Squeezing Drops: Force Measurements of the Cassie-to-Wenzel Transition

Area of Science:

• Materials Science
• Surface Science
• Fluid Dynamics

Background:

• Superhydrophobic surfaces exhibit remarkable liquid repellency and self-cleaning properties, crucial for applications.
• These properties depend on the metastable Cassie state, which can transition to the Wenzel state, compromising functionality.
• Maintaining the Cassie state is vital for the practical use of superhydrophobic materials.

Purpose of the Study:

• To experimentally investigate the Cassie-to-Wenzel transition on superhydrophobic micropillar surfaces.
• To quantify the forces and pressures involved in this transition.
• To compare experimental findings with existing theoretical predictions.

Main Methods:

• Utilizing a transparent superhydrophobic force probe to squeeze water drops on micropillar surfaces.
• Employing top-view optics to monitor the drop's contact area during compression.
• Measuring the force exerted by the drop and comparing it with calculated capillary pressure.

Main Results:

• The Cassie-to-Wenzel transition was identified by a sharp force decrease and abrupt contact area change.
• Experimental forces closely matched calculated capillary pressure forces.
• Measured impalement pressures were consistently lower than three literature predictions.

Conclusions:

• The study provides experimental data on the Cassie-to-Wenzel transition dynamics.
• Observed lower impalement pressures suggest potential mechanisms like microdroplet coalescence influencing the transition.
• Findings are critical for designing stable superhydrophobic surfaces for industrial applications.