Menthyl acetate powered self-propelled Janus sponge Marangoni motors with self-maintaining surface tension gradients and active mixing.

Hypothesis: Small scale Marangoni motors, which self-generate motion by inducing surface tension gradients on water interfaces through release of surface-active "fuels", have recently been proposed as self-powered mixing devices for low volume fluids. Such devices however, often show self-limiting lifespans due to the rapid saturation of surface-active agents. A potential solution to this is the use volatile surface-active agents which do not persist in their environment. Here we investigate menthyl acetate (MA) as a safe, inexpensive and non-persistent fuel for Marangoni motors.

Experiments: MA was loaded asymmetrically into millimeter scale silicone sponges. Menthyl acetate reacts slowly with water to produce the volatile surface-active menthol, which induces surface tension gradients across the sponge to drive motion by the Marangoni effect. Videos were taken and trajectories determined by custom software. Mixing was assessed by the ability of Marangoni motors to homogenize milliliter scale aqueous solutions containing colloidal sediments.

Findings: Marangoni motors, loaded with asymmetric "Janus" distributions of menthyl acetate show velocities and rotational speeds up to 30 mm s and 500 RPM respectively, with their functional lifetimes scaling linearly with fuel volume. We show these devices are capable of enhanced mixing of solutions at orders of magnitude greater rates than diffusion alone.