Contact Angles and Hysteresis on Soft Surfaces

Area of Science:

• Materials Science
• Surface Science
• Polymer Science

Background:

• Wetting phenomena are influenced by substrate properties.
• Understanding contact angle hysteresis is crucial for applications involving liquid-substrate interactions.
• Soft materials exhibit unique wetting behaviors due to their deformability.

Purpose of the Study:

• To investigate the relationship between the mechanical properties of soft substrates and contact angle hysteresis.
• To quantify how changes in substrate modulus affect wetting behavior.
• To determine the threshold modulus for significant deformation-induced contact angle variation.

Main Methods:

• Wetting experiments using water and ethylene glycol on elastomeric substrates.
• Varying substrate stiffness by peroxide crosslinking of natural and butadiene rubber.
• Characterization of surface chemistry using attenuated total reflecting infrared spectroscopy and X-ray photoelectron spectroscopy.
• Surface roughness analysis via atomic force microscopy.

Main Results:

• Contact angle hysteresis varied significantly with the bulk tensile modulus (E) of the elastomers.
• Soft, uncrosslinked butadiene rubber showed high hysteresis (69°) due to visible ridge formation.
• Increased crosslinking (e.g., 2% peroxide) led to a ~50-fold increase in modulus and reduced hysteresis to 22°.
• Deformation effects on contact angles were prominent when E < 5 MPa.

Conclusions:

• The mechanical properties, specifically the bulk tensile modulus, are critical determinants of contact angle hysteresis on soft substrates.
• Deformation at the contact line, evidenced by ridge formation, plays a key role in hysteresis.
• A modulus below 5 MPa is identified as a threshold for significant deformation-induced variations in contact angles.