The collision of two liquid drops is both an applied question (in rain formation or combustion, for example) and a beautiful basic situation, where impact involves liquid phases, making this problem worth studying in its own right. In a stimulating paper, Planchette, Lorenceau & Brenn (J. Fluid Mech., this issue, vol. 702, 2012, pp. 5–25) consider collisions between oil and water, which often lead to water drops protected by a shell of oil. By looking at the deformations during impact, they characterize the dynamical conditions leading to single encapsulation, and derive a criterion for avoiding fragmentation.

Since the advent of cluster computing over 10 years ago there has been a steady output of new and better direct numerical simulation of homogeneous, isotropic turbulence with spectra and lower-order statistics converging to experiments and many phenomenological models. The next step is to directly compare these simulations to new models and new mathematics, employing the simulated data sets in novel ways, especially when experimental results do not exist or are poorly converged. For example, many of the higher-order moments predicted by the models converge slowly in experiments. The solution with a simulation is to do what an experiment cannot. The calculation and analysis of Yeung, Donzis & Sreenivasan (J. Fluid Mech., this issue, vol. 700, 2012, pp. 5–15) represents the vanguard of new simulations and new numerical analysis that will fill this gap. Where individual higher-order moments of the vorticity squared (the enstrophy) and kinetic energy dissipation might be converging slowly, they have focused upon ratios between different moments that have better convergence properties. This allows them to more fully explore the statistical distributions that eventually must be modelled. This approach is consistent with recent mathematics that focuses upon temporal intermittency rather than spatial intermittency. The principle is that when the flow is nearly singular, during ‘bad’ phases, when global properties can go up and down by many orders of magnitude, if appropriate ratios are taken, convergence rates should improve. Furthermore, in future analysis it might be possible to use these ratios to gain new insights into the intermittency and regularity properties of the underlying equations.

Fish escaping from predators exhibit amazing acceleration. The hydrodynamic mechanisms employed to power this acceleration are highlighted in the paper by Gazzola, van Rees & Koumoutsakos (J. Fluid Mech., this issue, vol. 698, 2012, pp. 5–17), showing that the fish bends its entire body and caudal fin, in order to entrap and then accelerate as large a mass of water as possible. It is also shown that hydrodynamic optimization drives the fast-start kinematics.

By what process can droplets be extracted out of the sea? This is an old problem, well-documented by precise field measurements of the size distribution of the spray aerosols. Lhuissier & Villermaux (J. Fluid Mech., this issue, vol. 696, 2012, pp. 5–44) study and characterize the bursting of an emerging bubble. They show that this single type of event can, by itself, generate the droplet size distributions in a sea spray. This is a remarkable result showing how, in a complex system, a statistical distribution can be entirely produced by the dynamics of one dominant phenomenon.

Buckling instabilities of thin sheets or plates of viscous fluid occur in situations ranging from food and polymer processing to geology. Slim, Teichman & Mahadevan (J. Fluid Mech., this issue, vol. 694, 2012, pp. 5–28) study numerically the buckling of a sheared viscous plate floating on a denser fluid using three approaches: a classical ‘thin viscous plate’ model; full numerical solution of the three-dimensional Stokes equations; and a novel ‘advection-augmented’ thin-plate model that accounts (in an asymptotically inconsistent way) for the advection of perturbations by the background shear flow. The advection-augmented thin-plate model is markedly superior to the classical one in its ability to reproduce the predictions of the Stokes solution, illustrating the utility of judicious violations of asymptotic consistency in fluid-mechanical models.

Salt fingers are a form of double-diffusive convection that can occur in a wide variety of fluid systems, ranging from stellar interiors and oceans to magma chambers. Their amplitude has long been difficult to quantify, and a variety of mechanisms have been proposed. Radko & Smith (J. Fluid Mech., this issue, vol. 692, 2012, pp. 5–27) have developed a new theory that balances the basic growth rate with that of secondary instabilities that act on the finite amplitude fingers. Their approach promises a way forward for computationally challenging systems with vastly different scales of decay for momentum, heat and dissolved substances.

The splash resulting from the impact of a drop onto a pool is a particularly beautiful manifestation of a canonical problem, where a mass of fluid breaks up into smaller pieces. Despite over a century of experimental study, the splashing mechanics have eluded full description, the details often being obscured by the very rapid motions and small length scales involved. Zhang et al. (J. Fluid Mech., vol. 690, 2012, pp. 5–15) introduce a powerful new tool to the experimental arsenal, when they apply X-ray imaging to study the fine ejecta sheets which emerge during the earliest contact of the drop. Their images reveal hidden details and complex underlying dynamics, which will directly affect the size and velocity of the splashing droplets.

