistry in the system we describe is trivial, and t h e solution can-be regenerated and used indefinitely by adding a few drops of base. Discovery of the phenomenon has other significance as well. Its occurrence may complicate the interpretation of true chemical spatial structures unless precautions are taken (7). The comparatively straightforward visualization technique may prove of use in fluid dynamics, where buoyancy instability is an active area of research; the basic physics has application to atmospheric motion at both local and large-scale levels, to vertical ocean currents, and to motion of Earth's liquid mantle. On a smaller scale, convective patterns are important in
paint technology, where the roughness of a freshly painted surface is thought to be due to convective cells "frozen" into the laver as the paint dries ( I ) . Appendix
For the cover photographs, we used a 5-mm-deep layer of solution in an uncovered 100- X 50-mm Petri dish. The solution was prepared hy diluting 15 mL of saturated aqueous bromcresol green indicator t o 40 mL with 5 X loe4 M NaOH. The indicator solution was filtered prior to use to eliminate specks of undissolved indicator. After placing the solution on the light table, we covered the Petri dish for 10 s
with a 12-cm filter paper soaked in 6 M HCI, carefully removed the paper, and took photographs a t 204 intervals. The blue color of the indicator solution could be regenerated for further experiments by adding a minimum amount of base (1-2 drops of 0.1 M NaOH).
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4. Avnu,D.;Kagan,M.Notunuirs. 1983,70,36l;~ofure198~307,717. 5. Avnir, D.; Kagan, M.: Rosa,W . Cham.Phys.L?tI.198%135,177. B O ~ C P.: ~D ~~W ~~ IG.; ,, D.:K ~ ~ ~ J.~stat.phya. Y ~ Y 1987.48, . IMI.
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7. Field, R J.~ m 4 r . 1985,73,142. S ~
Volume 66
Number 3
March 1989
211
