Artificial microswimmers display adaptive locomotion by autonomously morphing in response to physical changes in the environment.

Bacteriophage viruses, one of the most abundant entities in our planet, lack the ability to move independently. Instead, they crowd fluid environments in anticipation of a random encounter with a bacterium. Once they 'land' on the cell body of their victim, they are able to eject their genetic material inside the host cell. Many phage species, however, first attach to the flagellar filaments of bacteria. Being immotile, these so-called flagellotropic phages still manage to reach the cell body for infection, and the process by which they move up the flagellar filament has intrigued the scientific community for decades. In 1973, Berg and Anderson (Nature, 245, 380-382) proposed the nut-and-bolt mechanism in which, similarly to a rotated nut that is able to move along a bolt, the phage wraps itself around a flagellar filament possessing helical grooves (due to the helical rows of flagellin molecules) and exploits the rotation of the flagellar filament in order to passively travel along it. One of the main evidence for this mechanism is the fact that mutants of bacterial species such as Escherichia coli and Salmonella typhimurium that possess straight flagellar filaments with a preserved helical groove structure can still be infected by their relative phages. Using two distinct approaches to address the short-range interactions between phages and flagellar filaments, we provide here a first-principle theoretical model for the nut-and-bolt mechanism applicable to mutants possessing straight flagellar filaments. Our model is fully analytical, is able to predict the speed of translocation of a bacteriophage along a flagellar filament as a function of the geometry of both phage and bacterium, the rotation rate of the flagellar filament, and the handedness of the helical grooves, and is consistent with past experimental observations.

Magnetic microrobots that swim through liquid media are of interest for minimally invasive medical procedures, bioengineering, and manufacturing. Many of the envisaged applications, such as micromanipulation and targeted cargo delivery, necessitate the use and adequate control of multiple microrobots, which will increase the velocity, robustness, and efficacy of a procedure. While various methods involving heterogeneous geometries, magnetic properties, and surface chemistries have been proposed to enhance independent control, the main challenge has been that the motion between all microswimmers remains coupled through the global control signal of the magnetic field. Katsamba and Lauga [Phys. Rev. Appl. 5, 064019 (2016)] proposed transchiral microrobots, a theoretical design with magnetized spirals of opposite handedness. The competition between the spirals can be tuned to give an intrinsic nonlinearity that each device can function only within a given band of frequencies. This allows individual microrobots to be selectively controlled by varying the frequency of the rotating magnetic field. Here, we present the experimental realization and characterization of transchiral micromotors composed of independently driven magnetic helices. We show a swimming micromotor that yields negligible net motion until a critical frequency is reached and a micromotor that changes its translation direction as a function of the frequency of the rotating magnetic field. This work demonstrates a crucial step toward completely decoupled and addressable swimming magnetic microrobots.

One of the aspirations for artificial microswimmers is their application in non-invasive medicine. For any practical use, adequate mechanisms enabling control of multiple artificial swimmers will be of paramount importance. Here we theoretically propose a multi-helical, freely-jointed motor as a novel selective control mechanism. We show that the nonlinear step-out behavior of a magnetized helix driven by a rotating magnetic field can be exploited, when used in conjunction with other helices, to obtain a velocity profile that is non-negligible only within a chosen interval of operating frequencies. Specifically, the force balance between the competing opposite-handed helices is tuned to give no net motion at low frequencies (tug-of-war) while in the middle frequency range the magnitude, and potentially the sign, of the swimming velocity can be adjusted by varying the driving frequency. We illustrate this idea on a two-helix system and demonstrate how to generalize to N helices, both numerically and theoretically. We then explain how to solve the inverse problem and design an artificial swimmer with an arbitrarily-complex velocity vs. frequency relationship. We finish by discussing potential experimental implementation.

Flexible filaments moving in viscous fluids are ubiquitous in the natural microscopic world. For example, the swimming of bacteria and spermatozoa as well as important physiological functions at organ-level, such as the cilia-induced motion of mucus in the lungs, or individual cell-level, such as actin filaments or microtubules, all employ flexible filaments moving in viscous fluids. As a result of fluid-structure interactions, a variety of nonlinear phenomena may arise in the dynamics of such moving flexible filaments. In this paper we derive the mathematical tools required to study filament-driven propulsion in the asymptotic limit of stiff filaments. Motion in the rigid limit leads to hydrodynamic loads which deform the filament and impact the filament propulsion. We first derive the general mathematical formulation and then apply it to the case of a helical filament, a situation relevant for the swimming of flagellated bacteria and for the transport of artificial, magnetically actuated motors. We find that, as a result of flexibility, the helical filament is either stretched or compressed (conforming previous studies) and its axis also bends, a new result which we interpret physically. We then explore and interpret the dependence of the perturbed propulsion speed due to the deformation on the relevant dimensionless dynamic and geometric parameters.

Autophoretic microswimmers self-propel via surface interactions with a surrounding solute fuel. Chemically-active filaments are an exciting new microswimmer design that augments traditional autophoretic microswimmers, such as spherical Janus particles, with...

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