Active droplets in microfluidic confinement

Active droplets generate self-propelled motion through physicochemical interactions with surfactants present in the surrounding medium. The motion of such droplets is strongly influenced by the surfactant distribution around the droplets. In this study, we focus on the modulation of the surfactant distribution around active droplets, arising from the interplay between advective–diffusive transport of surfactants (characterized by the Péclet number) and the level of geometric confinement (characterized by the droplet-to-channel size ratio). We experimentally observe the active motion of an oil droplet in a surfactant-laden aqueous medium and validate our findings with finite element-based numerical simulations. We find that an active droplet undergoes a transition from centerline motion, to oscillatory motion, and to off-centered motion, which can be independently controlled by both parameters. We observe that the propulsion speed increases with increasing confinement ratio in the centerline motion regime, whereas it decreases with increasing confinement ratio in the off-centered regime. Furthermore, we find that in the off-centered regime, the droplet's lateral position relative to the channel centerline can also be tuned by these two factors. Overall, this study emphasizes that the motion dynamics of active swimmers can be modulated with the control parameters, which can have potential applications in targeted drug/cargo delivery, biosensors, and lab-on-a-chip devices.