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Introduction

Chemical, colloidal, crystal, silica, or silicate gardens (Fig. 1) are familiar to every child with a chemistry set. As their very name indicates, their forms are similar to those observed in living organisms, which provoked a flurry of interest a century ago from investigators in search of the origin of life [1,2,3]. Unfortunately, as the necessary notions of biochemistry were then lacking, that proved to be a false trail. However, almost a hundred years later, and several centuries on from the first observations of so-called metallic trees by the early chemists such as Glauber [4,5], chemical gardens remain incompletely understood. In this paper we present the results of experiments observed with Mach-Zehnder interferometry designed to understand the formation and growth of the hollow tubes that are the basic structures in chemical gardens. We discuss the fluid dynamics implicated in their morphogenesis, and highlight the dynamical processes underlying their characteristic patterns.

Chemical gardens are obtained from the precipitation reaction on adding crystals of soluble metal salts to aqueous solutions containing anions such as aluminates, borates, carbonates, chromates, cyanoferrates, phosphates, or silicates [6]. Alternatively, a solution of the metal salt may be used instead of the solid, when the reaction takes place at the interface between the two liquids [7,8]. In spite of their chemical diversity, the common characteristic of all these reactions is the precipitation of a semipermeable colloidal membrane, across which osmosis occurs. Such precipitation membranes, first studied by Traube in the middle of the nineteenth century [9], were employed a few years later by Pfeffer in his investigations into osmosis [7], and in this way played a fundamental rôle in the quantitative understanding of that phenomenon, which culminated with van't Hoff's law of osmotic pressure, formulated in 1887 [10,11].

Among all the chemical-garden reactions, those that have been most studied are from silicate solutions -- hence the terms silica or silicate gardens. However, even in this case the basic mechanisms of morphogenesis remain unclear, and we choose here to study these in depth. The chemistry of sodium silicate and its behaviour in solution -- as water glass -- is rather complex [12]. Adding a crystal of a soluble metal salt to water glass provokes a reaction producing the hydrous metal silicate, which is deposited as a colloidal gel around the crystal [13]. The gel acts as a semipermeable membrane, through which water and excess hydroxide ions are drawn under osmotic pressure. Under this process the crystal continues to dissolve, and the membrane surrounding it to stretch, until the latter ruptures, ejecting a jet of fluid into the surrounding solution. At each of the points of rupture there develop tubular fibres. In the right conditions, these can grow to many centimetres in length. Thus are formed the characteristic chemical-garden growths. Most metals, except the alkali metals of group 1 of the periodic table, whose silicates are very soluble, produce growths [14]. The compositions of two silicate-garden precipitates have recently been studied in detail: those formed from aluminium nitrate [15,16], and from copper nitrate [17]. The former precipitate is a material with a hierarchical structure on the nanoscale, consisting of silica nanotubes clustered together over several orders, surrounded by aluminosilicate and aluminium hydroxide. The latter precipitate is more crystalline, being formed in part of crystalline copper hydroxide nitrate, together with amorphous silica.

Apart from their purely scientific fascination as spectacular examples of pattern formation, chemical gardens may be implicated in practical problems that involve the precipitation of a colloidal gel membrane separating two aqueous solutions of different compositions. The importance for cement technology arises from understanding the hydration of Portland cement. The chemistry involved in the formation of Portland cement may be seen as a type of silicate-garden system [18,19,20,21,22]. While in the usual silicate garden a crystal of a soluble metal salt is immersed in a solution of sodium silicate, in Portland cement solid calcium silicates are immersed in water, when a membrane of calcium silicate hydrate is formed around them. Because of its semipermeable character, osmosis creates an excess pressure in the volume enclosed by the membrane that causes its rupture, and jets of solution are ejected that react with the outer solution to precipitate further material as thin tubular filaments of calcium silicate hydrate. The fabrication of the cement consists of the formation of a mesh of these filaments. The Portland cement system is then analogous to a reverse silicate garden; one in which sodium silicate grains are immersed in a metal ion solution. A further example of a possible practical application of chemical gardens is to metal corrosion in aqueous environments, in which corrosion products on the metal surface may be colloidal in character [23,6,13], and corrosion tubes can form on the surface of rusting iron or steel [24,25,26,27,28]. Lastly, in lead-acid battery technology, a membrane-osmosis model of battery paste has been suggested [29]. In an interesting example of the cyclic nature of science, a century on from the research of Leduc [1], Herrera [2], and others that linked chemical gardens to the origin of life through their similarity in morphology, the wheel has turned full circle. Recent work speculates that chemical-garden type membranes produced in precipitates in submarine vents early in our planet's history would be natural containers within which the controlled environment may have provided the impetus for the emergence of life on Earth [30].

Investigations of chemical gardens have concentrated up to now on understanding the chemistry producing the membrane, and the mechanism of its rupture. Here, we look at the physics giving rise to the pattern formation.


next up previous
Next: Materials and Methods Up: Formation of Chemical Gardens Previous: Formation of Chemical Gardens
Julyan Cartwright 2002-12-13