Self-assembly is an important process in biological systems and also a promising avenue toward dynamic and responsive micro- and nano-technologies. This study discusses the non-equilibrium self-assembly of inherently magnetic bacteria oriented perpendicular to a solid surface. An interplay between hydrodynamic and magnetic interactions leads to stable three-dimensional clusters in the long-time regime, which may be programmatically assembled, disassembled, and translated across a surface. The implications of the findings for the rational design of non-equilibrium self-assembly in general are discussed.

One of the most common swimming strategies employed by microorganisms is based on the use of rotating helical filaments, called flagella, that are powered by molecular motors. Determining the physical properties of this propulsive system is crucial to understanding the behavior of these organisms. Furthermore, the ability to dynamically monitor the activity of the flagellar motor is a valuable indicator of the overall energetics of the cell. In this work, inherently magnetic bacteria confined in micromagnetic CoFe traps are used to directly and noninvasively determine the flagellar thrust force and swimming speed of motile cells. The technique permits determination of the ratio of propulsive force/swimming speed (the hydrodynamic resistance) and the power output of the flagellar motor for individual cells over extended time periods. Cells subjected to ultraviolet radiation are observed to experience exponential decays in power output as a function of exposure time. By noninvasively measuring thrust, velocity, and power output over time at a single-cell level, this technique can serve as the foundation for fundamental studies of bacterial hydrodynamics and also provides a novel, to our knowledge, tether-free probe of single-cell energetics over time.