Through billions of years of evolution, microorganisms mastered unique swimming behaviors to thrive in complex fluid environments. Limitations in nanofabrication have thus far hindered the ability to design and program synthetic swimmers with the same abilities. Here we encode multi-behavioral responses in microscopic self-propelled tori using nanoscale 3D printing. We show experimentally and theoretically that the tori continuously transition between two primary swimming modes in response to a magnetic field. The tori also manipulated and transported other artificial swimmers, bimetallic nanorods, as well as passive colloidal particles. In the first behavioral mode, the tori accumulated and transported nanorods; in the second mode, nanorods aligned along the toriʼs self-generated streamlines. Our results indicate that such shape-programmed microswimmers have a potential to manipulate biological active matter, e.g. bacteria or cells.
To navigate in complex fluid environments, swimming organisms like fish or bacteria often reorient their bodies antiparallel or against the flow, more commonly known as rheotaxis.
Active particles are capable of self-propelling by consuming energy from the environment. [1] The particles may be living, such as bacteria, or synthetic, such as bimetallic rods or spherical Janus particles. Due to persistent energy input, active materials are examples of out-of-equilibrium systems. They exhibit variety of intriguing phenomena such as the onset of collective behavior, [2,3] reduction of effective viscosity, [4-6] extraction of useful energy, [7-9] and enhanced mixing. [10-12] Typically, active microswimmers show a preferred orientation which determines the self-propulsion direction. Distribution of active particle orientation may have a significant impact on the macroscopic properties of the active material. It was shown in refs. [13-15] that reduction of effective viscosity in the suspension of active microswimmers, exemplified by bacteria, may be explained by a specific form of orientational distribution with respect to the background shear flow. In refs. [16,17] authors showed how the orientation of active microswimmers in the background shear flow leads to the formation of depletion regions, where particles' number density is significantly lower than the average value. Chemically-driven synthetic microswimmers, mimicking motility of living microorganisms , were first introduced by Paxton et al. [18] Since then the repertoire of synthetic microswimmers, as well as mechanisms which can be used to activate their self-propulsion, have significantly expanded. [1] The importance of synthetic microswimmers is twofold. On the one hand, their development leads to a variety of potential applications, for example in medicine [19,20] and materials science. [21] On the other hand, their study sheds new light on fundamental motility mechanisms and biological self-organization.
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