Active mixing by self-propelled Janus sponge Marangoni motors with self-maintaining surface tension gradients.

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 could lead to drastic rate improvements in processes reliant on diffusion limited reactions, without reliance on externally powered actuation, allowing for off-grid applications. Such devices however, often show self-limiting lifespans due to the rapid saturation of surface-active agents, resulting in a loss of motion far before depletion of their “fuel”. A potential solution to this is the use surface-active agents which do not persist in their environment. Here we investigate menthyl acetate as a safe, inexpensive and convenient fuel for Marangoni motors. Menthyl acetate is a hydrophobic oil which reacts slowly with water to produce menthol, a high vapor pressure surface active agent. We demonstrate millimeter scale silicone sponge Marangoni motors, loaded with asymmetric “Janus” distributions of menthyl acetate show velocities and rotational speeds up to 30 mm per second 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.