Jean-Paul Lecomte

Critical parameters for the partial coalescence of a droplet

The partial coalescence of a droplet onto a planar liquid-liquid interface is investigated experimentally by tuning the viscosities of both liquids. The problem mainly depends on four dimensionless parameters: The Bond number (gravity vs surface tension), the Ohnesorge numbers (viscosity in both fluids vs surface tension), and the density relative difference. The ratio between the daughter droplet size and the mother droplet size is investigated as a function of these dimensionless numbers. Global quantities such as the available surface energy of the droplet have been measured during the coalescence. The capillary waves propagation and damping are studied in detail. The relation between these waves and the partial coalescence is discussed. Additional viscous mechanisms are proposed in order to explain the asymmetric role played by both viscosities.

Foam control in fermentation bioprocess: from simple aeration tests to bioreactor.

In this article, we describe the development of a simple laboratory test for the effective screening of foam control agents on a selected fermentation system, the mass production of Yarrowia lipolytica. Aeration testing is based on sparging air in the foaming medium allowing partial reproduction of the gas-liquid hydrodynamic encountered in bioreactors. “Dynamic sparge test”, for which measurements are made during foam formation, was used to compare the capacity of three antifoams, based on different technologies, to control the foam produced in the fermentation broth. The selected foam control agents were: (1) an organic antifoam (TEGO AFKS911), (2) a siliconebased emulsion containing in situ treated silica (DC-1520) and (3) a silicone/organic blend silica-free formulation. The testing results demonstrated dramatic differences among them and showed that the capacity of TEGO AFKS911 and DC-1520 to control the foam generated in the fermentation broth decreases as a function of fermentation time. This occurred to a much lesser extent for the silicone/organic blend formulation. These results were correlated with the change of the foam nature and the increase of foam stability of the fermentation broth with culture time. The increase in protein content as a function of growth time was correlated with an increase in foam stability and antifoam consumption. A “synthetic fermentation broth” was also developed, by adding both proteins and microorganism to the culture medium. This allowed us to mimic the fermentation broth, shown by the similar antifoams behaviour, and is therefore a simple methodology useful for the selection of appropriate antifoams.