Tapio Ala-Nissilä (Aalto and Loughborough University)
Controlled motion of micro and nanomotors in a fluid environment is a promising tool in biology and biomedicine. Fuel-free controlled propulsion and steering in aqueous solutions have been experimentally demonstrated at the microscale by taking advantage of the coupled rotational and translational motion. The challenge in the controlled propulsion and maneuverability at the nanoscale is overcoming thermal effects which can alter the direction of motion and interfere with the propulsion. The hybrid lattice-Boltzmann -- Molecular Dynamics method with full hydrodynamic interactions and thermal fluctuations  is used to optimize the helical shapes and to demonstrate that controlled propulsion and maneuverability is possible for helically shaped structures at a sufficiently high Peclet number, a ratio of the diffusive and propulsive timescales. The magnetic helical structure interacts with a rotating magnetic field. The interaction induces a torque that propels the helix in the fluid through the coupled rotational and translational motion. The Peclet number and the propulsive velocity are quantified at various field frequencies of the rotating magnetic field. The propulsive velocities are observed to be linear with the field frequencies up to a certain step-out frequency which depends on the helical structures' rotational viscous drag coefficient and the magnitude of the product of the magnetic field strength and the magnetization of the helix. In the presence of thermal fluctuations, we demonstrate that the direction of motion of nanohelices may be altered and that the helices can be guided to follow a pre-defined trajectory .  S.T. Ollila, C. Denniston, M. Karttunen, and T. Ala-Nissila, J. Chem. Phys 134, 064902 (2011); Santtu T.T. Ollila, Cristopher J. Smith, Tapio Ala-Nissila, and Colin Denniston, Multiscale Modeling & Simulation 11, 213-243 (2013).  M.M.T. Alcanzare, V. Thakore, M. Karttunen, S.T.T. Ollila, and T. Ala-Nissila, Soft Matter 13, 2148 (2017); M.M.T. Alcanzare, M. Karttunen, and T. Ala-Nissila, to appear in Soft Matter (2019).