RESUMEN
Typical bodily and environmental fluids encountered by biological swimmers consist of dissolved macromolecules such as proteins or polymers, rendering them even non-Newtonian at times. Active droplets mimic the essential propulsive characteristics of several biological swimmers, and serve as ideal model systems to widen our understanding of their locomotive strategies. Here, we investigate the motion of a micellar solubilization driven active oil droplet in an aqueous medium consisting of polymers as macromolecular solutes. Experiments reveal extreme sensitivity of the droplet motion to the presence of macromolecules in its ambient medium. Through in situ visualization of the self-generated chemical field around the droplet, we notice unexpectedly high diffusivity of the filled micelles in the presence of high molecular weight polymeric solutes. This highlights the breakdown of continuum approximation due to a significant size difference between the macromolecular solutes and the micelles. It is shown that the Péclet number, defined based on the experimentally determined filled micelle diffusivity (taking into account the local solvent viscosity) successfully captures the transition from smooth to jittery propulsion mode for both molecular and macromolecular solutes. With an increase in macromolecular solute concentration, particle image velocimetry reveals another mode switching from the conventional pusher mode to puller mode of propulsion, characterized by a more persistent droplet motion. By doping the ambient medium with suitable choice of macromolecules, our experiments unveil a novel route to orchestrate complex transitions in active droplet propulsion.
RESUMEN
The motion of biological swimmers in typical bodily fluids is modelled using a system of micellar solubilization driven active droplets in a viscoelastic polymeric solution. The viscoelastic nature of the medium, as perceived by the moving droplet, characterized by the Deborah number (De), is tuned by varying the surfactant (fuel) and polymer concentration in the ambient medium. At moderate De, the droplet exhibits a steady deformed shape, markedly different from the spherical shape observed in Newtonian media. A theoretical analysis based on the normal stress balance at the interface is shown to accurately predict the droplet shape. With a further increase in De, time-periodic deformation accompanied by an oscillatory transition in swimming mode is observed. The study unveils the hitherto unexplored rich complexity in the motion of active droplets in viscoelastic fluids.
