The results of collaborative work between University of Birmingham and University College London on Effect of surfactant dynamics on microfluidic emulsification have been presented at the 31st Conference of the European Colloid & Interface Society (Madrid, September 3-8th, 2017).
Abstract
The microfluidic emulsification is a rapidly growing scientific area providing an advantage of formation of uniform drops of micrometer size and manipulating them in controlled way. Surfactants are broadly used in emulsification processes to stabilize drops. In microfluidic, where drops are formed on the time scale of milliseconds, dynamic effects related to surfactant transfer and adsorption become extremely important. In this study, using high resolution high-speed video-recording, we explore in a flow-focusing microfluidic device the effects of surfactants on the dynamic regimes of aqueous drop formation in an organic phase, the size of formed drops and their coalescence in the microchannel.
It is shown that dynamic interfacial tension is a key parameter for controlled drop production. Under the same flow rates of continuous and dispersed phases, addition of surfactant often changes the flow regime from dripping to jetting, what results in a wider drop size distribution. Using data on dynamic surface tension of the surfactant of interest, the range of flow rates for production of uniform surfactant-laden drops in the dripping regime can be estimated from the flow map of surfactant-free system.
The size of formed drops increases with the increase of ratio of flow rates of dispersed to continuous phase, f. For the drops with size smaller than the channel width the growth is slow, proportional to f0.1, whereas for plugs with size larger than the channel width the size is proportional to f. The power law exponent is practically independent of surfactant type.
It is found that the presence of ionic surfactant can facilitate drop coalescence in the channel, if the drop size is larger than the channel depth. This effect can be a result of surfactant redistribution under the high shear stress near the channel wall.
The effect of surfactant on the flow patterns is also studied using a two-colour PIV technique. The technique measures velocity profiles in both continuous and dispersed phases simultaneously, thus revealing the effects that the presence of surfactants has on the local flow fields.
The results of study carried out in University of Birmingham on Understanding of interaction between drop formation, polarity of surfactant and electrical field for the production of water in oil emulsion have been presented at the 10th World Congress of Chemical Engineering (Barcelona, October 1-5th, 2017).
Abstract
Emulsions are ubiquitous in food, examples being milk, butter, margarine, sauces and spreads. The taste of those products, their rheology, shelf life and cost depend to a large extent on the stabiliser used, volume fraction of dispersed phase, drop size and size distribution. One of the ways to improve the emulsification process is using electrical fields. In particular it enables decreasing of the size of dispersed phase and reducing the emulsification time [1] as well as energy consumption [2].
To optimise the use of electrical field in emulsification a better understanding of drop formation under electrical field conditions is needed. The regularities of formation of pure water drops in oil are quite well understood [3, 4], however usually emulsions are stabilised by surfactant being polar or ionic substance and can also contain salts, which affect interaction of forming drop with electric field.
Here we present the results of an experimental study on formation of surfactant-laden drops in presence of electric field. Three surfactants: non-ionic, Triton X-100; anionic, sodium dodecyl sulphate and cationic, dodecyltrimethylammonium bromide are investigated at different concentrations. To distinguish the effect of electric conductivity from the effect of surfactant/electric conductivity, a comparison is proposed using aqueous solution with the addiction of sodium bromide. Lytol is used as the continuous oil phase. Effect of increasing electric field on drop shape, size and frequency as well as transition from dripping to spraying is investigated.
