Significance Turing proposed that intercellular reaction-diffusion of molecules is responsible for morphogenesis. The impact of this paradigm has been profound. We exploit an abiological experimental system of emulsion drops containing the Belousov–Zhabotinsky reactants ideally suited to test Turing’s theory. Our experiments verify Turing’s thesis of the chemical basis of morphogenesis and reveal a pattern, not previously predicted by theory, which we explain by extending Turing’s model to include heterogeneity. Quantitative experimental results obtained using this artificial cellular system establish the strengths and weaknesses of the Turing model, applicable to biology and materials science alike, and pinpoint which directions are required for improvement.
We experimentally and computationally study the dynamics of interacting oscillating Belousov-Zhabotinsky (BZ) droplets of $120 mm diameter separated by perfluorinated oil and arranged in a onedimensional array (1D). The coupling between BZ droplets is dominated by inhibition and is strongest at low concentrations of malonic acid (MA) and small droplet separations. A microfluidic chip is used for mixing the BZ reactants, forming monodisperse droplets by flow-focusing and directing them into a hydrophobized 100 mm diameter capillary. For samples composed of many drops and in the absence of well defined initial conditions, the anti-phase attractor, in which adjacent droplets oscillate 180 outof-phase, is observed for strong coupling. When the coupling strength is reduced the initial transients in the phase difference between neighboring droplets persist until the BZ reactants are exhausted. In order to make quantitative comparison with theory, we use photosensitive Ru(bipy) 3 2+ -catalyzed BZ droplets and set both boundary and initial conditions of arrays of small numbers of oscillating BZ droplets with a programmable illumination source. In these small collections of droplets, transient patterns decay rapidly and we observe several more complex attractors, including ones in which some adjacent droplets are in-phase. Excellent agreement between experiment and numerical simulations is achieved.
We study the dynamical behavior of one-dimensional arrays of ∼100 μm diameter aqueous droplets containing the oscillatory Belousov-Zhabotinsky (BZ) reaction, separated by narrow gaps of a fluorinated oil. In this closed system, the malonic acid concentration decreases as the reaction proceeds. Starting with a low initial malonic acid concentration, we observe a series of attractors as a function of time in the following order: anti-phase attractors; in-phase attractors, which evolve into traveling waves; and mixed modes that contain either regions of in-phase droplets separated by anti-phase oscillators, or in-phase oscillators combined with non-oscillatory droplets. Most of the observations are consistent with numerical chemical models of the BZ reaction in which components that participate in the excitatory (bromine dioxide and bromous acid) and inhibitory (bromine) pathways diffuse between the droplets. The models are used to quantitatively assess the inter-drop coupling strength as a function of drop separation, drop size and malonic acid concentration. To experimentally establish the mechanism of excitatory coupling between the BZ droplets, we verify the transport through the fluorinated oil of chlorine dioxide and several weak acids, including malonic acid.
After a polymerizable hydrogelator self-assembles in water to form molecular nanofibers, post-self-assembly cross-linking allows the catalyst of the Belousov-Zhabotinsky (BZ) reaction to be attached to the nanofibers, resulting in a hydrogel that exhibits concentration oscillations, spiral waves, and concentric waves. In addition to the first report of the observation of BZ spiral waves in a hydrogel that covalently integrates the catalyst, we suggest a new approach to developing active soft materials as chemical oscillators and for exploring the correlation between molecular structure and far-from-equilibrium dynamics.
While living systems have developed highly efficient ways to convert chemical energy (e.g., ATP hydrolysis) to mechanical motion (e.g., movement of muscle), it remains a challenge to build muscle-like biomimetic systems to generate mechanical force directly from chemical reactions. Here we show that a continuous flow of reactant solution leads to by far the largest volume change to date in autonomous active gels driven by the Belousov-Zhabotinsky reaction. These results demonstrate that microfluidics offers a useful and facile experimental approach to optimize the conditions (e.g., fabrication methods, counterions, flow rates, concentrations of reagents) for chemomechanical transduction in active materials. This work thus provides much needed insights and methods for the development of chemomechanically active systems based on combining soft materials and microfluidic systems.
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