Rapidly rotating dynamos, relevant to the origin of the Earth magnetic field, are difficult to model owing to the extreme parameter regimes that occur in their dynamics. Numerical models alone fail to approach the correct regime. However, progress can be achieved by combining numerical and analytical methods. This can offer a better understanding of the variety of behaviour observed near the onset of dynamo action, as seen in the recent study of Sreenivasan & Jones (J. Fluid Mech., this issue, vol. 688, 2011, pp. 5–30).

In the following featured article, Boyer, Pouliquen & Guazzelli (J. Fluid Mech., this issue, vol. 686, 2011, pp 5–25) measure the normal stresses in a suspension of non-colloidal rigid spheres. They use the classical rod-climbing experiment, except that for interesting reasons the free surface near to the rotating rod does not rise but dips down. Careful techniques reveal that the normal stresses occur only above a volume concentration of 22 %. Over a period of hours the measurements drift, typical of many observations of suspensions. This is due to particles slowly migrating away from the rotating rod. A model of the migration gives good predictions of the observed changes.

Maintaining the stratification of the ocean requires deep mixing. Part of the energy that provides this mixing is transferred over topography from the surface tides into the internal tides. Numerous theoretical, numerical and experimental studies have aimed at quantifying the energy transfer of this conversion mechanism. Maas (J. Fluid Mech., this issue, vol. 684, 2011, pp. 5–24) constructs model topographies with localized response and no energy transfer into the propagating internal tide, a surprising result that raises questions about models of tidal conversion.

The conditions for the appearance of instabilities in systems of helical vortices constitute an intriguing problem that still remains partly unsolved. The experimental study of Felli, Camussi & Di Felice (J. Fluid Mech., this issue, vol. 682, 2011, pp. 5–53) has shed new light on some of the basic mechanisms governing the instability mechanisms.

When a wave breaks, the tip forms a liquid sheet which impinges the base and creates an air cavity which breaks into bubbles. Gomez-Ledesma, Kiger & Duncan (J. Fluid Mech., this issue, vol. 680, 2011, pp. 5–30) have conducted a nice experiment on this problem, enabling them to discuss both the inclination of the jet and the effect of its translation. This work has interesting links with other transient cavities.

Many swimming microorganisms are able to propel themselves by the organized beating motion of numerous short flagella or cilia attached to their body surface. For their small size and the inherently viscous nature of the motion, this mechanism is very effective and they can swim several body lengths per second. The quest has been to see if artificial cilia may be developed and if the strategy of cilia propulsion can be used in microfluidic devices to transport fluids in a localized and controllable manner. Babataheri et al. (J. Fluid Mech., this issue, vol. 678, 2011, pp. 5–13) explore the response of chains of small paramagnetic beads that are elastically bonded together to form artificial cilia. The chain or fleximag is tethered to the surface and driven by external magnetic fields, responding also to both fluid and elastic forces. A key observation from their experiments and model is that for a simple planar-forcing strategy there is a hidden symmetry that limits the net transport of fluid.

How internal flow states can influence the global transport properties in a turbulent system has always been an intriguing question. Weiss & Ahlers (J. Fluid Mech., this issue, vol. 676, 2011, pp. 5–40) have provided an example by measuring the instantaneous Nusselt number in turbulent Rayleigh-Bénard convection and correlating it to the different modes of large-scale flow.

Direct numerical simulations have been performed by Mayer, Von Terzi & Fasel (J. Fluid Mech., this issue, vol. 674, 2011, pp. 5–42) to demonstrate that oblique-mode breakdown leads to fully turbulent flow for a Mach 3 flat-plate boundary layer. Since very low level of initial disturbances is required for this transition scenario, oblique-mode breakdown is the most potent mechanism for transition in two-dimensional supersonic boundary layers in low-disturbance environments relevant to flight.

Liquid sheets are a generic occurrence in nature. They provide a key pathway by which liquid matter breaks up and is dispersed into smaller entities, as in the formation of sprays. However, the stability of liquid sheets is surprisingly subtle, and involves a range of physical effects, without which a quantitative understanding is impossible. As a result, even the linear stability of liquid sheets has remained open for over half a decade. Now, by a combination of experiment and thorough theoretical analysis by Tammisola et al. (J. Fluid Mech., this issue, vol. 672, 2011, pp. 5–32), we have finally gained a solid understanding of the key factors that control the initial stages of sheet breakup.

