Interstitial Water Enhances Sliding Friction.

This study examines how surfaces with different water contact angles (wettability) affect dry and underwater adhesion and friction. These studies were conducted by bringing a deformable hydrophobic poly(dimethylsiloxane) lens in contact with surfaces of gradient wettability. On the basis of our adhesion and friction results, we divide the results in three regions. In region I (water contact angles greater than 80°), the dry adhesion is lower than underwater adhesion. In contrast, in region III, (water contact angles less than 50°), the dry adhesion is higher than underwater adhesion. For surfaces with water contact angles between 50 and 80° (region II), the dry and wet adhesion values are comparable. Interestingly, in this region II, the underwater coefficient of friction (COF) values are higher than those in regions I and III. We have used surface-sensitive sum frequency generation (SFG) spectroscopy to probe whether the contact interface in static conditions and during dynamic sliding is dry or wet. The SFG results reveal that the contact is dry in region I. If this dry contact is maintained, the underwater COF follows the trend of adhesion hysteresis in dry conditions (adhesion hysteresis decreases with an increase in water contact angles). In region III, the contact is wet and the underwater COF follows the trend for adhesion hysteresis in wet conditions (adhesion hysteresis increases with an increase in water contact angles). By knowing whether the contact interfaces are dry or wet, we can relate the trends in COF with the trends in adhesion hysteresis. For conditions where the contact interfaces have both dry and wet patches (region II), the COF values are higher than those in completely dry conditions, suggesting that a partially lubricated system can exhibit a higher COF.

Multicomponent reassembly of terpyridine-based materials: quantitative metallomacrocyclic rearrangement.

Mixing of metallocyclic trimers and tetramers in an exact 1 : 1.5 stoichiometry provided new supramolecular triangles in quantitative yields. Characterization of the new hetero-nuclear metallomacrocycles was achieved by (1)H, 2D-COSY, 2D-NOESY, and (13)C NMR spectroscopy, along with ESI and TWIM mass spectrometry. Gradient tandem MS (gMS(2)) provided insight into the stabilities of the binuclear structures.

Consequences of water between two hydrophobic surfaces on adhesion and wetting.

The contact of two hydrophobic surfaces in water is of importance in biology, catalysis, material science, and geology. A tenet of hydrophobic attraction is the release of an ordered water layer, leading to a dry contact between two hydrophobic surfaces. Although the water-free contact has been inferred from numerous experimental and theoretical studies, this has not been directly measured. 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 structures 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-Dupré 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. This study exemplifies the importance of interstitial water in controlling adhesion and wetting.