Research into Multi-scale Examination of MultiPHase physIcs in flowS (MEMPHIS)

Posted by Nina Kovalchuk | June - 29 - 2017 | 0 Comment

Results on drop coalescence and mixing have been presented at two international conferences.

1. Drops coalescence and mixing in microchannel by N.M.Kovalchuk, E. Nowak, Z.Schofield, D. Vigolo, and M.J.H. Simmons was presented at 9th International Symposium on Mixing in Industrial processes in Birmingham, UK

Abstract

Emulsification is a process which enables a high level of homogeneity to be obtained in the mixing of immiscible liquids.  In most cases emulsification occurs under conditions of high shear stress, which can also bring dispersed drops together and facilitate their coalescence.  The average size of drops in the emulsion and the drop size distribution in the final product depends on the equilibrium between drops breakage and coalescence.  Coalescence of drops in a stored emulsion is also a highly undesirable event, as it leads eventually to the emulsion destabilization by creaming/sedimentation leading to complete phase separation.  Emulsions are stabilised against coalescence by surfactants, but the behaviour of surfactant during drop interactions, especially under shear stress is not well understood. Therefore the study of the coalescence event for two drops is of high interest and practical importance for optimization of emulsification process and improving emulsion life time.

On the other hand in many applications, in particular in microfluidics, where drops can serve as micro-reactors, coalescence enables reagents to be brought together and react under well controlled conditions. In this case not only conditions enabling bespoke coalescence, but also mixing of the drops content accompanying the coalescence is of great importance.

The aim of this study is to investigate in detail the process of coalescence of two drops under conditions of controlled shear stress in a constrained geometry. The drops are formed in a microfluidic flow focusing device. The coalescence is studied depending on drop size, confinement, flow rates of dispersed and continuous phase and the presence of surfactant. In particular it was found that drops in the microchannel can accelerate towards each other and coalesce even at surfactant concentrations of multiple times the critical micelle concentration (cmc), see Fig. 1.

To study the flow field within the droplets during coalescence and the mixing of their contents afterwards, we exploit a novel optical technique, Ghost Particle Velocimetry (GPV). The dispersed phase is seeded with nanoparticles to produce a ‘speckle’ pattern. This is obtained by the reduction of the numerical aperture of the condenser lens of an optical microscope to observe the light scattered by the nanoparticles and captured using a high frame speed camera. The signal can then be enhanced by subtracting the median of few hundred images from each frame.  A cross-correlation algorithm is then used to extract the velocity profile. The GPV technique permits the detection of the speckle pattern close to the interface where the coalescence takes place.  Moreover, three-dimensional velocity information can be extracted by performing GPV measurements on different planes.

Fig.1

Fig. 1 Coalescence of drops in microchannel. Disperse phase is 52 % glycerol/48 % water mixture with surfactant dodecyltrimethylammonium bromide (7.5 cmc), continuous phase is a silicone oil.

2. Bulk and interfacial flows in the coalescence of surfactant-laden and surfactant-free drops by Emilia Nowak, Nina Kovalchuk, Zhihua Xie, Chris Pain, Omar Matar, and Mark Simmons was presented at 7th Bubble and Drop conference in Lyon, France.

Abstract

Coalescence of drops was studied for more than a century with different focuses, for instance on the time before merging occurs [1], strongly related film drainage from between the drops [2] or kinetics of the bridge expansion [3]. The flow accompanying the merging of drops in binary coalescence events has been investigated in the literature. The coalescence of drops of different properties, however, has received far less attention despite its importance to a plethora of industrial applications; these include tightly-controlled merging of reactants, different emulsions, and the formation of bespoke multiphase structures, which deliver specific function via novel manufacturing routes.

This study is aimed at providing a thorough understanding of the merging process immediately after the rupture of the thin liquid film separating the drops initially. Coalescence of two aqueous drops, one containing surfactant and the other surfactant-free, in silicone oils of various viscosities, was studied. It is observed that the surfactant-free drop intrudes into the surfactant-laden drop in the form of a penetrating jet whose speed increases and average radius decreases with increasing outer phase viscosity. Mixing patterns within the coalescing drops are due to the force imbalance caused by capillary pressure difference and surfactant-induced Marangoni stresses; the intensity of the convective bulk motion is also influenced by the viscosity of the outer phase. Numerical simulations provide a deeper insight into the liquid redistribution during the merging. Their results are in good agreement with the experimental data and will be discussed during the talk.

References:

1. Nielsen, L.E., Wall, R., Adams, G., Coalescence of liquid drops at oil-water interfaces. Journal of Colloid Science, 1958. 13(5): p. 441-458.

2. Jones, A.F., Wilson, S.D.R., The film drainage problem in droplet coalescence. Journal of Fluid Mechanics, 1978. 87(2): p. 263-288.

3. Thoroddsen, S.T., Takehara, K., Etoh, T.G., The coalescence speed of a pendent and a sessile drop. Journal of Fluid Mechanics, 2005. 527: p. 85-114.

 

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