To understand in details this dramatic effect, experiments have been performed at ESPCI Paris on a model system, with smell not as pleasant as pastis, but easier to control : mix of water, acetone and a polymer that is soluble into acetone, and not in water ; kinetics (over 1 second, since emulsification is quite fast) was followed by ultrafast small angle X-ray scattering at ESRF Grenoble to measure the droplets radii ; numerical simulations were done at LPS Orsay to analyze the data, and to study the mechanisms.
Which scenario came out from these analyses ?... well, droplets acquire electrical charges because of the hydroxide ions HO<sup–, and coalescence must then stop when charges on the droplets are too large ; radius distributions become narrow because droplets of different charges are more likely to merge than droplets of similar charges. All right. Then, we had to check these mechanisms, that is to compare evolutions of the averaged droplet radius
At this point, numerical simulations of kinetics of coalescence have been useful. The experimental data were recovered in the simulations for the first second of physical time. Then, beyond that time, distribution widths increased still and always as the square root of
Then, was the exact solution a kind of anecdote ? Actually, not really so, since it allowed us to understand that this change in the scaling laws was the expression of a general behavior – the theory of the universal fluctuations – and that the machinery of that effect has things in common with other, totally different, systems. However, this aspect will need another section to be developed…
Contacts :
Robert Botet
Roger Kevin
Bernard Cabane
References :
Coalescence of Repelling Colloidal Droplets: A Route to Monodisperse Populations
R. Kevin, R. Botet and B. Cabane
Langmuir 29, 5689 (2013).
Universal Fluctuations
R. Botet and M. Ploszajczak
World Sci. ed, Singapore (2002).