Self-assembled precipitation membranes and the implications for natural sciences

Far from thermodynamic equilibrium, many precipitation reactions can generate complex membrane structures. Such membranes are of great research interest in fields ranging from chemical engineering to geophysics, and even biology where they are thought to have played a vital role in the origin of life. Usually, the transport of chemicals by combined buoyancy, osmotic and diffusive mechanisms, support the precipitation reaction. In order to study these transport processes across a growing selective membrane, we use reactions forming chemical gardens. We focus on four studies: one in a micro-fluidic reactor where flow is forced by a pump and others in a Hele-Shaw cell where the flow is driven by the membrane itself. In the first, with externally forced flow, the growth of a wavy precipitate membrane is observed. We establish that its growth is controlled by transverse diffusion and dispersion of the ions in solution. We develop a precipitation model, taking into account diffusion of ions through the precipitate and through an adjacent gel layer. Results from our theory are in excellent agreement with the measurements and show that a wavy precipitate surface can enhance the transverse transport of ions by extracting energy from a longitudinal flow field. In the second study, the chemical gardens are formed in a horizontal Hele-Shaw cell. We examine the changes of the membrane morphology associated with the concentration of reactants. We also survey the growth rate of membrane, which is determined by the osmotic flow as well as by concentration effects. The motion of the fluid is visualized in order to understand the transport process. The pressure inside the membrane structure is measured and different patterns of pressure changes are identified. A pressure-concentration model is proposed to explain the harmonic pressure changes of this system. In our third study, we observe that a chemical garden confined to two dimensions is a clock reaction involving a phase change, so that after a reproducible and controllable induction period it explodes. The explosion of chemical garden is caused by the decreasing permeability of membrane, owing to the gradual blocking of its pores by the precipitate. A pressure-concentration-thickness model is developed to analyse the explosive system. In our final study, we return to a classic chemical garden where gravity force is of relevance. Oscillatory growth of tubes in the vertical direction is witnessed. The chemical gardens explode at a late phase of experiments, with longer life times than the corresponding horizontal cases. We also observe descending flow with a surrounding precipitation structure, which is controlled by gravity.