# Dispersion of OH Radicals in Applications Related to Fear-Free Dentistry Using Cold Plasma
## 3 Results and Discussion
For example, the results for tooth 6 (canine tooth) were significantly different from other molar and premolar teeth, which was attributed to the much sharper tip and smaller size of tooth 6. Figure 4: Spatial distribution of OH radicals over tooth (**a**) 1, (**b**) 2, (**c**) 3, (**d**) 4, (**e**) 5 and (**f**) 6 for nozzle to tooth distance of 4 mm. (Note: the scale bar shows the normalized concentration of the OH radicals). Figure 5: Spatial distribution of H\({}_{2}\)O\({}_{2}\) species over tooth (**a**) 1, (**b**) 2, (**c**) 3, (**d**) 4, (**e**) 5 and (**f**) 6 for nozzle to tooth distance of 2 mm. (Note: the scale bar shows the normalized concentration of the H\({}_{2}\)O\({}_{2}\) species). To ascertain the steady-state in the studied system, the speed at 1 mm below the center of the nozzle was separately determined for all six teeth. The temporal variations of speeds at 1 mm below the center of the nozzle are shown in Figure 8a,b for 2 and 4 mm nozzle to tooth distances, respectively. The speed started to increase with time up to 1 ms, and became constant for the longer exposure time, which demonstrated the steady-state in the studied system. Moreover, the determined speeds varied for different treated teeth. Following the contact between the carrier gas and the tooth, the flow would get distorted and a velocity field would be generated (i.e., eddies would be formed) around the tooth. The distribution of this velocity field in three-dimensional (3D) space depended on the geometry of the treated tooth (or in other words how the flow was distorted), and led to variations in the speed (the magnitude of velocity in 3D space) at a point from the tooth. Figure 6: Spatial distribution of H\({}_{2}\)O\({}_{2}\) species over tooth (**a**) 1, (**b**) 2, (**c**) 3, (**d**) 4, (**e**) 5 and (**f**) 6 for nozzle to tooth distance of 4 mm. (Note: the scale bar shows the normalized concentration of the H\({}_{2}\)O\({}_{2}\) species). Figure 7: Temporal variations of normalized concentration of OH radicals and H\({}_{2}\)O\({}_{2}\) species averaged over a treated tooth for nozzle to tooth distance of (**a**) 2 and (**b**) 4 mm. Figure 9a,b shows variations of normalized concentrations of OH radicals at 1 mm below the center of the nozzle for all six teeth at nozzle to tooth distances of 2 and 4 mm, respectively. The normalized concentrations started to increase rapidly up to about 1 in \(\sim\)1 ms, beyond which the steady-state condition was achieved. Small variations for different teeth were noticed, which were attributed to the distortion of the flow and variations in the velocity field around different teeth under CAP treatment. Such variations were governed by the convection of the OH radicals, since the velocity field determined the dispersion and distribution of these species, which was mathematically shown in Equation (2), where the velocity field \(u\) was present. The results in Figure 9 imply that the movement of the OH radicals reached the steady-state as well. Figure 10 shows the normalized concentrations of the OH radicals around the four sides at 1 mm below the nozzle, which were obtained for the nozzle to tooth distance of 2 mm. The results revealed anisotropy of dispersion and distribution of OH radicals underneath the nozzle and variations of the dispersion for different teeth. Interestingly, the normalized concentrations away from the center of the nozzle were lower than 1, which meant that most discharged OH radicals were concentrated at the center of the CAP plume. This feature was particularly useful considering that very narrow regions in teeth were often treated in dentistry. For \(t<\)1 ms, the normalized concentrations significantly varied  
Figure 8: Temporal variations of speed at 1 mm below the center of the nozzle for nozzle to tooth distance of (**a**) 2 and (**b**) 4 mm. Figure 2:0 Variations of normalized concentration of OH radicals at 1 mm below the center of the nozzle for nozzle to tooth distances of (**a**) 2 and (**b**) 4 mm. with time.