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CPP: Fachverband Chemische Physik und Polymerphysik

CPP 10: Droplets, Wetting, and Microfluidics (joint session DY/CPP)

CPP 10.2: Vortrag

Montag, 9. März 2026, 15:15–15:30, ZEU/0118

Air layers and wetting under drops impacting on pre-wetted surfaces — •Kirsten Harth and Shiva Moradimehr — Fachbereich Technik, Technische Hochschule Brandenburg, Germany

Drop impact at low Weber numbers causes the formation of a (temporary) air cushion between the drop liquid and the substrate. On hard, dry substrates, its qualitative thickness profile and the location of the thinnest point are strongly governed by the Weber number. For impact on bulk, soft surfaces, like PDMS gels, it was found that impacts on softer substrates entrap more air [1]. Are there similar effects of thin liquid films?

We address the air layer shapes, entrapped air volume and wetting underneath droplets impacting on a hard substrate pre-wetted by microscopic oil layers. We control the Weber number, the oil layer thickness (in the range of micrometers), the oil viscosity, and in that way the *softness* of the substrate. Despite the thinness of the film, we observe clear effects of the oil layer on the volume and drainage of the air entrapment, its profile shapes, and the wetting behavior.

[1] K.R. Langley et al., Soft Matter 16, 5702 (2020)

We acknowledge funding by DFG within SPP2171 (HA8467/2-1, 2-2).

Keywords: drop impact; pre-wetted surfaces; air entrapment; high-speed interferometry