Abstract
The confluence of droplet-compartmentalised chemical systems and architectures composed of interacting droplets points towards a novel technology mimicking core features of the cellular architecture that dominates biology. A key challenge to achieve such a droplet technology is long-term stability in conjunction with interdroplet communication. Here, we probed the parameter space of the Belousov-Zhabotinsky (BZ) medium, an extensively studied model for non-equilibrium chemical reactions, pipetted as 2.5 mm droplets in hexadecane oil. The presence of asolectin lipids enabled the formation of arrays of contacted BZ droplets, of which the wave patterns were characterised over time. We utilised laser-cut acrylic templates with over 40 linear oil-filled slots in which arrays are formed by pipetting droplets of the desired BZ composition, enabling parallel experiments and automated image analysis. Using variations of conventional malonic acid BZ medium, wave propagation over droplet-droplet interfaces was not observed. However, a BZ medium containing both malonic acid and 1,4-cyclohexanedione was found to enable inter-droplet wave propagation. We anticipate that the chemical excitation properties of this mixed-substrate BZ medium, in combination with the droplet stability of the networks demonstrated here for nearly 400 droplets in a template-defined topology, will facilitate the development of scalable functional droplet networks.
Highlights
Some microscale structuring methods impose physical boundaries on a continuous BZ medium, for example 1,4-cyclohexanedione (CHD) BZ confined in polydimethylsiloxane (PDMS) networks, where the local surface-to-volume ratio affects the extent of bromine diffusion into the PDMS matrix[5]
For BZ droplet plugs of 1–7 nL volume and interdroplet octane plugs in the range of 100–400 μm, stable anti-phase oscillation between neighbouring droplets or stationary Turing patterns were observed depending on the malonic acid (MA) concentration, which was attributed to inhibitory droplet-droplet coupling facilitated by diffusion of bromine through the oil phase[9,10,11]
Inhibitory coupling through oil gaps was not possible because the oil-solubilised surfactant served as a bromine scavenger, but excitatory waves could propagate through the 2D BZ droplet network at droplet-droplet contact points, which was attributed to excitatory BZ species diffusing over an inter-droplet surfactant bilayer[15,16]
Summary
Some microscale structuring methods impose physical boundaries on a continuous BZ medium, for example 1,4-cyclohexanedione (CHD) BZ confined in polydimethylsiloxane (PDMS) networks, where the local surface-to-volume ratio affects the extent of bromine diffusion into the PDMS matrix[5]. Guzowski et al observed coupling between microfluidically produced pairs of MA BZ droplets in hexadecane with asolectin lipids, manifested as larger droplets of ≈0.9 μL volume increasing the inherent self-oscillation frequency of smaller droplets (down to ≈0.1 μL), which are postulated to contain a lower concentration of oil-soluble reaction species because of a higher surface-to-volume ratio[17]. This coupling was explained by diffusion of excitatory species, HBrO2, over the asolectin lipid bilayer at the droplet-droplet interface[17]. It has been a challenge to achieve stability for contacting BZ droplets-in-oil[22], preventing systematic exploration of functional droplet networks
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