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.

Liquid rings can be generated in the Leidenfrost state using liquid oxygen of low boiling point ($- 183~\textdegree \mathrm{C} $) and high magnetic susceptibility, allowing one to ‘sculpt’ the liquid into a ring shape using an annular magnet. When the magnetic field is turned off, the ring shrinks back into a puddle with a constant acceleration. A potential flow approach accurately describes the dynamics of closure with an equation reminiscent of the Rayleigh–Plesset equation for the collapse of transient cavities.

We study the behaviour of elongated puddles deposited on non-wetting substrates. Such liquid strips retract and adopt circular shapes after a few oscillations. Their thickness and horizontal surface area remain constant during this reorganization, so that the energy of the system is only lowered by minimizing the length of the contour (and the corresponding surface energy); despite the large scale of the experiments (several centimetres), motion is driven by surface tension. We focus on the retraction stage, and show that its velocity results from a balance between the capillary driving force and inertia, due to the frictionless motion on non-wetting substrates. As a consequence, the retraction velocity has a special Taylor–Culick structure, where the puddle width replaces the usual thickness.

We discuss how a solid textured with well-defined micropillars entrains a film when extracted out of a bath of wetting liquid. At low withdrawal velocity V, it is shown experimentally that the film exactly fills the gap between the pillars; its thickness hd is independent of V and corresponds to the pillar height hp. At larger velocity, hd slowly increases with V and tends towards the Landau–Levich–Derjaguin (LLD) thickness hLLD observed on a flat solid. We model the entrainment by adapting the LLD theory to a double layer consisting of liquid trapped inside the texture and covered by a free film. This model allows us to understand quantitatively our different observations and to predict the transition between hp and hLLD.

We study the shape and dynamics of cavities created by the explosion of firecrackers at the surface of a large pool of water. Without confinement, the explosion generates a hemispherical air cavity which grows, reaches a maximum size and collapses in a generic w-shape to form a final central jet. When a rigid open tube confines the firecracker, the explosion produces a cylindrical cavity that expands without ever escaping the free end of the tube. We discuss a potential flow model, which captures most of these features.

We study the capillary rise of wetting liquids in the corners of different geometries and show that the meniscus rises without limit following the universal law: h(t)/a ≈ (γt/ηa)1/3, where γ and η stand for the surface tension and viscosity of the liquid while is the capillary length, based on the liquid density ρ and gravity g. This law is universal in the sense that it does not depend on the geometry of the corner.

We study the trajectory and the maximum diving depth of floating axisymmetric streamlined bodies impacting water with a vertical velocity. Three different types of underwater trajectory can be observed. For a centre of mass of the projectile located close to its leading edge, the trajectory is either straight at low velocity or y-shaped at high velocity. When the centre of mass is far from the leading edge, the trajectory has a U-shape, independent of the initial velocity. We first characterize experimentally the aerodynamic properties of the projectile and then solve the equations of motion to recover the three types of trajectories. We finally discuss the transitions between the different regimes.

We study the thickness ${h}_{d} $ of the liquid film left on a wet surface after scraping it with an elastic wiper (length $L$, rigidity $B$) moved at a velocity $V$. The scraper is clamped vertically at a given distance above the substrate, and ${h}_{d} $ is maximal when the tip of the scraper is just tangent to the surface. We show experimentally and theoretically that this maximum thickness is ${h}_{\mathit{max}} \simeq 0. 33L \mathop{ (\eta V{L}^{2} / B)}\nolimits ^{3/ 4} , $ where $\eta $ is the liquid viscosity. The deposition law is found to be sensitive to the shape of the wiper: the film thickness can also be tuned by using wipers with a permanent curvature, and varying this curvature.

A plate placed above a porous substrate through which air is blown can levitate if the airflow is strong enough. We first model the flow needed for taking off, and then examine how an asymmetric texture etched on the porous surface induces directional motion of the hovercraft. We discuss how the texture design impacts the propelling efficiency, and how it can be used to manipulate these frictionless objects both in translation and in rotation.