PhD Student
Associate member of the GRK2516
Group: Prof. Friederike Schmid, JGU Physics
Contact: E-mail

Research Project: Theory

The interactions of droplets with soft surfaces are of crucial importance for various applications such as self-cleaning, spray coating, anti-icing, or antifouling. In our project, we study the statics and kinetics of droplets on substrates covered by lubricated polymer brushes or gels by simulation and experiment. Our experimental approach involves observing water droplets in contact with PDMS gels lubricated with chemically identical silicone oil, measuring the effective surface tension using the pendant droplet method. Complementing our experiments, we employ molecular dynamics simulations, where we deposit a simple liquid on a polymer brush and model the lubricant as a polymeric liquid.

At the experimental level, the results showcase a clear evolution of the effective surface tension with time, highlighting the dynamic nature of the system. Our simulations reveal the formation of a wetting ridge and the cloaking of the droplet by the lubricant, which creeps up and completely covers the droplet surface. We quantify the cloaking in terms of its thickness, showing a cloaking transition that sets in with the fraction of lubricant present [1]. Our investigation also sheds light on the dependence of the contact angles on the droplet size and the possible effect of line tension. We further explore the evolution of the cloak over time and compare it to the experiment, discussing the underlying mechanisms behind the cloak's progression and comparing different possible scenarios for the evolution.

Figure 1. Snapshots from the simulation of a spherical droplet on a lubricated brush. Red particles belong to the grafted chains, yellow to the lubricant (free chains), and blue to the simple liquid. The arrow points towards increasing fraction of lubricant, showcasing the transition from uncloaked droplet on the dry brush, to partial coverage on a moderately lubricated brush, to fully cloaked on a decently lubricated brush.

References

[1] R.G.M. Badr, L. Hauer, F. Schmid, D. Vollmer. Cloaking Transition of Droplets on Lubricated Brushes. Phys. Chem. B 2022, 126, 36, 7047–7058. DOI: https://doi.org/10.1021/acs.jpcb.2c04640