Active Brownian Motion

Self-propelled microswimmers hold tremendous potential as autonomous agents to localize, pick-up and deliver nanoscopic objects, e.g., in bioremediation, drug-delivery and gene-therapy. Until now their behaviour has only been studied in homogenous environments. We demonstrate how they navigate through environments presenting complex spatial features, which more closely mimic the conditions inside cells, living organisms and future lab-on-a-chip devices. For example, when driven by an external force through patterned environment, some swimmers do not follow this force but steer along directions being clear of obstacles. This complex response to the environment can be exploited to characterize and sort, for example, chemotactic bacteria avoiding the need of stable chemical gradients.

We realized such experiments using a new kind of microswimmers whose active motion is due to the local demixing of a critical binary liquid mixture and can be easily tuned by illumination. We need such microswimmers because we can easily study them as a function for their swimming behavior, without altering their other (physical, chemical, etc…) properties. In comparison to other mechanisms proposed in the literature, the main advantage of our mechanism is that very low light intensities can be used permitting us to exclude optical forces and to accurately tune the active Brownian motion of the particle. Furthermore, since the demixing of the critical mixture is only local, the “fuel” driving the microswimmer regenerates once the particle has moved to a different region of the sample.

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