Dynamic Solid Surface Tension Causes Droplet Pinning and Depinning

Area of Science:

• Soft matter physics
• Polymer science
• Surface science

Background:

• The contact line of liquid drops on solids generates sharp surface traction, enabling the study of localized deformations in soft polymer networks.
• Stick-slip instability, a phenomenon occurring above a critical velocity, involves periodic depinning of the contact line from its wetting ridge, with its underlying mechanism remaining unclear.

Purpose of the Study:

• To visualize the dynamic wetting ridge during water spreading on PDMS gels.
• To elucidate the mechanism behind the stick-slip instability in soft polymer networks.

Main Methods:

• Direct visualization of the dynamic wetting ridge using water spreading on PDMS (polydimethylsiloxane) gels.
• Time-resolved measurements of solid deformation.
• Experimental confirmation of a derived depinning criterion.

Main Results:

• The opening angle of the wetting ridge was observed to increase with speed.
• This phenomenon was attributed to a dynamic increase in the solid's surface tensions, rather than bulk rheology.
• A novel criterion for depinning was derived and experimentally validated.

Conclusions:

• The study reveals a connection between stick-slip processes and previously unidentified dynamic surface effects.
• The findings provide a new understanding of the mechanism driving stick-slip instability in soft polymer systems.
• Dynamic surface tension changes play a crucial role in the depinning of liquid contact lines on soft solids.