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Synthetic microswimmers offer models for cell motility and their tunability makes them promising candidates for biomedical applications. Here we measure the acoustic propulsion of bimetallic micro-rods that, when trapped at the nodal plane of a MHz acoustic resonator, swim with speeds of up to 300 microns per second. While past acoustic streaming models predict speeds that are more than one order of magnitude smaller than our measurements, we demonstrate that the acoustic locomotion of the rods is driven by a viscous, non-reciprocal mechanism relying on shape anisotropy akin to that used by swimming cells and that reproduces our data with no adjustable parameters.

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.

Peritrichous bacteria swim through rotation of a helical bundle of filaments. Here, physical mechanisms that contribute to its formation in terms of direct and indirect hydrodynamic interactions are categorized. A simple analytical singularity model shows that indirect effects generically dominate at all except the very last stages of bundling. A numerical elastohydrodynamic model allows for a detailed analysis along the entire length of the filaments.

Swimming microorganisms self-propel in viscous fluids using traveling wavelike deformations of appendages called flagella. A long-wavelength mathematical model is used to show that the flagellar motion maximizing the propulsive force for a given amount of energy dissipated in the fluid is waves traveling with constant speed (potentially on a curved flagellar centerline), with propulsion always in the direction opposite to the wave.

The low-Reynolds number hydrodynamics of slender ribbons is accurately captured by slender-ribbon theory, an asymptotic solution to the Stokes equation which assumes that the three length scales characterising the ribbons are well separated. We show in this paper that the force distribution across the width of an isolated ribbon located in a infinite fluid can be determined analytically, irrespective of the ribbon's shape. This, in turn, reduces the surface integrals in the slender-ribbon theory equations to a line integral analogous to the one arising in slender-body theory to determine the dynamics of filaments. This result is then used to derive analytical solutions to the motion of a rigid plate ellipsoid and a ribbon torus and to propose a ribbon resistive-force theory, thereby extending the resistive-force theory for slender filaments.

The Stokes flow created by regularized stokeslets (point forces) is analyzed. The authors determine the error made in the approximation compared to the singular solution, show how a source dipole appears generically in the far field, and propose an improved regularized solution.

The bacterium Helicobacter pylori causes ulcers in the stomach of humans by invading mucus layers protecting epithelial cells. It does so by chemically changing the rheological properties of the mucus from a high-viscosity gel to a low-viscosity solution in which it may self-propel. We develop a two-fluid model for this process of swimming under self-generated confinement. We solve exactly for the flow and the locomotion speed of a spherical swimmer located in a spherically symmetric system of two Newtonian fluids whose boundary moves with the swimmer. We also treat separately the special case of an immobile outer fluid. In all cases, we characterise the flow fields, their spatial decay, and the impact of both the viscosity ratio and the degree of confinement on the locomotion speed of the model swimmer. The spatial decay of the flow retains the same power-law decay as for locomotion in a single fluid but with a decreased magnitude. Independently of the assumption chosen to characterise the impact of confinement on the actuation applied by the swimmer, its locomotion speed always decreases with an increase in the degree of confinement. Our modelling results suggest that a low-viscosity region of at least six times the effective swimmer size is required to lead to swimming with speeds similar to locomotion in an infinite fluid, corresponding to a region of size above $\approx 25~\mu$m for Helicobacter pylori., Comment: 37 pages, 12 figures, 1 table

Swimming of Spiroplasma is not as kinky as one might suspect. The reversibility of Stokes flow combined with the symmetry of the swimming gait result in straight trajectories.

Solutions to the Stokes equations written in terms of a small number of hydrodynamic image singularities have been a useful tool in theoretical and numerical computations for nearly fifty years. In this article, we extend the catalogue of known solutions by deriving the flow expressions due to a general point torque and point source in the presence of a stationary sphere with either a no-slip or a stress-free (no shear) boundary condition. For an axisymmetric point torque and a no-slip sphere the image system simplifies to a single image point torque, reminiscent of the solution for a point charge outside an equipotential sphere in electrostatics. By symmetry, this also gives a simple representation of the solution due to an axisymmetric point torque inside a rigid spherical shell. In all remaining cases, the solution can be described by a collection of physically intuitive point and line singularities. Our results will be useful for the theoretical modelling of the propulsion of microswimmers and efficient numerical implementation of far-field hydrodynamic interactions in this geometry., Comment: 30 pages, 8 figures