Abstract
To explore and react to their environment, living micro-swimmers have developed sophisticated strategies for locomotion - in particular, motility with multiple gaits. To understand the physical principles associated with such a behavioural variability,synthetic model systems capable of mimicking it are needed. Here, we demonstrate bimodal gait switching in autophoretic droplet swimmers. This minimal experimental system is isotropic at rest, a symmetry that can be spontaneously broken due to the nonlinear coupling between hydrodynamic and chemical fields, inducing a variety of flow patterns that lead to different propulsive modes. We report a dynamical transition from quasi-ballistic to bimodal chaotic motion, controlled by the viscosity of the swimming medium. By simultaneous visualisation of the chemical and hydrodynamic fields, supported quantitatively by an advection-diffusion model, we show that higher hydrodynamic modes become excitable with increasing viscosity, while the recurrent mode-switching is driven by the droplet's interaction with self-generated chemical gradients. We further demonstrate that this gradient interaction results in anomalous diffusive swimming akin to self-avoiding spatial exploration strategies observed in nature.
Highlights
In response to physical constraints in nature, microorganisms have adapted and developed various locomotion strategies
To visualize the chemical and hydrodynamic fields involved in the droplet activity, we directly image the chemical field of swollen micelles by adding the hydrophobic dye Nile Red to the oil phase [Figs. 1(c) and 1(d); see Appendix A 5 and Supplemental Video S1 in [29] ]
Since we observe in experiments with Pe ≳ 100 that the active droplet experiences sustained periods of dynamical arrest during which it remains stationary with a surrounding extensile flow [Fig. 3(c)], it appears that the n 1⁄4 2 mode can evolve from a nonquiescent state and prevail in a similar Peclet regime as derived from the performed stability analysis
Summary
In response to physical constraints in nature, microorganisms have adapted and developed various locomotion strategies. A steric stress [4], octoflagellate microalgae exhibiting run-stop-shock motility with enhanced mechanosensitivity [5], and starfish larvae maximizing fluid mixing, and thereby nutrition uptake, through rapid changes of ciliary beating patterns [6] Such intricate gait-switching dynamics [7,8] enable organisms to navigate in external flows [9,10], to follow gradients [11], or to efficiently explore their environment [12,13]. Interfacial activity spontaneously breaks the symmetry, allowing for the emergence of different flow patterns depending on the environmental parameters We use such active droplets as model systems to demonstrate the physical principles guiding the emergence of multimodal motility in response to changes in environmental conditions. The visualization technique and the findings presented here lay the groundwork for future investigations of emergent dynamics in active phoretic matter
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