|Title||Emulsification in microfluidic Y- and T-junctions|
|Source||Wageningen University. Promotor(en): Remko Boom, co-promotor(en): Karin Schroen. - [S.l. : S.n. - ISBN 9789085854579 - 150|
Food Process Engineering
|Publication type||Dissertation, internally prepared|
|Keyword(s)||emulsies - emulgering - emulgeren - druppelgrootte - emulsions - emulsification - emulsifying - droplet size|
|Abstract||On a daily basis, we encounter many emulsion-based products such as butter or sun cream, which consist of oil droplets in water, or water droplets in oil. Traditionally, these emulsions are produced with systems that allow a high throughput, but yield a broad droplet size distribution. Therefore, the industry is interested in emulsification techniques that give more monodisperse emulsions, such as emulsification with microfluidic devices, i.e. defined geometries with channel diameters in the order of several to hundreds of micrometers.
The goal of this thesis was to develop a new microfluidic emulsification technique that has the potential to be scaled up for the production of large volumes of monodisperse emulsion. We chose to study shear-driven microfluidic devices, i.e. T- and Y-junctions, due to their high productivity per junction and their potential for mass-parallelisation. However, reliable application of these junctions is only possible when the droplet-formation mechanism and droplet size determining parameters are fully understood. Therefore, we took a single junction both as a starting and as a focal point of this thesis.
The thesis starts off by indicating and quantifying the parameters that determine droplet size in microfluidic T-junctions. In literature, (monodisperse) emulsification at T-junctions is studied for a broad range of channel dimensions, flow rates, and materials. However, it is not yet clear which parameters determine the droplet size. Therefore, in Chapter 2 statistical analysis is used to quantitatively relate droplet size data from various literature sources. For T-junctions it is found that emulsion droplet size of drops, discs, and plugs can be described by a two-step model consisting of a droplet growth and a droplet detachment step. This suggests that an emulsion droplet grows until a certain volume is reached, after which it starts to detach. The channel dimensions determine droplet growth, while the continuous- and the disperse-phase flow rate determine the abating time (i.e. the fast decrease of the neck resulting in detachment).
In the remainder of this thesis microfluidic (flat) Y-junctions are discussed; they resemble T-junctions, but are hardly studied in literature. In Chapter 3, emulsification of hexadecane in various ethanol-water mixtures at different process conditions, i.e. flow rates and static interfacial tensions, is experimentally investigated. We focus on droplet formation at the Y-junction or downstream without the incipient droplet blocking the downstream channel (i.e. the dripping and the jetting regime). For Y-junctions, the droplet size is described with a force balance between the interfacial tension force and the shear force at the point where the incipient droplet is kept to the bulk by a neck. It is found that the droplet size at Y-junctions is determined by the interfacial tension, the channel dimensions, and the viscosity and flow rate of the continuous phase; but not by the flow rate of the disperse phase. This makes operation of Y-junctions intrinsically easier than T-junctions, for which the flow rates of both phases need to be (accurately) controlled.
Where Chapter 3 concentrates on process conditions, in Chapter 4 the effect of (Y-) junction design on the droplet size is investigated. In five different Y-junction geometries and one T-junction with a depth of 5 m, hexadecane is emulsified in ethanol-water mixtures at a given static interfacial tension and at various process conditions, e.g. flow rates. For the various Y-junctions, no effect on droplet size is observed from the junction angle and the length(s) and/or the width(s) of the microchannel(s). In contrast, significant differences are observed between T- and Y-junctions.
In Chapter 5, the force balance, found in Chapter 3, is extended by including the effect of the viscosity of the disperse phase and a broader range of viscosities and/or flow rates of the continuous phase. The force balance is mainly adapted by rewriting the shear force from the drag force on a sphere to the drag force on the cross-sectional area of the squeezed incipient droplet (head). It is found that the emulsion droplet size at Y-junctions is determined by the interfacial tension, the channel dimensions, the viscosity, density, and flow rate of the continuous phase, and the resistance with the wall. The influence of the viscosity of the disperse phase and the viscosity ratio were found negligible, just as the disperse-phase flow rate.
The first five chapters show that droplet size at microfluidic Y-junctions is strongly influenced by the interfacial tension and therefore it is important to quantify its value under dynamic conditions. Traditional tensiometric techniques do not allow interfacial tension measurement under the conditions applied in Y-junctions: high shear and droplet formation in less than milliseconds. Therefore, in Chapter 6, (monodisperse) emulsification at microfluidic Y-junctions is proposed as a new tensiometric technique. A calibration curve is derived for hexadecane in various ethanol-water mixtures with a range of static interfacial tensions. Subsequently, this curve is used to estimate the apparent dynamic interfacial tension for solutions with the surfactants SDS or Synperonic PEF108. The apparent dynamic interfacial tension is found to be determined by the flow rates of the continuous and disperse phase, the surfactant and its concentration. In addition, we showed that surfactant transport in Y-junctions is dominated by convection.
In Chapter 7, the thesis is concluded by comparing emulsification with microfluidic Y-junctions to other shear-driven microfluidic geometries with cross-flow membrane emulsification as a benchmark technology. Especially, the negligible effect of the flow rate and the viscosity of the disperse phase on the droplet size makes microfluidic Y-junctions unique. To illustrate the large-scale feasibility of microfluidic Y-junctions, typical emulsification device volumes and required areas to process 1 m3h-1 of disperse phase were calculated. The requirements are found to be comparable to values obtained from literature for membranes and microsieves. The energy input of the current microfluidic Y-junction design is comparable to traditional emulsification techniques, but since there is room for optimisation, we are hopeful that these values may well be reduced.