Twenty sectioned enamel blocks, with 2 × 2 mm2 buccal enamel surface, were obtained from ten sound extracted primary incisors. The incisors were stored in distilled water at 4 °C prior to use for no longer than one week. Each incisor was sliced longitudinally parallel to its long axis to get two enamel blocks by a cutting machine under running water. All enamel blocks were embedded in light-cured resin (Filtek Z250, 3M ESPE) onto an acrylic plate. The buccal surfaces were faced upwards to make sure most regions of the surface are parallel to the plate.
After embedding the enamel into the resin, the buccal surfaces of the enamel were polished using 1000- and 2000-grit SiC abrasive paper by a manually water-cooled low-speed polishing machine to obtain a flat surface without dentine exposure, and the specimen underwent ultrasonic washing in deionised water before and after polishing. Then, the twenty blocks were randomly divided into five groups (four experimental groups GA, GB, GC, GD, and one control group GE; seeTable 1 ), i.e., four blocks in one group, to immerse into the oral liquids or deionised water.
After embedding the enamel into the resin, the buccal surfaces of the enamel were polished using 1000- and 2000-grit SiC abrasive paper by a manually water-cooled low-speed polishing machine to obtain a flat surface without dentine exposure, and the specimen underwent ultrasonic washing in deionised water before and after polishing. Then, the twenty blocks were randomly divided into five groups (four experimental groups GA, GB, GC, GD, and one control group GE; see