Molecular restructuring kinetics and low-velocity wetting of n-alkane rotator phases by polar liquids

Advancing and receding contact angles were measured on n-alkane surfaces at substrate speeds below 300 μm/s for dimethyl sulfoxide, formamide, ethylene glycol, 2,2′-thiodiethanol, and glycerol. The existence of plastic crystalline or rotator phases prior to the melting transition for n-alkane solids was utilized to provide a range of substrate molecular freedom. Contact angles for liquid-substrate combinations demonstrated velocity dependence for high interface velocities resulting in steady-state hysteresis. For measurements made on a commercial paraffin substrate in transition between the ordered crystalline and R_II rotator phases corresponding to a high mechanical loss tangeent, contact angles converged to apparent equilibrium values at low velocities. However, no such relaxation was observed for the ordered crystalline phase, and thus significant differences in relaxation behavior between tested liquids on the paraffin wax were not found for substrates possessing high and low molecular freedom. Differences in restructuring kinetics between liquids were observed on a C₂₁-C₂₃ binary n-alkane blend in the R_I rotator phase with a moderate level of molecular freedom. The order of relaxation toward equilibrium contact angles was consistent with the order of liquid-phase molecular correlation times estimated from nuclear magnetic resonance spin-lattice relaxation, T₁ inversion recovery measurements. The exception was glycerol, which had the longest correlation time but demonstrated greater convergence of measured advancing and receding contact angles than expected. We speculate that contributions originating from viscous forces due to glycerol's high capillary number may explain this behavior. Results of this study also demonstrate that water provides for anomalous wetting on n-alkane surfaces, a topic that will be addressed in a future publication.