A simple catch: Fluctuations enable hydrodynamic trapping of microrollers by obstacles

Abstract

It is known that obstacles can hydrodynamically trap bacteria and synthetic microswimmers in orbits, where the trapping time heavily depends on the swimmer flow field and noise is needed to escape the trap. Here, we use experiments and simulations to investigate the trapping of microrollers by obstacles. Microrollers are rotating particles close to a bottom surface, which have a prescribed propulsion direction imposed by an external rotating magnetic field. The flow field that drives their motion is quite different from previously studied swimmers. We found that the trapping time can be controlled by modifying the obstacle size or the colloid-obstacle repulsive potential. We detail the mechanisms of the trapping and find two remarkable features: The microroller is confined in the wake of the obstacle, and it can only enter the trap with Brownian motion. While noise is usually needed to escape traps in dynamical systems, here, we show that it is the only means to reach the hydrodynamic attractor.

Restructuring a passive colloidal suspension using a rotationally driven particle

“…The passive particles are spherical and made of polystyrene (Bangs laboratoryr, FSPP005) with a density of 1.06 g cm À3 and a mean diameter of = 2.07 AE 0.15 mm. The microrollers in the experiment are described in detail in Sprinkle et al 37 and van der Wee et al 38 The microroller has a mean diameter of 2.1 AE 0.1 mm and a permanent magnetic dipole as it is comprised of a hematite cube within a spherical polymer matrix, see Fig. 1(b).…”

Micro/Nanorobotic Swarms: From Fundamentals to Functionalities

“…Patterning the topography of a workspace or device is another approach to guide the generation of micro/nanorobotic swarms. The interactions between micro/nanoagents and those between micro/nanoagents and nearby surfaces have been investigated − ,, and applied to trigger swarm behaviors. ,,,, Self-propelling Janus particles were demonstrated to work collectively surrounding a microgear (Figure E) . Without the gear, the Janus particles moved randomly.…”

“…To date, different types of external fields have been applied to actuate micro/nanoagents (Figure ), including magnetic, − electric, − optical, − and acoustic fields. − In addition to these fields, chemical reaction − and topographical patterns − have also been investigated to power and guide the agents, respectively. Existing reviews have focused on individual micro/nanoagents, including their evolution, fabrication and functionalization, − modeling and motion control, − biomedical applications, − and environmental applications. − Compared with a single agent, micro/nanorobotic swarms have higher robustness (e.g., higher tolerance to the failure of a few agents), higher motion dexterity and flexibility, and better scalability.…”

Nanomotor-enhanced transport of passive Brownian particles in porous media

“…For example, active biological matter often coexists with inactive obstacles, and the mixing of two species can affect both active swimming and passive diffusion (23), which has become a core interest in the field. Self-propulsion of artificial active colloids is reported to control self-assembly (24,25) and directional transport (26)(27)(28) of passive particles based on hydrodynamic interactions (29). For example, researchers observed demixing of active-passive mixtures in a confined 2D environment and leveraged this phenomenon to separate two types of colloids into different chambers (30).…”