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Effects of buoyancy

It is evident that buoyant plumes are a major feature of the dynamics of chemical gardens. This may be confirmed by orienting the Hele-Shaw growth cell horizontally rather than vertically, or by placing a crystal at the surface of a solution of sodium silicate. If the latter operation is performed carefully, surface tension will keep the crystal from sinking. In this case, tubes do not form in solutions in which they would normally do so, and instead the growth is in the form of a cup around the underside of the crystal, as Fig. 8a demonstrates, which may gradually be enlarged downwards by the osmotically driven flow of liquid into it [6]. Furthermore, if the crystal is attached to the wall of the chamber, as shown in Fig. 8b, instead of being placed at its base, tubes grow from it as normal, but only upwards and not downwards.

If the effects of buoyancy are removed, the remaining driving force for chemical-garden formation is osmotic pressure, which dilates the initial membrane and powers the jet of fluid that emerges under pressure from a break. This naturally leads one to ask what form a chemical garden would take in the absense of buoyancy; or what is the same thing, in the absense of gravity. We should expect to find that free convection (buoyant plumes) has disappeared, and there remains only forced convection (osmotically powered jets). Chemical garden experiments have been performed at least twice in microgravity conditions, in which they can be studied in three dimensions. Stockwell and Williams [42] report growth in random directions, together with spirals for which they had no explanation. Unfortunately they provide few details of their experiments. Subsequently Jones and Walter [33,5] flew a further experiment, in which they found that the reaction occurred an order of magnitude slower than on ground, due to the absence of free convection. The absence of buoyancy-driven flow led to novel structures, as apart from the tubes seen on Earth, they found also evidence for spherical membranes and fingers, structures typical of Laplacian growth mechanisms such as viscous fingering. A preliminary model for the instability of a moving semipermeable membrane as may be applied to this case has been presented by Sørensen [43].


next up previous
Next: Discussion Up: Mechanism of growth Previous: Instabilities of tube formation
Julyan Cartwright 2002-12-13