Rolling Rebels: Counteractive transport of Microrollers in an obstacle lattice.

The ability to control transport at the microscale is crucial for a variety of applications, from controlling bacterial

flow for mitigating infection risks to navigation of microrobots for micro-surgical and drug delivery purposes.

Microrollers, which are microscopic particles driven into rolling motion by an external field, are viable candidates

for such transport purposes since they can be driven in a non-invasive manner. However, in the case of

complex microscale environments, such as in biologically relevant systems, accurately steering microrollers is

challenging due to hydrodynamic interactions between the microroller and the surrounding environment. In this

work, we use an ordered obstacle array as a model system and study the dynamics of a microroller suspended

between the obstacles. We observe a counterintuitive phenomenon: under certain conditions,

the microroller traverses the lattice in the direction opposite to that of its rolling motion. We show

that the microroller’s direction of motion depends on the array (static) and driving (dynamic) properties, enabling us to steer the microroller across the obstacles.

In addition, by controlling the driving dynamics of the microroller, we can control its entry into the array from outside, enabling guided navigation through porous structures based on activity. Our findings offer critical insights into precise control over microroller dynamics and

could be implemented to design strategies for steering microrollers in more complex and biologically relevant

systems, advancing applications in targeted drug delivery and microscale transport.