Substrate and Drop Dynamics During Impact and Coalescence on Soft Adaptive Surfaces

• Applicant: Dr. Kirsten Harth
• Subject Area: Statistical Physics, Nonlinear Dynamics, Complex Systems, Soft and Fluid Matter, Biological Physics ; Experimental Condensed Matter Physics
• Term: since 2019
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 422792849

Droplets impacting on soft materials are frequently encountered ineveryday life, e.g. water drops on skin during showering, dropsimpacting on fresh paint or on plant leaves, or simply drops impacting onto a previously wetted surface. Such systems often consist of a (thin) soft and adaptive surface layer on a stiff substrate. The soft layer, however thin, can crucially alter the behavior of contact lines, and particularly of droplets, on microscopic and macroscopic scales. Contrasting to this, research so farmainly focused on hard surfaces, liquid pools and miscible liquid surface layers (of usually non-microscopic depth). The coupled problem of static shapes of a droplet on visco-elastic substrates has been modeled extensively, and comparatively slow processes of contact line motion have been addressed. Recent research develops more towards studies of the interaction ofsoft substrates with rapidly moving contact lines, as e.g. encountered during drop impact. However, in those studies, the dynamics of the substrate, while it is the actual reason for all changes of the macroscopic contact line and droplet behavior, the actual substrate dynamics is not recorded and not analyzed – measuring the microscopic deformations of thin surface layers during rapid processes requires substantial experimental effort and sophisticated, new techniques. My project deals with two exemplary processes, drop impact on soft flexible layers and drop coalescence on fluid layers of limited depth. The substrates may consist of viscous, or viscoelastic liquids, or of viscoelastic gels. Standard side and top / bottom view high-speed imaging will be combined with a number of non-standard high-speed bottom view techniques,providing access to the substrate's deformations, strains or contact formation processes of the surface with the liquid, as well as possibly entrained microscopic gas layers. My setup and expertise will beavailable to the other participating groups of SPP-2171 throughcollaboration, currently planned for several adaptive substrates. I willcollaborate with theoreticians on analytical and numerical modeling ofmy experiments.

• DFG Programme: Priority Programmes
• Subproject of: SPP 2171: Dynamic wetting of flexible, adaptive and switchable surfaces