Rolling is a ubiquitous transport mode utilized by living organisms and engineered systems. However, rolling at the microscale has been constrained by the requirement of a physical boundary to break the spatial homogeneity of surrounding mediums, which limits its prospects for navigation to locations with no boundaries. Here, in the absence of real boundaries, we show that microswarms can execute rolling along virtual walls in liquids, impelled by a combination of magnetic and acoustic fields. A rotational magnetic field causes individual particles to self-assemble and rotate, while the pressure nodes of an acoustic standing wave field serve as virtual walls. The acoustic radiation force pushes the microswarms towards a virtual wall and provides the reaction force needed to break their fore-aft motion symmetry and induce rolling along arbitrary trajectories. The concept of reconfigurable virtual walls overcomes the fundamental limitation of a physical boundary being required for universal rolling movements.

Rolling is a ubiquitous mode of transport utilized by both living organisms and engineered systems. Rolling, on the microscale, has become particularly interesting for the manipulation of microswarms, since enacting such motion does not require special prefabrication techniques. However, rolling motion has to date been restricted by the need for a physical boundary to break the spatial homogeneity of surrounding mediums, which limits its prospects for microswarm navigation and cargo delivery to locations with no boundaries. Here, in the absence of real physical boundaries, we show that chain-shaped microswarms can undergo rolling motion along virtual walls in the aqueous medium, impelled by a combination of magnetic and acoustic fields. A rotational magnetic field causes individual particles to self-assemble and rotate, while the pressure nodes generated by an acoustic standing wave field serve as virtual walls. The acoustic radiation force pushes the rotating microswarms towards a virtual wall and provides the reaction force needed to break their fore-aft motion symmetry and induce rolling. We develop an experiment-supported theoretical model to quantify the net displacement generated by rolling. Finally, we demonstrate that rolling can be achieved along arbitrary trajectories by dynamically switching the orientation of the virtual walls and the rotational directions of the magnetic field. Consequently, the concept of reconfigurable virtual walls developed here overcomes the fundamental limitation of a physical boundary being required for universal rolling movements.