MATERIALS SCIENCE AND CHEMISTRY

Particle-stabilised emulsions have long been studied for their unique properties, which have a number of different industrial applications. Leveraging the petascale computing power of JSC high-performance computing system JUQUEEN, scientists of the Eindhoven University of Technology have been using simulations to investigate these systems, the results of which are now being picked up by experimental groups and realised in practice.

Particles as stabilisers for emulsions are used in the food and cosmetics industries, for crude oil recovery, and waste water treatment. However, the properties of such systems are only poorly understood. Challenges in this field are understanding the complex particleparticle and particle-fluid interactions. A team of researchers led by Professor Jens Harting of the Eindhoven University of Technology has been investigating the behaviour of individual particle-covered droplets, as well as the influence of the particle properties on the stability of emulsions.

Such systems are too complicated to be treated with theoretical models, while experimental trial-and-error searches for the best stabilisation method are both expensive and time consuming. “Our large scale simulations, which use combined lattice Boltzmann and molecular dynamics simulations, are able to cover the relevant parameters of such systems,” says Harting. “They have not only led to better fundamental knowledge about the systems, but may also directly lead to improved production or spark the interest of companies in using particle stabilised multiphase systems for their own products.”

Harting and his colleagues originally developed a computational method based on a combination of the lattice Boltzmann method to simulate fluid flows and the discrete element method to simulate suspended particles. “We combined the two and, from the beginning, it was clear that it would be an HPC application because of the different scales involved in the systems,” says Harting. Harting’s group has always made sure to write their codes with parallel architectures in mind, and received support in this from a number of centres including the Edinburgh Parallel Computing Centre, the Jülich Supercomputing Centre and SURFsara in Amsterdam. This put them in a unique position to be able to simulate these complex systems on large scales. “The world of HPC is always moving on to new architectures, and we have always had one eye on this,” he explains. “This has meant that we have only had to apply minimum effort to move on to the next set of machines, such as the new modular architectures with booster modules.”

Particle-stabilised emulsions are commonly used in various industrial applications. These emulsions can present in different forms, such as Pickering emulsions or bijels, which can be distinguished by their different topologies and rheology. Harting’s PRACE-supported project, called “COMFLOW – Colloids in multiphase flow”, covered a number of sub-topics. The first of these looked at how the distribution of droplet size and structure sizes in particle-stabilised emulsions is dependent on fluid parameters, such as viscosity, surface tension, particle shape, and particle concentration.

The team numerically investigated the effect of the volume fraction and the shape or wettability of the stabilising particles in mixtures of two fluids. For this, they used the well-established three-dimensional lattice Boltzmann method, extended to allow for the added colloidal particles with non-neutral wetting properties. “We obtained data on the domain sizes in the emulsions by using both structure functions and our own Hoshen-Kopelman cluster detection algorithm,” explains Harting. “We confirmed an inverse dependence between the concentration of particles and the average radius of the stabilised droplets. Furthermore, we were able to demonstrate the effect of particles detaching from interfaces on the emulsion properties and domain-size measurements, and identified various timescales in the emulsion formation in the case of anisotropic particles.”

Harting’s team also investigated how to dynamically change the properties of such emulsions from one state to the other, i.e. from a Pickering emulsion to a bijel. “We wanted to be able to switch back and forth between the two just by changing the contact angle, so that we would have a dynamic system whereby changing some property on the outside can quickly change the rheology and other properties,” explains Harting. “This could be useful for industrial purposes where, for instance, you may want to switch between low and high viscosity depending on the application.”

Following on from this work, the researchers developed a new mechanism for creating self-assembled porous media with highly tunable geometrical properties and permeabilities. This mechanism involves first allowing a particle-stabilised emulsion to form from a mixture of two fluids and colloidal particles. Then, either one fluid phase or the particle layer is solidified, which can be achieved by techniques such as polymerisation or freezing. “Based on computer simulations, we demonstrated that modifying only the particle wettability or concentration results in porous structures with a wide range of pore sizes and a permeability that can be varied by up to three orders of magnitude,” says Harting. “These could be perfect for use in chemical reactors or filters.”

The final topic that Harting and his colleagues looked at involved similar systems, but with particles at the fluid-fluid interface that have magnetic cores. With these systems, it is then possible to use a magnetic field to apply torque to the particles, and thus deform the interface and create very fine structures. Although these systems have not been studied extensively yet, there are a number of possible uses for them, including the printing of electronics and photovoltaics, as well as acting as an alternative electronic ink in e-readers.