In the present system, a pellet of copper sulfate was immersed in solutions with different concentrations of oxalate and the processes that lead to complex structures were observed. In each case, the copper dissolved into the surrounding solution [18]. Due to density differences, the more concentrated solution sinks to the bottom of the reaction dish and spreads across it to form a layer on the bottom. The bottom solution then moves outward from the pellet, while the upper solution moves towards the pellet.
At an oxalate concentration of 0.02 M during the movement of solutions, copper oxalate is formed and makes a thin finger-like precipitation pattern. At 0.05 M sodium oxalate, in addition to the fingers, a light blue disc-like structure made of CuC2O4 with a diameter of about 3 cm develops on the bottom of the dish (Figure 1A). When the oxalate concentrations increased above 0.05 M the disk becomes more pronounced and reaches 1 mm in thickness. At 0.11 M oxalate a dark blue crystalline “urchin” structure not observed at lower oxalate concentrations forms while the thin fingers are absent (Figure 1B & C). The dark blue crystal growth begins from many initiation points all around the disc and radiates in all directions. Chemical analysis confirms that these dark blue crystals are made from Na2Cu(C2O4)2 ×2H2O.
The light blue crystals which are the subunits of the thin fingers (Figure 1A) and light blue disk (Figure 1A &B) are formed by the following reaction:
In our system, at oxalate concentrations above 0.11 M, the CuC2O4(s) undergoes dissolution according to following reaction:
This reaction provides a necessary reactant for the formation of the dark blue Na2Cu(C2O4)2 ×2H2O crystals (Figure 1B & 1C) according to the following equation:
The precipitation patterns are substantially more complicated for 0.15-0.25 M oxalate as presented in Figure 2. Initially, a CuC2O4(s) precipitation disk forms. At the edge of this disk Na2Cu(C2O4)2 ×2H2O(s) structures very distinct from the dark blue crystals seen at 0.11 M oxalate (Figure 1B & 1C) form from a handful of initiation points (Figure 2A). Crystalline projections “fans” from these initiation points grow towards the center of the disk and eventually cover its entire surface (Figure 2B & 2C). During a period of approximately 10 hours, the CuC2O4(s) disk disappears altogether through transformation to the structure shown in Figure 2D. The volume of this structure increases by a factor of 10 in comparison with the structure presented on Figure 2A & B and the height changes from 2 mm to 20 mm. The stem of this “mushroom” structure is filled with a gel-like substance. The gel was removed from the solution, washed, and dried for analysis which determined that the composition is Na2Cu(C2O4)2 ×O2H2O.
We also investigated the microscopic structure of the precipitation patterns and found that each was constructed from a morphologically distinct crystal subunit (Figure 3A-D). The CuC2O4 crystals forming the light blue fingers (Figure 1A) are constructed from filaments (Figure 3A) and theCuC2O4precipitation disk is constructed from intricate spherical subunits (Figure 3B). The dark blue Na2Cu(C2O4)2 ×2H2O crystal (Figure 1B & 1C) subunits are seen in (Figure 3C). The fans (Figure 2B & 2C) are composed of other unique crystal structures (Figure 3D). Their formation is presumably controlled by system hydrodynamics where local changes in oxalate concentrations are causing changes in solution density and viscosity. Formation of the mushroom morphology is more complex. In the microscopic picture shown on (Figure 3E), one can see hollows that develop from the drying of the gel. The formed crystals are presented on (Figure 3F).
As is the case in biological systems, the formation of these complex structures involves many processes. Physical processes include the diffusion of copper ions from the pellet to the oxalate solution, establishment of a density gradient resulting in the solute convection of concentrated solution downwards and away from the pellet across the bottom of the dish while less dense solution moves toward the pellet. Chemical processes include CuSO4 dissolution, formation of CuC2O4 crystals with two different morphologies, formation of crystals of Na2Cu(C2O4)2 × 2H2O with two different morphologies and a gel. The CuC2O4(s) precipitation disk serves to organize crystal growth. For oxalate concentrations in the range of 0.15-0.25 M, we observe metamorphosis i.e., the transition of one form into another (see Figure 2B, 2D). Usually, in hierarchical constructions, one structure is built from others. In the presented experiments, however, there is a full metamorphosis to a different structure without elements of the first structure remaining.
In our experiments, the initial conditions have a “seed” arrangement which is somewhat analogous to biological morphogenesis. In this experiment, the copper sulfate “seed” is surrounded by an environment of sodium oxalate. This non-homogeneous arrangement establishes various potential gradients that define a network of chemical and physical processes. This network and its oxalate concentration dependent bifurcation points, shown in a schematic diagram (Figure 4), controls the precise spatio-temporal execution of chemical and physical processes which lead to the construction of complex precipitation patterns. The initially formed chemical structures establish subsequent potential differences and a cascade of processes develops. Small changes to system parameters (i.e. initial oxalate concentration) leads to switching the construction trajectory from one pathway in the processes network to another, leading to a radically different structure therefore we can control the system trajectory and which structures are formed.
The sophistication of processes and complexity of structures that are generated by this simple system has ramifications for our understanding of primitive life and evolutionary processes. Some complex structures do not need to have evolved in a slow, step by step fashion and could exist in the absence of large amounts of genetic information if organisms could nucleate spontaneous, complex processes. Small changes in genes that control nucleation conditions could also lead to drastic new structural forms.