The preparation of enamel specimens was identical to the previous experiments.19,20 Briefly, teeth crowns were separated from the roots using Isomet® Low Speed Saw (Buehler, Düsseldorf, Germany). The buccal sites of the specimens were embedded into the resin (Paladur, Heraeus Kulzer GmbH, Hanau, Germany) in two planar parallel molds. The thinner mold (200-μm thick) was removed, while the teeth in the thicker one (7.5-mm thick) were serially abraded under constant tap water cooling using Knuth Rotor machine (LabPol 21, Struers, Copenhagen, Denmark) with silicon carbide paper discs of grain size 18.3, 8, and 5 μm, 60 s each. The embedded enamel blocks were taken out of the molds before being polished for 60 s with 3-μm diamond abrasive on Struers polishing cloth under constant cooling (LaboPol-6, DP-Mol Polishing, DP-Stick HQ, Struers, Copenhagen, Denmark). Between two polishing steps and after the final polishing, all slabs were ultrasonicated for 1 min in tap water and rinsed. All prepared specimens had a flat-ground enamel area with 200-μm cut-off layer. Samples were stored in a mineral solution [, , and NaCl, 21] and underwent further polishing with a 1-μm diamond abrasive (60 s, LaboPol-6, DP-Mol Polishing, DP-Stick HQ, Struers) immediately before the experiment. Part of the exposed enamel area was covered with light curing fixation adhesive tape (Technovit, Heraeus Kulzer GmbH, Germany) to preserve the original enamel surface from the erosive treatment [reference nontreated area, Fig. 1(a)].