Weak thermal fluctuations impede steering of chiral magnetic nanobots

External fields are used for remote engineless and fuel-free steering of synthetic colloids. One promising method relies on spinning shaped (e.g., chiral) microbots by a rotating magnetic field, resulting in their propulsion due to viscous rotation-translation coupling. The emerging interest in driven nanobots capable of navigating within living cells and sub-micrometer interstitial spaces of crowded biological environments and subjected to inherent thermal noise, gives rise to the question of how small a steerable propeller can be. In the present paper, we theoretically study the effect of thermal fluctuations on torque-driven actuation of magnetic nanohelices in a viscous fluid. Using a combination of numerical and analytical methods, we demonstrate that weak rotational diffusion dramatically affects the orientation of the nanohelix, hindering its forced rotations and impeding propulsion. The results of Langevin simulations are in close agreement with the numerical solution of the orientational Fokker–Planck equation and with the approximate effective-field theory. Steering nanobots in crowded biological environments raises questions about the limits on the size of a steerable propeller under thermal noise. Here, the authors theoretically investigate torque-driven actuation of magnetic nanohelices in a viscous fluid, revealing that weak rotational diffusion significantly disrupts their orientation and propulsion, offering insights into the challenges of controllable steering at the nanoscale.