Effects of surface wettability and density change during solidification of paraffin phase change materials

Phase change materials (PCMs) are widely used for thermal energy storage and other applications. During the solidification of a PCM, a volume change occurs due to thermal expansion and structural change of the molecules. A concave shape may be created during the solidification shrinkage. This paper investigates the shrinkage profile formed during the solidification of paraffin wax and its relationship with the surface wettability (contact angle), considering the effect of density changes. The contact angle of a paraffin droplet on a surface indicates the extent of adhesion between the solid surface and paraffin. The experimental study examines a mixture of wax with nanoparticles, wax with surfactant, different coatings on the container surface, and different solidification temperatures. Enhanced adhesion and increased shrinkage height are observed due to the non-polar interaction between coated surfaces and paraffin, facilitated by induced dipole-induced dipole forces. The addition of 3% octadecylamine enhances adhesion between paraffin and the container, as driven by dipole–dipole forces between the hydroxyl group on epoxy resin-container surfaces and the hydrophilic amine group of octadecylamine. The shrinkage height increases with a decrease in the contact angle of paraffin on the surface. The shrinkage profile also changes with the solidification temperature. The highest shrinkage profile is achieved for the highest solidification temperature.