Magnetically powered helical nanoswimmers

Abstract

From flocking birds to migrating cells, ‘active matter’ is ubiquitous in the natural world. Almost all known life forms are based on self-propelled entities working collectively to create large-scale structures, networks, and movements. Artificially designed self-propelled objects can allow the study of active matter phenomena with a level of control that is not possible in natural, biological systems. With this motivation, we develop micro/nanoscale swimmers whose swimming mechanism is inspired by microscopic, flagellated bacteria. Among different ways of powering swimmers, the magnetic field deserves special mention due to its inherent biocompatibility, minimal dependence on the properties of the surrounding medium, and remote powering mechanism. Along with providing an insight into the non-equilibrium phenomena of active matter, the helical swimmers can also impact future biomedical practices with intelligent, multifunctional entities swarming toward a diseased site and delivering therapeutics with high accuracy. When an oscillating magnetic field is applied to helical structures, motility is induced in the form of back-and-forth motion, but the directionality is unspecified and thereby represents a zero force, zero torque, active colloid system. These are called reciprocal swimmers, and their degree of randomness in the reciprocal sequence plays an important role in determining their effective motility. We show the results at high activity levels where the degree of randomness is further affected by the presence of the surface, which in turn results in a non-monotonic increase of motility as a function of the magnetic drive. The magnetic swimmers show enhanced diffusivity compared to their passive counterparts, and their motility can be tuned externally. However, to achieve a self-propelled velocity, we use the ratchet principle to break reciprocal symmetry in time. The thermal ratchets can extract useful work from random fluctuations and are common on the molecular scale, such as motor protein. We use the ratchet principle to induce net motility in an externally powered magnetic colloid, which otherwise shows reciprocal (back and forth) motion. The swimmers show net motility with enhanced diffusivity, in agreement with numerical calculations. We further discuss the preliminary experimental results and modelling pertaining to collective dynamics of the helical magnetic nanoswimmers. Additionally, we have studied non-magnetic 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. Crucial aspects of studying the active swimmers pertain to their behaviour under different physical conditions. We demonstrate controlled manipulation of magnetic helices within two types of optical confinement: an optical bowl and a flat potential, both formed by manipulating an optical tweezer. The interaction of helical swimmers with optical confinement is modelled and further confirmed by experiments. Combining optical and magnetic forces in a single nanostructure can allow multiple investigations pertaining to colloidal physics, including micro-rheology, hydrodynamics and confinement effects. In summary, we envision that developing helical magnetic swimmers will provide a new model system to investigate fundamental non-equilibrium phenomena and play a vital role in developing intelligent theragnostic probes for biomedical applications.