External-energy-independent polymer capsule motors and their cooperative behaviors

The design and development of mobile nano-, micro-, and millimeter-scale autonomous systems have been perused over several decades. Here, we introduce a millimeter-sized polymer capsule motor with specific features and functionalities. It runs without any external energy sources or the consumption of external fuels such as H₂O₂ or glucose. The occurrence of motion is due to the asymmetric release of organic solvent from the capsule and the asymmetric change in the surface tension of the surrounding liquid. The capsule moves from a place of lower surface tension to a place of higher surface tension (Marangoni effect) in an attempt by the system to attain the desirable lowest-free-energy state. The operation of the motor is versatile in terms of the environment, as it moves on a wide variety of liquid/air interfaces, including water, sea water, organic solvent/water mixtures, and acids. A high-motion velocity was observed, with a travelling distance of over 20 meters. The manipulation of its motion was achieved upon functionalization with nickel powder and application of an external magnetic field. Long-range interaction behaviors and surface-cleaning effects due to the chemotaxis effect were also demonstrated when the capsule was functionalized with sodium dodecyl sulfate (SDS). We believe that a plethora of applications can be envisioned with this motor, such as cargo delivery, manipulation of matter, sensing and detection, biorecognition, and environmental remediation.