Understanding Elastomeric Contact Interfaces in the Presence of Water

The traction of tires in the rain or sticking a bandage on wet skin involves understanding the role of interfacial water in adhesion and friction. In most cases, the contact interfaces are not completely dry or wet and instead show patchy contact. Even for smooth surfaces, the contact can be patchy due to difficulty in draining the water upon contact. In this work, we have studied the contact interface between hydrophobic PDMS and hydrophilic sapphire substrate in dry and wet conditions using infrared-visible sum-frequency generation (SFG) spectroscopy and macroscopic Johnson-Kendall-Roberts (JKR) adhesion measurements. SFG spectroscopy provides information on interfacial water and shows direct spectral peaks associated with the surface -OH groups in contact with either water or PDMS elastomers. This molecular information can be directly correlated with the macroscopic work of adhesion, adhesion hysteresis, and friction. We will discuss how the presence of interfacial water is affected by normal load and velocity.