We consider the rotational motion of an elongated nanoscale object in a fluid under an external torque. The experimentally observed dynamics could be understood from analytical solutions of the Stokes equation, with explicit formulae derived for the dynamical states as a function of the object dimensions and the parameters defining the external torque. Under certain conditions, multiple analytical solutions to the Stokes equations exist, which have been investigated through numerical analysis of their stability against small perturbations and their sensitivity towards initial conditions. These experimental results and analytical formulae are general enough to be applicable to the rotational motion of any isolated elongated object at low Reynolds numbers, and could be useful in the design of non-spherical nanostructures for diverse applications pertaining to microfluidics and nanoscale propulsion technologies.

Conspectus Micro- and nanomotors are nonliving micro- and nanoparticles that are rendered motile by supplying energy from external sources, for example, through asymmetric chemical reactions or the application of electric, magnetic, optical, or acoustic fields. Their study is interesting for two reasons. First, nanomotors can impact future biomedical practices, where one envisions intelligent multifunctional nanomachines swarming toward a diseased site and delivering therapeutics with high accuracy. The second motivation stems from the prevalence of self-powered systems in nature, ranging from intracellular transport to human migration, which are nonequilibrium phenomena yet to be completely understood. Nanomotors provide a promising route toward the study of complex active matter phenomena with a well-defined and possibly reduced set of variables. Among different ways of powering nanomotors, magnetic field deserves a special mention because of its inherent biocompatibility, minimal dependence on properties of the surrounding medium, and remote powering mechanism. In particular, magnetically actuated propellers (MAPs), which are helical structures driven by rotating fields in fluids and gels, have been demonstrated to be highly suitable for various microfluidic and biotechnology applications. Unfortunately, this method of actuation requires direct application of mechanical torque by the applied field, implying that the system is driven and therefore cannot be considered self-propelled. To overcome this fundamental limitation, we discuss an alternate magnetic drive where the MAPs are powered by oscillating (not rotating) magnetic fields. This technique induces motility in the form of back-and-forth motion but allows the directionality to be unspecified, and therefore, it represents a zero-force, zero-torque active matter where the nanomotors behave effectively as self-propelled entities. The MAPs show enhanced diffusivity compared with their passive counterparts, and their motility can be tuned by altering the external magnetic drive, which establishes the suitability of the MAPs as model active particles. Enhancement of the diffusivity depends on the thermal noise as well as the inherent asymmetries of the individual motors, which could be well-understood through numerical simulations. In the presence of small direct-current fields and interactions with the surface, the swimmers can be maneuvered and subsequently positioned in an independent manner. Next, we discuss experimental results pertaining to the collective dynamics of these helical magnetic nanoswimmers. We have studied nonmagnetic tracer beads suspended in a medium containing many swimmers and found the diffusivity of the beads to increase under magnetic actuation, akin to measurements performed in dense bacterial suspensions. In summary, we envision that rendering the system of MAPs active will not only provide a new model system to investigate fundamental nonequilibrium phenomena but also play a vital role in the development of intelligent theranostic...

There is considerable interest in powering and maneuvering nanostructures remotely in fluidic media using noninvasive fuel-free methods, for which small homogeneous magnetic fields are ideally suited. Current strategies include helical propulsion of chiral nanostructures, cilia-like motion of flexible filaments, and surface assisted translation of asymmetric colloidal doublets and magnetic nanorods, in all of which the individual structures are moved in a particular direction that is completely tied to the characteristics of the driving fields. As we show in this paper, when we use appropriate magnetic field configurations and actuation time scales, it is possible to maneuver geometrically identical nanostructures in different directions, and subsequently position them at arbitrary locations with respect to each other. The method reported here requires proximity of the nanomotors to a solid surface, and could be useful in applications that require remote and independent control over individual components in microfluidic environments.

We report on the development of a system of micron-sized reciprocal swimmers that can be powered with small homogeneous magnetic fields, and whose motion resembles that of a helical flagellum moving back and forth. We have measured the diffusivities of the swimmers to be higher compared to nonactuated objects of identical dimensions at long time scales, in accordance with the theoretical predictions made by Lauga [Phys. Rev. Lett. 106, 178101 (2011)]. Randomness in the reciprocity of the actuation strokes was found to have a strong influence on the enhancement of the diffusivity, which has been investigated with numerical calculations.

N-Heterocyclic carbenes (NHCs) with a methylene-linked aryloxide side arm constitute a flexible bidentate ligand platform whose usability is partly hindered as their alkali metal salts, the primary ligand transfer agents, are prone to carbene deactivation by 1,2-benzyl migration. Reacting an imidazolium precursor of this ligand class [HO-4,6-Bu t 2-C6H2-2-CH2{CH(NCHCHNAr)}]Br [LH 2Br; Ar = 2,6-Pr i 2-C6H3 (Dipp)] with Ti(NMe2)4 in 1:1 ratio readily gives the monoligated titanium complex [(L)Ti(NMe2)2Br] (1). Reacting 1 with an additional 1 equiv of LH 2Br shows an interesting fragmentation behavior of L–, distinct from that 1,2-migration, and gives the cationic Ti complex [(LH)Ti{κ2-(O-4,6-Bu t 2-C6H2-2-CH2NMe2)}(NMe2)Br]Br ([2]Br). This shows the vulnerability of the NHC–Ti motif in 1 and presents a rare case in which an imidazolium moiety acts as a leaving group, departing as an aryl imidazole. The nature of the NHC–Ti bond in 1 and its conversion into [2]Br are probed by computational analyses. In addition, 1 is established as a catalyst for the ring-opening polymerization of ε-caprolactone (CL), where it exhibits high activity and good control over polymer growth under ambient conditions. A kinetic analysis suggests the classic coordination–insertion mechanism with a typical first-order dependence on CL concentration, while the end group characterization indicates a bifunctional nature of the NHC–Ti combo in which the labile NHC makes the nucleophilic attack in the ring-opening initiation step.