Electrotaxis behavior of droplets composed of aqueous Belousov-Zhabotinsky solutions suspended in oil phase

Abstract

Taxis is ubiquitous in biological and physical chemistry systems as a response to various external stimulations. We prepared aqueous droplets containing Belousov-Zhabotinsky (BZ) solutions suspended on an oleic acid oil phase subject to DC electric field and found that these BZ droplets undergo chemically driven translational motion towards the negative electrode under DC electric field. This electrotaxis phenomenon originates from the field-induced inhomogeneous distribution of reactants, in particular Br$$^{-}$$ - ions, and consequently the biased location of the leading centers towards the positive electrode. We define the ’leading center’ (LC) as a specific location within the droplet where the BZ chemical wave (target pattern) is initiated. The chemical wave generated from the LC propagates passing the droplet center of mass and creates a gradient of interfacial tension when reaching the droplet-oil interface on the other side, resulting in a momentum exchange between the droplet and oil phases which drives the droplet motion in the direction of the electric field. A greater electric field strength renders a more substantial electrotaxis effect.

The excitable nature of polymerizing actin and the Belousov-Zhabotinsky reaction

“…The five ingredients form a fivedimensional parameter space, mapping the emergent patterns based on the possible combinations of initial concentrations. On top of that, reaction dynamics can be heavily impacted by environmental factors such as temperature (Carballido-Landeira and Epstein, 2010;Howell et al, 2021) or externally applied fields (Agladze and De Kepper, 1992;Blank and Soo, 2003;Back et al, 2023), or by the spatial dimensions of the reaction container (Toth et al, 1994;Steinbock and Müller, 1998).…”

“…When encapsulated within droplets, the BZ reaction can cause self-propelled motion reminiscent of cell migration (Kitahata et al, 2002;Suematsu et al, 2016;Back et al, 2023). This movement is driven by Marangoni flows, which arise driven by gradients in surface tension along the interface between the internal solution and the surrounding oil, as illustrated in Figure 2B.…”

“…cell migration, the pursuit of these gradients is referred to as taxis. Electrotaxis, the ability of cells to follow electrical fields, can be observed in both droplets and cells (Zajdel et al, 2020;Back et al, 2023). While the underlying mechanism in cells is not fully understood, the propagation direction of polymerizing actin waves also align with an applied electrical field (Yang et al, 2022).…”

“…An applied electric field triggers symmetry breaking and influences the speed of the emergent waves. Either accelerating or decelerating them depending on their propagation direction relative to the direction of the field (Agladze and De Kepper, 1992;Back et al, 2023). An applied electric field biases the motion and direction of a BZ droplet.…”