Péclet (Pe) dependent dynamics and interactions in chemically active droplets



Artificially propelled droplets are captivating active soft matter systems with the potential to mimic biological motion and have the potential to be used as as advanced micro-cargo transporters. In this study, we investigate the motion of micellar solubilization-driven oil (5CB) droplets in an aqueous medium containing the surfactant TTAB. The process of micellar solubilization induces a spontaneous asymmetry in interfacial surfactant concentration, generating an interfacial tension gradient that propels the droplet through imbalanced Marangoni stresses. During propulsion, the droplets release a chemical field of filled micelles, referred to as solutes, whose advection-diffusion behavior—characterized by the non-dimensional Péclet number (Pe)—significantly influences droplet motion.

By introducing macromolecular (high molecular weight PEO) additives into the aqueous phase, we demonstrate for the first time a Pe-dependent transition in droplet swimming behavior. At high Pe, droplets exhibit a conventional pusher mode with random trajectories, while at low Pe, they transition to a puller mode characterized by more persistent motion. Our findings further reveal that tuning Pe provides a mechanism to modulate the pair-interactions between droplets by altering the underlying chemical and hydrodynamic fields. At low Pe, interactions are dominated by chemo-repulsive effects that prevent physical contact and, in multi-body systems, promote the emergence of ordered structures. Conversely, at higher Pe, hydrodynamic interactions become predominant, enabling physical engagement between droplets. Additionally, we establish that chemo-repulsive deflection or rebounding of droplets is influenced by their inherent chemical polarity, governed by Pe, irrespective of their approach orientation. Overall, by demonstrating the capability to adjust Pe, we demonstrate the ability to selectively manipulate chemical and hydrodynamic inter-droplet interactions, which play a pivotal role in shaping the emergent collective behavior of the system.

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