The transition from a compact liquid volume to a set of dispersed smaller drops, namely the process of atomization, often involves as a transient stage the change of the liquid topology into a sheet shape. This transition is sometimes enforced by specific man-made devices, and also occurs spontaneously as a result of various impacts and blow-ups. Then, the sheet has to destabilize in some way to induce another transition towards the formation of threads or ligaments, the ultimate objects whose breakup sets the size distribution of the final drops in the spray, depending on their state of corrugation while they fragment.The physical processes that lead to liquid sheet fragmentation are investigated for two regimes: the first one corresponds to a situation where the interaction with the surrounding medium is negligible and a second for which shear at the liquid/air interface induces undulations of the liquid sheet.
Situations where liquid interfaces, flat or curved, that are destabilized and breaks into droplets after being subjected to an impulsive acceleration are also investigated.

Liquid sheet

Smooth liquid sheet

Smooth liquid sheet.

The formation and fragmentation of liquid sheets resulting from the oblique collision of two identical cylindrical jets is investigated. The liquid expands radially from the impacting point forming a sheet in the form of a bay leaf bounded by a thicker rim. The sheet shape, rim size and liquid velocity field are quantified and represented analytically. External harmonic perturbations of the injection conditions reveal the nature of the rim destabilization and of its coupling with the sheet. Flow perturbations in the incident jets lead to sheet thickness modulations which trigger the fragmentation of the rim via the formation of liquid ligaments whose dynamics is described. The breakup of these ligaments induce both the shape and width of the drop size distribution in the spray formed by this process.

N. Bremond, E. Villermaux, 2006, Atomization by jet impact. J. Fluid Mech., 549, p. 273-306.

Undulating liquid sheet

Undulating liquid sheet.

The fragmentation of a laminar undulating liquid sheet flowing in quiescent air is investigated. Combining various observations and measurements we propose a sequential atomization scenario describing the overall sheet-drop transition in this configuration. The undulation results from a controlled primary Kelvin-Helmholtz instability. As the liquid travels through the undulating pattern, it experiences transient accelerations perpendicular to the sheet. These accelerations trigger a secondary instability responsible for the amplification of spanwise thickness modulations of the sheet. This mechanism, called the 'wavy corridor', is responsible for the sheet free edge indentations from which liquid ligaments emerge and break, forming drops. The final drop size distribution is of a Gamma type characterized by a unique parameter independent of the operating conditions once drop sizes are normalized by their mean.

N. Bremond, C. Clanet, E. Villermaux, 2007, Atomization of undulating liquid sheets. J. Fluid Mech., 585, p. 421-456.

Impulsive film

Bursting of a thin liquid film.

The breakup of a free thin liquid film subjected to an impulsive acceleration is investigated. A soap film is stretched on a frame at the exit of a shock tube. As the shock impacts the film, the film accelerates within a very short time and detaches from the frame at a constant velocity function of the shock strength. The liquid thickness modulations amplify and eventually the film is perforated with a number of holes, subsequently growing in radius and connecting to each other. The initially connex film is left in the form of a web of liquid ligaments which break into droplets. Both the hole density and formation time depend on the film velocity. We analyse these observations with an impulsive Rayleigh-Taylor instability incorporating liquid surface tension. It is shown to account for both the mode selection and its associated time of growth, providing a criterion for the film bursting time and hole density.

N. Bremond, E. Villermaux, 2005, Bursting thin liquid films. J. Fluid Mech., 524, p. 121-130.

Impulsive jet

Impulsive jet.

A tube filled with a perfectly wetting liquid falls axially under its own weight. In its gravity-free reference frame, the liquid interface is deformed by surface tension into a hemispherical shape. On impact of the tube on a rigid floor, the interface curvature reverses violently, forming a concentrated jet. If the contact angle at the tube wall is such that the interface is flat, the liquid rebounds as a whole with the tube, with no deformation. We analyse this phenomenon using an impulse pressure description, providing an exact description of the initial liquid velocity field at the impact, supported by high-speed image velocimetry measurements. This initial dynamics is insensitive to liquid surface tension and viscosity.

A. Antkowiak, N. Bremond, S. Le Dizès, E. Villermaux, 2007, Short-term dynamics of a density interface following an impact. J. Fluid Mech., 577, p. 241-250.

A. Antkowiak, N. Bremond, J. Duplat, S. Le Dizès, E. Villermaux, 2007, Cavity jets. Phys. Fluids, 19, 091112, (Gallery of fluid motion).