Petter Johansson

Interests

I am working on exploring the dynamics of wetting using molecular simulations. Several theoretical models have been proposed to descripe the phenomenas, making different assumptions of boundary conditions and approaching the problem from different length scales. Although these models to various extents correctly predict wetting dynamics, they have to prescribe ad hoc boundary conditions which may not correspond to real physical processes. In particular it is unknown which molecular processes resolve the famed shear stress singularity which arises when macroscopic fluid dynamics are made to model wetting at a molecular scale.

A backtrace of molecules at the wetting edge

A backtrace of molecules at the wetting edge shows that molecules on a hydrogen bonding substrate never slides along the substrate.

Any molecular regime is difficult to probe in a laboratory setting, but is the principal component of molecular dynamics (MD) simulations. Furthermore, MD is an ab initiomethod and makes no assumptions of boundary conditions on a system. Thus it has seen a lot of use in probing liquid systems of nanometer length scales and test the viability of wetting models in that regime. However, due to the lack of available computational power there has been little work done on realistic systems that are large enough to exhibit both nano- and macroscopic phenomena.

Using modern large scale computing resources and MD software I am studying droplet spreading experiments with systems that are large enough to bridge these two regimes. They consist of droplets with a radius of 50 nm (containing more than 1.2 million water molecules) and semi-realistic substrates which mimic realistic electrostatics. I hope to use these systems to shed some light on how molecular processes influence large scale wetting dynamics.

The flow field of a droplet spreading on a substrate

The flow field of a droplet spreading on a substrate. The spatial resolution is 0.25 nm and the flow is an average taken over a time frame of 10 ps.

Selected Publications

Background

Studied theoretical physics at Umeå University from 2006 to 2012, graduating with a Master’s degree. For my Bachelor’s thesis I studied the effect of spin-orbit coupling on Langmuir waves in plasmas, while my Master’s thesis concerned strong field Quantum Electrodynamics, examining pair annihilation in intense laser fields. As part of my final year, I studied abroad at Universität Heidelberg during the fall of 2010.

Joined the TCB Lab in 2012.