Run-and-tumble like motion of a camphor-infused Marangoni swimmer

'Run-and-tumble' (RT) motion has been a subject of intense research for several decades. Many organisms, such as bacteria, perform such motion in the presence or absence of local chemical concentration gradients and it is found to be advantageous in search processes. Although there are previous reports involving the successful design of non-living self-propelled particles exhibiting such motion in the presence of external stimuli (chemical/mechanical), RT motion with 'rest' has not yet been observed for autonomous non-living active particles. We have designed a swimmer that performs motion using a combination of 'run', 'tumble', and 'rest' states with stochastic transitions. In the present scenario, it arises solely due to self-generated local surface tension gradients. We quantify the residence time statistics by analyzing the swimmer trajectories from the experimental data, which suggests that the 'rest' and 'tumble' states are more frequent than 'run'. Then, we quantify the motion properties by computing the mean squared displacement, which shows that the swimmer performs ballistic motion on a short time scale and then slows down due to tumbling and resting. To validate the observed transport properties, we introduce a minimal model of a chiral active Brownian particle, stochastically switching between three internal states. The model parameters were extracted from the experiments, which rendered a good agreement between the experiments and simulations.

Surface tension gradient invoked path selection

Certain active particles execute continuous self-propelled motion at the air-water interface due to Marangoni forces. Here, we report motion of camphor and pentanol infused disks in an L-shaped two path and a Y-shaped three path channel. These channels are open to the sink. In both the cases, these channels have asymmetry in arm-lengths while the arm-widths are equal. The active disks are introduced in the decision region (center) of the channel from where they elect to move along a particular path towards the sink. We observe that these active disks show a preference for reaching the sink along the minimal (in length) path. The camphor disk prefers the minimal path 80% of the time in the two-path and 68% of the time in the three-path channels, while for the pentanol disk, the preference rates for the minimal path selection are 88% and 74%, respectively. Furthermore, we perform numerical analysis to validate the experimental observations.

Spreading dynamics of reactive surfactants driven by Marangoni convection

We consider the spreading dynamics of some insoluble surface-active species along an aqueous interface. The model includes both diffusion, Marangoni convection and first-order reaction kinetics. An exact solution of the nonlinear transport equations is derived in the regime of large Schmidt number, where viscous effects are dominant. We demonstrate that the variance of the surfactant distribution increases linearly with time, providing an unambiguous definition for the enhanced diffusion coefficient observed in the experiments. The model thus presents new insight regarding the actuation of camphor grains at the water-air interface.

Rotational motion of a camphor disk in a circular region

In a two-dimensional axisymmetric system, the system symmetry allows rotational or oscillatory motion as stable stationary motion for a symmetric self-propelled particle. In the present paper, we studied the motion of a camphor disk confined in a two-dimensional circular region. By reducing the mathematical model describing the dynamics of the motion of a camphor disk and the concentration field of camphor molecules on a water surface, we analyzed the reduced equations around a bifurcation point where the rest state at the center of the system becomes unstable. As a result, we found that rotational motion is stably realized through the double-Hopf bifurcation from the rest state. The theoretical results were confirmed by numerical calculation and corresponded well to the experimental results.