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.

The generation of foam during the course of a bioprocess remains a major technological challenge to be resolved and needs further investigation. The foaming tendency of the cultivation media used in bioreactors induces various direct,that is microbial cells stripping and contamination, as well as indirect adverse effects, that is modification of the properties of the medium subsequent to the addition of chemical antifoam leading to toxic effects at the level of the microbial metabolism and fouling of the downstream processing equipment. In this work, the mechanisms leading to foam formation are described at the molecular level (adsorption of surface‐active molecule at the gas–liquid interface), and also at the process level. In view of the potential foam‐associated problems, special attention is given to the level of the compatibility of the intensification of the gas–liquid operation in bioreactors. Due to these considerations, both the chemical and mechanical antifoam techniques have been evolved. Numerous new antifoam formulations and original combinations of chemical antifoams have been especially designed in order to meet the specific requirements of bioprocesses. Original mechanical techniques to prevent foam formation in bioreactors have been elaborated from combined knowledge involving the fluid dynamics of gas–liquid dispersion and interfacial processes (e.g. the “stirring as foam disruption” concept).

A two-phase partitioning bioreactor to treat gas effluents polluted by volatile organic compound has been developed. In this work, both the mass transfer of isopropylbenzene (IPB) and oxygen have been considered in relation to their influence on the hydrodynamics of the reactor and the type of silicone oils used as a second phase. The synergistic effect of silicone oil and stirrer speed on the global oxygen mass transfer coefficient (KLa) and gas holdup (up to 12%) have been investigated. The addition of 10% of low viscosity silicone oil (10 cSt) in the reactor does not significantly affect the oxygen transfer rate. The very high solubility of IPB in the silicone oil leads to an enhancement of driving force term, especially for high fraction of silicone oil. However, it does not seem useful to exceed a volume fraction of 10% since KLa(IPB) decreases sharply at higher proportions of silicone oil. KLa(IPB) and KLaO2 evolve in the same way with the proportion of silicone oil. These results confirm the potentialities of our bioreactor to improve both the oxygen and pollutant gas transfer in the field of the treatment of gaseous pollutants, even for highly concentrated effluents.

Processing convenience and formulation flexibility frequently require the delivery of the silicone oils as emulsions. The shelf life of the latter is achieved kinetically, in the most cases via the addition of surfactants. On the other hand, surfactants are the subject of increasing scrutiny with regard to their environmental impact. The goal of this study is to formulate silicone oils in surfactant‐free emulsions and to demonstrate the controlled release of the active silicone material. A non‐aqueous silicone emulsion comprising of a continuous phase of a polar organic liquid, having droplets of silicone antifoam compound dispersed therein, have been developed. These systems are stabilised by (fractal) waxy particles which play a dual role. They act as Pickering stabilisers and in the same time they form an elastic network in the continuous phase, providing a creaming stability of more than a year. A triggered release of the (antifoam) silicone active can be achieved via heating above the melting temperature of the waxy particles. This is demonstrated by the fact that no antifoam activity has been observed at temperatures below ca. 60°C, while at temperature of above 65–70°C a strong antifoam effect has been obtained.

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 silicone-based 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.