Moving wetting ridges on ultrasoft gels

Area of Science:

• Soft matter physics
• Surface science
• Polymer science

Background:

• Wetting phenomena on soft solids are crucial for natural and technological applications.
• Soft polymer gels serve as a model system for studying static and dynamic wetting.
• Existing theoretical models for wetting ridges on gels lack consensus and require experimental validation.

Purpose of the Study:

• To experimentally investigate the shapes of moving wetting ridges on ultrasoft polymer gels with high spatiotemporal resolution.
• To compare experimental findings with predictions from linear viscoelastocapillary theory.
• To evaluate the validity of different theoretical models, including elastocapillary scaling and capillary extraction.

Main Methods:

• High-resolution measurements of wetting ridge shapes on polydimethylsiloxane (PDMS) gels with varying stiffness (approx. 100 Pa).
• Experiments conducted with different wetting phases: water, FC-70, and air.
• Comparison of experimental data with asymptotic behaviors of linear viscoelastocapillary theory.

Main Results:

• Commonly used elastocapillary scaling does not accurately collapse wetting ridge shapes.
• Elastocapillary scaling provides a viable prediction for dynamic ridge angles under small normal forces.
• Neither debated theoretical model fully explains the observed phenomena quantitatively.
• Capillary extraction of an oil skirt emerges as the most plausible mechanism.

Conclusions:

• The study highlights limitations in current theoretical descriptions of dynamic wetting on soft gels.
• Experimental evidence suggests capillary extraction is a dominant factor in wetting ridge formation.
• Further theoretical development is needed to fully capture the complex surface mechanics of soft solids during wetting.