Spreading on viscoelastic solids: are contact angles selected by Neumann's law?

Area of Science:

• Soft Matter Physics
• Fluid Dynamics
• Surface Science

Background:

• Liquid drop spreading on viscoelastic substrates is hindered by significant energy dissipation within the solid.
• Understanding the dynamics of moving wetting ridges is crucial but challenging due to difficulties in quantifying underlying viscoelastic deformations.
• Previous hypotheses suggested a link between wetting ridge rotation and dynamic liquid contact angle.

Purpose of the Study:

• To experimentally visualize and quantify the dynamic wetting ridge during liquid drop spreading on soft substrates.
• To investigate the validity of Neumann's law for moving contact lines on viscoelastic materials.
• To develop a new theoretical framework accounting for surface strain effects, including the Shuttleworth effect, in soft wetting dynamics.

Main Methods:

• Direct experimental visualization of the dynamic wetting ridge using shadowgraphic imaging.
• Simultaneous measurement of the dynamic liquid contact angle.
• Development of a new theoretical model incorporating surface strain and boundary conditions at the contact line.

Main Results:

• The wetting ridge rotation was observed to precisely follow the dynamic liquid contact angle, confirming prior hypotheses.
• Experimental evidence supports the continued applicability of Neumann's law despite contact line motion.
• Moving contact lines were found to induce variable surface tension in the substrate, necessitating a revised theoretical approach.

Conclusions:

• Neumann's law remains valid for dynamic wetting ridges on soft substrates, even with contact line movement.
• The study introduces a novel dynamical theory for soft wetting that incorporates surface strain and the Shuttleworth effect.
• The findings provide a more comprehensive understanding of liquid-substrate interactions in soft matter systems.