Abstract

Far from the thermodynamic equilibrium, many precipitation reactions can create 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 a chemical-garden reaction in a micro-fluidic reactor. We focus on two studies in a horizontal Hele-Shaw cell: one where the flow is driven by the membrane itself and another where flow is forced by a pump. In the first, 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 the second study, 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 ions. 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.