ENGINEERING AND CFD

In SPH the computational domain is discretized with particles that are moving in time. This Lagrangian approach is very powerful when complex geometries with strong deformations can occur. As the governing equations for the particles are solved by particle-particle interactions it is straightforward to include complex physical models at phase interfaces where SPH particles of different phases interact with each other.

Surfactants are modelled with a transport equation that includes bulk diffusion, surface diffusion, adsorption and desorption between an interface and the bulk phase. As the surfactant concentration at an interface has an effect on the local surface tension the flow evolution and the surfactant concentration field are coupled leading to very complex physics.

Since all particle methods rely on particle-particle interactions their computational effort is strongly increased compared to more traditional mesh-based simulation tools. Thus it is important to use efficient algorithms and to make use of High Performance Computing. The scientists implemented their method in a FORTRAN-Code as a client application using the PPM-Library developed at the ETH Zürich [2]. This platform offers a fully parallelized “particle-method-environment” for the user that handles all details of communication for the usage of thousands of CPUs.

Results
When drops are exposed to extensional flows they are deformed. Depending on the flow conditions and the physical properties of the fluids the drop can either deform to a steady ellipsoid or break-up into two or more droplets. This phenomenon was already studied decades ago and is well understood. When surfactants occur at the drop interface the behaviour can strongly differ from a clean interface. Due to surface concentration gradients the surface tension on the interface can vary locally and induce so-called Marangoni forces that manipulate the break-up process.

Exemplarily shown in Fig. 1 is a stretched drop close before breakup where the interface is coloured with the local interface concentration of surfactant. The same experiment for a clean interface results in a similar breakup but much later.

Surfactants do not only change the dynamics of a breakup mode but can also strongly alter the breakup type itself. Fig. 2 shows the tipstreaming phenomenon due to a low diffusive surfactant. Here, the surfactant accumulates mostly at the tips of the stretched drop and the resulting Marangoni-forces suppress the main break-up mode but produce these very thin fluid filaments that leave the drop. This phenomenon was first found in experiments and is of high interest as the thickness of these fluid filaments can be much smaller than any flow focusing device can currently achieve.