Emulsification with microstructures

Abstract

A dispersion is a multiphase product in which at least one phase is dispersed into another phase. In emulsions, those phases are immiscible liquids, for example oil and water. Many products which we use in everyday life are emulsions. One can think of ointments, paints, or sun protection cream, but the largest variety of emulsion-based products can be found in food: butter, mayonnaise, cream, milk, salad dressings (like vinaigrettes), soups, sauces, ice cream, and margarine are some examples. Emulsions are generally thermodynamically unstable and they tend to separate into the original phases in due time. The droplet size and the droplet size distribution are two important properties of an emulsion that have influence on the stability, but also the taste and appearance. Traditional emulsification techniques are based on disrupting larger droplets in coarse pre-mix emulsions into smaller ones using intense force fields. A lot of energy is put into the product; almost all energy dissipates into heat and only a very small fraction is used to form the emulsion. In addition, the resulting emulsions have a wide droplet size distribution. New microtechnological emulsification techniques hold a large advantage over conventional techniques as they can produce narrowly dispersed emulsions, while using the energy much more efficiently for the formation of droplets. This implies that the process is much milder, therewith allowing production of shear and temperature sensitive products such as multiple emulsions, which have great potential for reduced calorie products or encapsulation of (healthy) components but cannot be produced with traditional methods. With these microtechnological techniques, droplets of the desired size are directly generated with a device that is structured on the same scale as the droplets itself. In general, one can divide these emulsification techniques into two categories based on the driving force: shear-induced and spontaneous droplet generation. In the first category, cross- or co-flow of the continuous phase is used to snap off droplets. In the second category, interfacial tension forces cause droplet snap off without the need of a flow field. These latter devices show the highest potential for the production of monodisperse emulsions. Given the potential of these emulsions for a variety of purposes, this thesis focuses on these microtechnological emulsification systems. The goal of this thesis was to gain a thorough understanding of the principles and dynamics of spontaneous droplet generation, and to use this understanding to design better devices, for improved product properties at reasonable throughput. In Chapter 2, experimental observations and extensive Computational Fluid Dynamics simulations of a so-called terrace system are combined into an analytical predictive model, based the analysis of local pressures in the device. This model was shown to have good predictive value and to give direct insight in the relevant process parameters. Based on that, the terrace structure itself could be evaluated, and various design rules could be derived. The analytical model was also used as a steppingstone to gain insight in complex phenomena such as dynamic interfacial tension effects and contact angle (surface properties) of the used microdevices. In Chapter 3, the ratio of the viscosities of the dispersed and the continuous phases was found to be an important parameter for droplet formation in terrace-based systems. At high viscosity ratios, the droplet size is constant, but below a critical value droplet size increases, and below a minimum value no droplet formation is possible. These characteristic ratios were found to be dependent on the device geometry, both experimentally and through CFD simulation. The inflow of continuous phase is relatively difficult at low viscosity ratios, resulting in steep pressure gradients on the terrace structure which result in more oil supply during droplet formation, which in turn results in larger droplets. Detailed understanding of the existing spontaneous droplet formation in microchannel systems resulted in a new design: Edge-based Droplet GEneration (EDGE), which is described in Chapter 4. In EDGE, droplets are formed simultaneously along the edge of a wide but shallow nozzle, in contrast to the terrace systems discussed in Chapters 2 and 3 where only one droplet is formed at a time. With EDGE, narrowly dispersed emulsion droplets were formed in a broad pressure range. The system seems self-regulating: a disturbance in the flow pattern does not influence the droplet formation. The droplet size scales with the height of the droplet formation unit by a factor 5.5 – 6.5. Fundamental aspects related to EDGE are discussed in Chapter 5; especially the filling of the plateau was found to be essential for multiple droplet formation. Crucial for this is the difference in pressure needed for the dispersed phase to invade the plateau and the pressure needed to flow over the edge at the other end. In addition, the nozzle needs to have a relatively high hydrodynamic resistance compared to the whole system, which ensures even oil supply to the plateau and complete filling. The droplet formation locations along the nozzle were found to be regularly distributed along the length of the nozzle, and to be dependent on plateau depth. The EDGE process was simulated with CFD, which revealed that the pressure distribution on the plateau is influenced only very locally by droplet formation, therewith explaining the robust behavior of EDGE systems. Also for EDGE, viscosity ratio was found to be an important parameter. Behavior similar to that described for terrace structures (Chapter 3) was found, therewith also explaining the differences in size between liquid droplets in emulsification and gas bubbles in foaming. The minimum droplet size that can be obtained with EDGE could be derived from a geometric model that includes contact angle effects. EDGE has an advantage over microchannel systems mainly due to its simple structure, robustness, and parallelization possibilities as discussed in Chapter 6. An important design criterion when parallelizing droplet formation units is designing both oil supply and emulsion drainage channels properly, i.e. minimizing the pressure drops in these channels to obtain similar pressure conditions for each nozzle. The shape of the plateau may further be tuned to the properties of the phases (especially viscosity is important), to obtain a more stable process. Many microfluidic droplet generation devices can generate equally sized droplets, but emulsions with droplets below 10 micrometer are not common. Similarly, many devices have limitations in the types of ingredients that can be used, due to their small dimensions. Unfortunately, food-grade ingredients often pose problems. We found in Chapter 7 that EDGE is rather tolerant in the type of ingredients used. Some examples of food-grade dispersions are shown; for example, sunflower oil in whey protein concentrate gave an emulsion with an average droplet size of about 7 micron and very stable process operation. Furthermore, food-grade double emulsions (again 7 micron droplet size) were produced, and foams of whey protein (with a bubble size of 30 micron). This indicates the suitability of EDGE for products that otherwise cannot be made, and together with the sizes of scaled-up versions, this shows that EDGE could be a truly interesting new technology for the food industry. The last chapter in this thesis discusses the research strategy followed in this project. The combination of experiments with computational modeling of fluid dynamics, and relatively simple analytical or geometric models is shown to be a very effective method for investigating complex systems, to obtain fast process understanding and for further development of the devices. EDGE has several possibilities to increase production capacity, and this together with the advances in precision engineering is expected to make large-scale application of emulsification with EDGE microstructures a possibility in the near future. <br/