Droplets of an aqueous phase placed on a very hydrophobic, waxy surface bead-up rather than spread, forming a sessile drop with a relatively large contact angle at the edge of the drop. Surfactant molecules, when dissolved in the aqueous phase, can facilitate the wetting of an aqueous drop on a hydrophobic surface. One class of surfactants, superwetters, can cause aqueous droplets to move very rapidly over a hydrophobic surface, thereby completely wetting the surface (superspreading). A recent numerical study of the hydrodynamics of superspreading by Karapetsas, Craster & Matar (J. Fluid Mech., this issue, vol. 670, 2011, pp. 5–37) provides a clear explanation of how these surfactants cause such a dramatic change in wetting behaviour. The study shows that large spreading rates occur when the surfactant can transfer directly from the air/aqueous to the aqueous/hydrophobic solid interface at the contact line. This transfer reduces the concentration of surfactant on the fluid interface, which would otherwise be elevated due to the advection accompanying the drop spreading. The reduced concentration creates a Marangoni force along the fluid surface in the direction of spreading, and a concave rim in the vicinity of the contact line with a large dynamic contact angle. Both of these effects act to increase the spreading rate. The molecular structure of the superwetters allows them to assemble on a hydrophobic surface, enabling the direct transfer from the fluid to the solid surface at the contact line.

Nature is often an inspiration for scientists and especially so in fluid dynamics. We observe and admire the beauty of birds and fish as they move through the air and water, and wonder how these forms of locomotion evolved and whether they are optimised for efficiency. A common feature of this locomotion is that the thrust is generated by the flapping of a wing or fin, creating an unsteady flow. Other less readily observed animals such as salps or squid eject a pulsed jet, raising the question of whether there is an advantage of this unsteady forcing over a steady jet. This is the question addressed by Ruiz, Whittlesey & Dabiri (J. Fluid Mech., this issue, vol. 668, 2011, pp. 5–32) who have carried out detailed flow and power-consumption measurements with a self-propelled vehicle in water. The vehicle has a novel propulsion mechanism that allows a comparison of the efficiency of a pulsed and steady jet to be compared. They show that significant increases in efficiency are possible with the pulsed jet, even allowing for the additional power needed to create the pulsed flow. They also show that vortex rings are produced by the unsteady jet and that the additional entrainment of ambient fluid into the ring and the higher pressure at the front of the ring are the cause of this increased efficiency.

Turbulence is widely considered one of the most important and most difficult unsolved problems in classical physics. It is also the area of fluid mechanics where the greatest effort is exerted, the most papers published and, some would argue, the least progress made. Although direct numerical simulation is becoming an increasingly valuable tool, there remains a need for high-quality experiments to underpin our theoretical and numerical progress. Such statements apply equally to the ‘classical’ problem of homogeneous isotropic turbulence and to turbulence in its many other guises. Of particular interest is turbulence in a rotating system, where it is well known that the influence of rotation leads to the development of anisotropy and the elongation of scales parallel to the rotation axis. Moisy et al. (J. Fluid Mech., 2010, this issue, vol. 666, pp. 5–35) present new experiments in the free decay of grid-generated turbulence in a rotating system. They investigate the emergence of anisotropy from essentially isotropic initial conditions. While it is well known that rotation suppresses velocity gradients parallel to the rotation axis, Moisy et al. (2010) uncover some startling and previously overlooked implications.

Abdominal aortic aneurysm (AAA), a common vascular disease among the adult population, forms in the portion of the aorta below the renal arteries and upstream of its bifurcation into the two iliac arteries. While the precise cause of this vascular disease is still unknown, it is believed to be multi-factorial and predominantly degenerative, arising through a complex interplay among several biological factors as well as from specific local changes in the haemodynamic stresses on the vessel's wall. Using a simple mechanical model to simulate the difference in the stiffness of the aorta and iliac arteries, Duclaux, Gallaire & Clanet (J. Fluid Mech., 2010, this issue, vol. 664, pp. 5–32) propose a scaling argument for the transition between homogeneous and inhomogeneous deformation of an elastic tubular membrane that offers a plausible explanation for the observed localization of the AAAs. While neglecting long-term tissue remodelling and other important biological processes, the fluid mechanics model of Duclaux et al. (2010) appears to be consistent with some known associated risk factors.