title: Controlled propulsion and separation of helical particles at the nanoscale.
abstract: Controlling the motion of nano and microscale objects in a fluid environment is a key factor in designing optimized tiny machines that perform mechanical tasks such as transport of drugs or genetic material in cells, fluid mixing to accelerate chemical reactions, and cargo transport in microfluidic chips. Directed motion is made possible by the coupled translational and rotational motion of asymmetric particles. A current challenge in achieving directed and controlled motion at the nanoscale lies in overcoming random Brownian motion due to thermal fluctuations in the fluid. We use a hybrid lattice-Boltzmann molecular dynamics method with full hydrodynamic interactions and thermal fluctuations to demonstrate that controlled propulsion of individual nanohelices in an aqueous environment is possible. We optimize the propulsion velocity and the efficiency of externally driven nanohelices. We quantify the importance of the thermal effects on the directed motion by calculating the Péclet number for various shapes, number of turns and pitch lengths of the helices. Consistent with the experimental microscale separation of chiral objects, our results indicate that in the presence of thermal fluctuations at Péclet numbers >10, chiral particles follow the direction of propagation according to its handedness and the direction of the applied torque making separation of chiral particles possible at the nanoscale. Our results provide criteria for the design and control of helical machines at the nanoscale.
authors.affiliation: COMP CoE at the Department of Applied Physics, Aalto University School of Science, P.O. Box 11000, FIN-00076 Aalto, Espoo, Finland. maria.alcanzare@aalto.fi.
authors.forename: Maria Michiko T
authors.identifier: http://orcid.org/0000-0002-0757-8441
authors.initials: MM
authors.lastname: Alcanzare
authors.affiliation: COMP CoE at the Department of Applied Physics, Aalto University School of Science, P.O. Box 11000, FIN-00076 Aalto, Espoo, Finland. maria.alcanzare@aalto.fi.
authors.forename: Vaibhav
authors.initials: V
authors.lastname: Thakore
authors.affiliation: Varian Medical Systems Finland, Paciuksenkatu 21, 00270 Helsinki, Finland.
authors.forename: Santtu T T
authors.initials: ST
authors.lastname: Ollila
authors.affiliation: Department of Mathematics and Computer Science & Institute for Complex Molecular Systems, Eindhoven University of Technology, P.O. Box 513, MetaForum 5600 MB, The Netherlands and Department of Chemistry & Applied Mathematics, Western University, 1151 Richmond Street, London, Ontario, Canada N6A 5B7.
authors.forename: Mikko
authors.initials: M
authors.lastname: Karttunen
authors.affiliation: COMP CoE at the Department of Applied Physics, Aalto University School of Science, P.O. Box 11000, FIN-00076 Aalto, Espoo, Finland. maria.alcanzare@aalto.fi and Department of Physics, Brown University, Box 1843, 182 Hope Street Barus & Holley, Providence, Rhode Island 02912-1843, USA and Department of Mathematical Science and Department of Physics, Loughborough University, Loughborough, LE11 3TU, UK.
authors.forename: Tapio
authors.initials: T
authors.lastname: Ala-Nissila