This is an open access article published under an ACS AuthorChoice License, which permits copying and redistribution of the article or any adaptations for non-commercial purposes. Details of the licence are available here: http://pubs.acs.org/page/policy/authorchoice_termsofuse.htmlChemical gardens are perhaps the best example in chemistry of a\ud self-organizing nonequilibrium process that creates complex\ud structures. Many different chemical systems and materials can\ud form these self-assembling structures, which span at least 8\ud orders of magnitude in size, from nanometers to meters. Key to\ud this marvel is the self-propagation under fluid advection of\ud reaction zones forming semipermeable precipitation membranes\ud that maintain steep concentration gradients, with osmosis and\ud buoyancy as the driving forces for fluid flow. Chemical gardens\ud have been studied from the alchemists onward, but now in the\ud 21st century we are beginning to understand how they can lead\ud us to a new domain of self-organized structures of semipermeable\ud membranes and amorphous as well as polycrystalline solids\ud produced at the interface of chemistry, fluid dynamics, and\ud materials science. We propose to call this emerging field\ud chemobrionics

Buoyancy-driven instabilities of a horizontal interface between two miscible solutions in the gravity field are theoretically studied in porous media and Hele-Shaw cells (two glass plates separated by a thin gap). Beyond the classical Rayleigh–Taylor (RT) and double diffusive (DD) instabilities that can affect such two-layer stratifications right at the initial time of contact, diffusive-layer convection (DLC) as well as delayed-double diffusive (DDD) instabilities can set in at a later time when differential diffusion effects act upon the evolving density profile starting from an initial step-function profile between the two miscible solutions. The conditions for these instabilities to occur can therefore be obtained only by considering time evolving base-state profiles. To do so, we perform a linear stability analysis based on a quasi-steady-state approximation (QSSA) as well as nonlinear simulations of a diffusion–convection model to classify and analyse all possible buoyancy-driven instabilities of a stratification of a solution of a given solute A on top of another miscible solution of a species B. Our theoretical model couples Darcy's law to evolution equations for the concentration of species A and B ruling the density of the miscible solutions. The parameters of the problem are a buoyancy ratio R quantifying the ratio of the relative contribution of B and A to the density as well as δ, the ratio of diffusion coefficients of these two species. We classify the region of RT, DD, DDD and DLC instabilities in the (R, δ) plane as a function of the elapsed time and show that, asymptotically, the unstable domain is much larger than the one captured on the basis of linear base-state profiles which can only obtain stability thresholds for the RT and DD instabilities. In addition the QSSA allows one to determine the critical time at which an initially stable stratification of A above B can become unstable with regard to a DDD or DLC mechanism when starting from initial step function profiles. Nonlinear dynamics are also analysed by a numerical integration of the full nonlinear model in order to understand the influence of R and δ on the dynamics.

This paper presents a method to form micron-sized droplets in an aqueous two-phase system (ATPS) and to subsequently polymerize the droplets to produce hydrogel beads. Owing to the low interfacial tension in ATPS, droplets do not easily form spontaneously. We enforce the formation of drops by perturbing an otherwise stable jet that forms at the junction where the two aqueous streams meet. This is done by actuating a piezo-electric bending disc integrated in our device. The influence of forcing amplitude and frequency on jet breakup is described and related to the size of monodisperse droplets with a diameter in the range between 30 and 60 μm. Rapid on-chip polymerization of derivatized dextran inside the droplets created monodisperse hydrogel particles. This work shows how droplet-based microfluidics can be used in all-aqueous, surfactant-free, organic-solvent-free biocompatible two-phase environment.

Significance: Chemical gardens are plant-like mineral forms existing in nature at various scales. Even though they have been described for more than three centuries thanks to their beauty and analogies with biological forms, their growth mechanisms remain largely not understood. To gain insight into their formation processes, we develop an innovative strategy to study these systems in which chemical gardens grow in a quasi-2D confined geometry upon injecting one reagent solution into the other. The advantage of this procedure over previous works is to allow studying, in controlled and reproducible conditions, the effects of variable concentrations and injection rates on the selected morphology.

When one solution of reactant A is displacing another miscible solution of reactant B, a miscible product C can be generated in the contact zone if a simple A + B → C chemical reaction takes place. Depending on the relative effect of A, B and C on the viscosity, different viscous fingering (VF) instabilities can be observed. In this context, a linear stability analysis of this reaction–diffusion–convection problem under the quasi-steady-state approximation is performed to classify the various possible instability scenarios. To do so, we determine the criteria for an instability, obtain dispersion curves both at initial contact time using an analytical implicit solution and at later times via numerical stability analysis. Along with recovering known results for non-reactive systems where the displacement of a more viscous fluid by a less viscous one leads to a VF instability, it is found that in the presence of a chemical reaction, injecting a more viscous fluid into a less viscous fluid can also lead to instabilities. This occurs when the chemical reaction leads to the build up of non-monotonic viscosity profiles. Various instability scenarios are classified in a parameter plane spanned by R_b and R_c representing the log-mobility ratios of the viscosities of the B and C solution respectively with respect to that of the injected solution of A. A parametric study of the influence on stability of the Damköhler number and of the time elapsed after contact of the two reactive solutions is also conducted.

We present a microfluidic method to compartmentalize aqueous polymer solutions within water-in-water microdroplets, which are continuously generated without using organic solvents or surfactants. Phase separation inside the drops yields all-aqueous core-shell structures (water-in-water-in-water), as we demonstrate using the aqueous two phase system of polyethylene glycol and dextran.