Microorganisms such as bacteria and many eukaryotic cells propel themselves with hair-like structures known as flagella, which can exhibit a variety of structures and movement patterns. For example, bacterial flagella are helically shaped and driven at their bases by a reversible rotary engine, which rotates the attached flagellum to give a motion similar to that of a corkscrew. In contrast, eukaryotic cells use flagella that resemble elastic rods and exhibit a beating motion: internally generated stresses give rise to a series of bends that propagate towards the tip. In contrast to this variety of swimming strategies encountered in nature, a controlled swimming motion of artificial micrometre-sized structures has not yet been realized. Here we show that a linear chain of colloidal magnetic particles linked by DNA and attached to a red blood cell can act as a flexible artificial flagellum. The filament aligns with an external uniform magnetic field and is readily actuated by oscillating a transverse field. We find that the actuation induces a beating pattern that propels the structure, and that the external fields can be adjusted to control the velocity and the direction of motion.

Micropatterned surfaces have been studied extensively as model systems to understand influences of topographic or chemical heterogeneities on wetting phenomena. Such surfaces yield specific wetting or hydrodynamic effects, for example, ultrahydrophobic surfaces, 'fakir' droplets, tunable electrowetting, slip in the presence of surface heterogeneities and so on. In addition, chemical patterns allow control of the locus, size and shape of droplets by pinning the contact lines at predetermined locations. Applications include the design of 'self-cleaning' surfaces and hydrophilic spots to automate the deposition of probes on DNA chips. Here, we discuss wetting on topographically patterned but chemically homogeneous surfaces and demonstrate mechanisms of shape selection during imbibition of the texture. We obtain different deterministic final shapes of the spreading droplets, including octagons, squares, hexagons and circles. The shape selection depends on the topographic features and the liquid through its equilibrium contact angle. Considerations of the dynamics provide a 'shape' diagram that summarizes our observations and suggest rules for a designer's tool box.

Inertial lift forces are exploited within inertial microfluidic devices to position, segregate, and sort particles or droplets. However the forces and their focusing positions can currently only be predicted by numerical simulations, making rational device design very difficult. Here we develop theory for the forces on particles in microchannel geometries. We use numerical experiments to dissect the dominant balances within the Navier-Stokes equations and derive an asymptotic model to predict the lateral force on the particle as a function of particle size. Our asymptotic model is valid for a wide array of particle sizes and Reynolds numbers, and allows us to predict how focusing position depends on particle size.

Following a novel realization of low-Reynolds-number swimming (Dreyfus et al., Nature, vol. 436, 2005, p. 862), in which self-assembled filaments of paramagnetic micron-sized beads are tethered to red blood cells and then induced to swim under crossed uniform and oscillating magnetic fields, the dynamics of magnetoelastic filaments is studied. The filament is modelled as a slender elastica driven by a magnetic body torque. The model is applied to experiments of Goubault et al. (Phys. Rev. Lett., vol. 91, 2003, art. 260802) to predict the lifetimes of metastable static filament conformations that are known to form under uniform fields. A second experimental swimming scenario, complementary to that of Dreyfus et al. (2005), is described: filaments are capable of swimming even if not tethered to red blood cells. Yet, if both ends of the filament are left free and the material and magnetic parameters are uniform along its length then application of an oscillating transverse field can only generate homogeneous torques, and net translation is prohibited by symmetry. It is shown that fore-aft symmetry is broken when variation of the bending stiffness along the filament is accounted for by including elastic defects, which produces results consistent with the swimming phenomenology.

The bacterium Bacillus subtilis produces the molecule surfactin, which is known to enhance the spreading of multicellular colonies on nutrient substrates by lowering the surface tension of the surrounding fluid, and to aid in the formation of aerial structures. Here we present experiments and a mathematical model that demonstrate how the differential accumulation rates induced by the geometry of the bacterial film give rise to surfactant waves. The spreading flux increases with increasing biofilm viscosity. Community associations are known to protect bacterial populations from environmental challenges such as predation, heat, or chemical stresses, and enable digestion of a broader range of nutritive sources. This study provides evidence of enhanced dispersal through cooperative motility, and points to nonintuitive methods for controlling the spread of biofilms.biofilms | thin-film hydrodynamics B acteria bind to surfaces and to liquid-air interfaces to form biofilms, thick mats of cells cemented together by exopolysaccharides. Biofilms endow pathogenic bacteria with enhanced virulence and resistance to antibiotics. Although many recent studies have focused on the adhesins that bind bacteria to each other and to abiotic substrates (1-3) and on the signaling pathways that initiate biofilm growth (4, 5), very little is known about the physical processes by which these colonies spread over or between nutrient sources. Previous studies have highlighted the role of individual cell motility and of passive processes such as the advection of cell aggregates in propagating biofilms (6), but such propagative mechanisms depend on dispersive fluid flows. In addition to being implicated in the development of aerial structures (7), in antagonistic interactions between colonies of different bacterial species (8) the biosurfactant surfactin is known to be necessary for the spreading of colonies of Bacillus subtilis in the absence of external fluid flows (9, 10). However, the physical consequences of the surfactant-like behavior of surfactin on the spreading of biofilms remains unknown.Here we show that gradients in the concentration of surfactin by cells in a liquid pellicle generates surface-tension gradients that drive cooperative spreading. The essential mechanism of surface-tension gradient-driven spreading is similar to the forced spreading of a thin film due to surface-tension gradients induced by temperature gradients (11,12) or by exogenous surfactants (13,14).In the present context, a surface-tension gradient develops because of the geometry of the bacterial biofilm: The bacterial pellicle is thinner at the edge than at the center. The surfactin produced by every cell moves rapidly to the air-fluid film interface, locally reducing the surface tension. Assuming the surfactin production rate is identical for each cell, the concentration of surfactant is greater at the center of the pellicle than at the edge, which results in a gradient in surface tension that drags the film outward, away from the center of the pellicle. T...