Fs Florian Günther

Numerical simulations of complex fluid-fluid interface dynamics

Interfaces between two fluids are ubiquitous and of special importance for industrial applications, e.g., stabilisation of emulsions. The dynamics of fluid-fluid interfaces is difficult to study because these interfaces are usually deformable and their shapes are not known a priori. Since experiments do not provide access to all observables of interest, computer simulations pose attractive alternatives to gain insight into the physics of interfaces. In the present article, we restrict ourselves to systems with dimensions comparable to the lateral interface extensions. We provide a critical discussion of three numerical schemes coupled to the lattice Boltzmann method as a solver for the hydrodynamics of the problem: (a) the immersed boundary method for the simulation of vesicles and capsules, the Shan-Chen pseudopotential approach for multi-component fluids in combination with (b) an additional advection-diffusion component for surfactant modelling and (c) a molecular dynamics algorithm for the simulation of nanoparticles acting as emulsifiers.

Effects of nanoparticles and surfactant on droplets in shear flow

We present three-dimensional numerical simulations, employing the well-established lattice Boltzmann method, and investigate similarities and differences between surfactants and nanoparticles as additives at a fluid–fluid interface. We report on their respective effects on the surface tension of such an interface. Next, we subject a fluid droplet to shear and explore the deformation properties of the droplet, its inclination angle relative to the shear flow, the dynamics of the particles at the interface, and the possibility of breakup. Particles are seen not to affect the surface tension of the interface, although they do change the overall interfacial free energy. The particles do not remain homogeneously distributed over the interface, but form clusters in preferred regions that are stable for as long as the shear is applied. However, although the overall structure remains stable, individual nanoparticles roam the droplet interface, with a frequency of revolution that is highest in the middle of the droplet interface, normal to the shear flow, and increases with capillary number. We recover Taylors law for small deformation of droplets when surfactant or particles are added to the droplet interface. The effect of surfactant is captured in the capillary number, but the inertia of adsorbed massive particles increases deformation at higher capillary number and eventually leads to easier breakup of the droplet.

Lattice Boltzmann simulations of anisotropic particles at liquid interfaces

Complex colloidal fluids, such as emulsions stabilized by particles with complex shapes, play an important role in many industrial applications. However, understanding their physics requires a study at sufficiently large length scales while still resolving the microscopic structure of a large number of particles and of the local hydrodynamics. Due to its high degree of locality, the lattice Boltzmann method, when combined with a molecular dynamics solver and parallelized on modern supercomputers, provides a tool that allows such studies. Still, running simulations on hundreds of thousands of cores is not trivial. We report on our practical experiences when employing large fractions of an IBM Blue Gene/P system for our simulations. Then, we extend our model for spherical particles in multicomponent flows to anisotropic ellipsoidal objects rendering the shape of, e.g., clay particles. The model is applied to a number of test cases including the adsorption of single particles at fluid interfaces and the formation and stabilization of Pickering emulsions or bijels.

Domain and droplet sizes in emulsions stabilized by colloidal particles.

Particle-stabilized 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. We numerically investigate the effect of the volume fraction and the uniform wettability of the stabilizing spherical particles in mixtures of two fluids. For this, we use the well-established three-dimensional lattice Boltzmann method, extended to allow for the added colloidal particles with non-neutral wetting properties. We obtain data on the domain sizes in the emulsions by using both structure functions and the Hoshen-Kopelman (HK) algorithm, and we demonstrate that both methods have their own (dis)advantages. We confirm an inverse dependence between the concentration of particles and the average radius of the stabilized droplets. Furthermore, we demonstrate the effect of particles detaching from interfaces on the emulsion properties and domain-size measurements.

Mesoscale simulations of anisotropic particles at fluid-fluid Interfaces

Anisotropic colloidal particles have attracted great interest in industrial applications, such as particle-stabilized emulsions and fabrication of self-assembled complex materials. However, our understanding of the fundamental properties of such systems is still limited. We combine the lattice Boltzmann method and molecular dynamics to simulate multicomponent fluids and suspended particles. We review two examples of our recent research on anisotropic particles. First, we study the ensemble of ellipsoidal particles at a spherical interface. The capillary interactions between the particles cause a local reordering on very long timescales leading to a continuous change in the interface configuration and to an increase of interfacial area. This effect can be utilized to counteract the thermodynamic instability of particle stabilized emulsions and thus offers the possibility to produce emulsions with exceptional stability. Second, we investigate the behaviour of magnetic Janus particles adsorbed at fluid-fluid interfaces interacting with an external magnetic field. We demonstrate that the strength of resulting capillary interactions can be tuned by altering the external field strength, opening up the possibility to create novel, reconfigurable materials.

Mesoscale Simulations of Fluid-Fluid Interfaces

Fluid-fluid interfaces appear in numerous systems of academic and industrial interest. Their dynamics is difficult to track since they are usually deformable and of not a priori known shape. Computer simulations pose an attractive way to gain insight into the physics of interfaces. In this report we restrict ourselves to two classes of interfaces and their simulation by means of numerical schemes coupled to the lattice Boltzmann method as a solver for the hydrodynamics of the problem. These are the immersed boundary method for the simulation of vesicles and capsules and the Shan-Chen pseudopotential approach for multi-component fluids in combination with a molecular dynamics algorithm for the simulation of nanoparticle stabilized emulsions. The advantage of these algorithms is their inherent locality allowing to develop highly scalable codes which can be used to harness the computational power of the currently largest available supercomputers.