💧 The first microseconds of droplet coalescence on a liquid surface

Thanks to a new numerical simulation model validated by innovative experimental techniques, a team of scientists has been able to study the initial phase of coalescence, that is, the fusion, of a droplet on the surface of a liquid with unprecedented spatial and temporal resolution. The results are published in the journal Physics of Fluids.

The coalescence of liquid droplets plays a key role in many industrial processes and applications involving emulsions or foams, chemical reactions between two liquids, or the dripping of droplets onto the surface of a reservoir. But its experimental study is difficult due to the very short time and space scales of the phenomenon in its initial phase. Numerical simulation, on the other hand, provides information that is inaccessible through experimental techniques.


A team from the Laboratory of Reactions and Process Engineering (LRGP, CNRS/University of Lorraine) has developed a numerical simulation model, validated by comparison with experimental results, which accounts with unmatched accuracy for the phenomena that occur when a droplet comes into contact with the surface of a reservoir of the same liquid.

The study was conducted with a Newtonian liquid (constant viscosity) – distilled water – and two viscoelastic liquids (variable viscosity) – polymer solutions in water. The simulations of droplet coalescence on the liquid surface were performed by coupling a statistical physics model (lattice Boltzmann method) with a rheological model for viscoelastic fluids (Oldroyd-B model).

To experimentally study coalescence and validate the simulation model, three techniques were implemented. An electrical conductance measurement, developed in the laboratory, gives the evolution of the coalescence zone between the droplet and the surface from the first microseconds. An ultra-fast camera (100,000 frames/s) allows visualization of the phenomenon. Finally, with a micro particle image velocimetry (μ-PIV) technique developed in the laboratory, coupled with the ultra-fast camera, the velocity fields of the fluids during coalescence were analyzed.

The results of the simulations and experimental measurements, obtained with a spatial resolution of 5.2 μm and a temporal resolution of 0.8 μs, are in good agreement. For both the Newtonian fluid and the viscoelastic fluids, the study revealed that the coalescence width, during the first microseconds of the phenomenon, varied linearly with time t, then followed a t^1/2 law.

These results represent a significant fundamental advance. In the long term, they should also enable the improvement of industrial processes, whether it is increasing the stability of an emulsion by avoiding coalescence, for example in the cosmetics industry, or intensifying oil-water separation by promoting coalescence, in the petroleum industry. Two areas in which LRGP already has research collaborations. A new study is underway to investigate coalescence involving, this time, an oily liquid phase.

Article available on the open archive repository HAL.