Microfluidic investigation of dynamic surfactant rearrangement on water-oil interfaces during single droplet formation

Kiratzis, Ioannis ORCID: 0000-0002-7924-9153 (2021). Microfluidic investigation of dynamic surfactant rearrangement on water-oil interfaces during single droplet formation. University of Birmingham. Ph.D.

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The formation of droplets within a continuous phase is most commonly referred to as dispersion and is a key process present in industry and nature. If the produced dispersion is stabilised then it is referred to as an emulsion. One method for the formation of emulsions involves the addition of surfactants that stabilise the interface to prevent drop coalescence. One of the challenges in emulsification is the competition between the rapid timescales of droplet break-up (interface formation) and surfactant mass transport. This suggests that emulsification might evolve under non-equilibrium conditions. This study investigates the dynamic aspects of emulsification using microfluidics, by studying the formation of an emulsion droplet by droplet in a range of capillary numbers from 10-4 to 10-1.
The process of droplet formation was studied using a flow focusing microfluidic device, with a channel width of 200 μm, for a range of surfactant solutions using continuous phases with a range of viscosities. The emulsion droplets formed were found to decrease in size with an increase in the quantity of surfactant in the system, the continuous phase flow rate and the continuous phase viscosity. Notably it was shown that different solutions which possess the same equilibrium properties (e.g. solutions above the critical micelle concentration), produce droplets of different sizes suggesting that the dynamic (non-equilibrium) properties of the system dominate the governing physics. Finally, the satellite droplets formed from the liquid filaments between the primary drops increase in size with an increase in the surfactant concentration.
Emulsification was further investigated by examining the thinning rate of the liquid filament connecting the expanding droplet to the bulk of the dispersed phase. In all cases the thinning rate follows two stages: initially, the thinning rate is slow and the surfactant solutions exhibit a thinning behaviour which is different from a surfactant free fluid. As the break-up process advances the thinning rate increases and the solutions with surfactant concentrations below the cmc exhibit a thinning behaviour similar to a surfactant free system. This presents more evidence that the droplet formation process evolves under non-equilibrium conditions when small quantities of surfactants are present and the process diverts from equilibrium as it reaches its final stages.
Furthermore, the flow fields in the dispersed phase were investigated during the droplet formation process using Ghost Particle Velocimetry (GPV), an optical velocimetry technique, with the aim of understanding how the presence of surfactant affects the local hydrodynamics. During the final stages of the break-up the presence of surfactant reduces fluid velocities. This suggests that inhomogeneities in surfactant concentration create gradients in surface tension causing interfacial (Marangoni) stresses which act in the opposite direction to the prevailing flow field. However, during the initial stages of droplet formation the presence of surfactants allows for higher fluid velocities that increase the fluid recirculation. The above results showed the impact of the presence of surfactant on the hydrodynamics of the dispersed phase during droplet formation.
This study contributes to the knowledge of how the mass transport of surfactant influences the hydrodynamics and interfacial behaviour over the short timescales of emulsification. It was demonstrated that when the surfactant quantities are small the systems display properties closer to a surfactant free system and the presence of Marangoni phenomena in the last stages of the process act against droplet formation. These interfacial stresses act to oppose the prevailing flow field.

Type of Work: Thesis (Doctorates > Ph.D.)
Award Type: Doctorates > Ph.D.
Licence: All rights reserved
College/Faculty: Colleges (2008 onwards) > College of Engineering & Physical Sciences
School or Department: School of Chemical Engineering
Funders: Engineering and Physical Sciences Research Council
Subjects: T Technology > TP Chemical technology
URI: http://etheses.bham.ac.uk/id/eprint/11698

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