The role of water is often overlooked in interfacial phenomena, but its presence influences many interfacial processes relevant to a number of scientific disciplines. Direct force measurements have offered the most insight into underwater surface related phenomena such as adhesion, wetting, and friction, and have provided molecular descriptions to the physical interactions taking place at the contact interface. Although the insight from these experiments maybe true, there lacks direct molecular confirmation of the assertions interpreted from these force measurements. To address this, we have studied the impact of water on adhesion, friction, and wetting by using a suite of complementary surface sensitive techniques. By using non-linear sum frequency generation spectroscopy, we are able to probe surfaces at a molecular level and connect these chemical details to better understand interfacial phenomena. This thesis focuses on three studies to better understand the role of water in interfacial phenomena.

For the first study, we focused on the contact of two hydrophobic surfaces in water. Here, we use surface sensitive sum frequency generation spectroscopy to directly probe the contact interface between hydrophobic poly-(dimethylsiloxane) (PDMS) and two hydrophobic surfaces (a self-assembled monolayer, OTS, and a polymer coating, PVNODC). We show that the interfacial structure for OTS and PVNODC are identical in dry contact but that they differ dramatically in wet contact. In water, the PVNODC surface partially rearranges at grain boundaries, trapping water at the contact interface leading to a 50% reduction in adhesion energy compared to OTS-PDMS contact. The Young-Dupre equation, used extensively to calculate the thermodynamic work of adhesion, predicts no differences between the adhesion energy for these two hydrophobic surfaces, indicating a failure of this well-known equation when there is a heterogeneous contact.

For the second study, we studied the role of water between two hydrophilic interfaces in sliding friction and adhesion. We achieve this by adsorbing cationic surfactant, cetyl trimethyl ammonium bromide (CTAB), on two hydrophobic surfaces, PDMS and a self assembled monolayer. Using surface sensitive sum frequency generation spectroscopy, we highlight a strongly coordinated ice-like water layer confined between two surfactant covered hydrophobic surfaces under hydration pressures. Such strongly coordinated water structure, that reduces the sliding friction, forms past the surfactant concentration needed for monolayer coverage. In addition, the surprising observance of a highly coordinated ice like layer of water between these two surfaces presents opportunities for developing a theoretical framework to the molecular behavior of confined water.

For the third study, we investigate underwater adhesion and friction for a hydrophobic lens in contact with plasma treated surfaces of different wettability. From these measurements, we aimed to guide the interfacial design of underwater adhesives by systematically varying the surface energy of the contact interface. We show underwater adhesion and friction is the highest between a hydrophobic lens and plasma treated surface for a water contact angle of 70 degrees.