Dynamic mechanical properties and low-velocity wetting behavior of plasticcrystalline states for n-alkane blends

Data presented here demonstrate a correlation between viscoelastic properti es and wetting behavior for an n-alkane, binary mixture, and several macroc rystalline paraffin waxes. Mesophases in the premelting region of n-alkane systems were identified using differential scanning calorimetry, and their level of viscoelasticity was characterized via dynamic mechanical spectrosc opy (DMS). Thermal analysis indicates that transitions between crystalline, plastic crystalline, and the isotropic phases for these systems coincide w ith peaks in loss tangent values measured using DMS. The combination of a b road range of viscoelastic properties and a relatively constant equilibrium contact angle over the premelting region provides a unique opportunity to study the relationship between substrate mechanical properties and wetting behavior. Dynamic contact angle measurements were performed for water on th ese surfaces using low substrate velocity (2-264,mum/s) Wilhelmy plate tens iometry. Advancing dynamic contact angles were found to have a velocity dep endence that was greatly enhanced far removed from phase transitions, while equilibrium values were observed for receding angles at all substrate velo cities and temperatures. Correlations were identified between loss tangent values for the substrate, phase transitions, and the magnitude and relaxati on kinetics of advancing dynamic contact angles. Results clearly demonstrat e that differences exist between wetting and dewetting mechanisms for water on n-alkane substrates. Advancing angle data could not be fit with models attributing contact angle behavior to substrate deformation, adsorption/des orption rates at the three-phase line, or hydrodynamic considerations. We s peculate that observed results are associated with enhanced molecular freed om, which can be gauged by the mechanical loss tangent. Findings from this study may provide insight to anomalous wetting behavior reported for other low-energy substrates.