Dependency of contact angle hysteresis on crystallinity for n-alkane substrates

Changes in dynamic contact angle for the wetting of heat-treated paraffin wax surfaces were reported. It was observed that the advancing angle remains relatively stable while the receding angle decreases with greater treatment. These changes appear to result primarily from a reduction in crystallinity, which was gauged via differential scanning calorimetry and X-ray diffraction. Surface roughness is also found to decrease with heat treatment, but its role in the reported phenomenon appears to be minimal. This is demonstrated by normalizing the roughness of coated plates with cold pressing. Along with global changes in crystallinity, modifications to the local crystallinity were observed using Raman confocal microscopy. Specifically, it is shown that the heat treatment increased the heterogeneities of the wax surfaces by producing micrometer-scale amorphous regions. These regions are believed to act as hydrophilic defects on the wetted surface, impacting almost exclusively receding angle measurements. The findings raise concerns over the use of semicrystalline oligomeric and polymeric materials as model substrates in wetting experiments. Results also identify a possible method for estimating the order of packing of straight-chain hydrocarbon derivatives at surfaces using dynamic contact angle measurements.