# Dispersion of OH Radicals in Applications Related to Fear-Free Dentistry Using Cold Plasma
## 3 Results and Discussion  
During CAP discharge towards a treated tooth, the OH radicals were transported through diffusion and convection from the outlet of the nozzle towards the surface of the tooth. The OH  
Figure 1: Modeled mandibular jaw with teeth under cold atmospheric plasma (CAP) discharge. Figure 2: Schematic diagram showing teeth 1 to 6 examined in the present study. radicals were distributed over the tooth, which led to a concentration gradient. The spatial variations of the concentration of the OH radicals over teeth 1 to 6 (see Figure 2) for the nozzle to tooth distance of 2 mm were obtained and shown in Figure 3. These snapshots were captured at \(t=10\) ms (i.e., the final timestep). Variations of OH concentrations over the six different exposed teeth for nozzle to tooth distance of 4 mm are shown in Figure 4. These snapshots were captured at \(t=10\) ms (i.e., the final timestep). To allow easier comparisons, orientations of the teeth were kept the same for both of the nozzle to tooth distances of 2 and 4 mm. Figures 3 and 4 showed that the OH concentration decreased, i.e., fewer OH radicals interacted with the tooth, as the nozzle to tooth distance increased. For longer nozzle to tooth distances, the area on a treated tooth exposed to OH radicals became smaller. Figure 5 also shows that fewer OH radicals could reach the treated tooth for longer nozzle to tooth distances if the speed and time duration were kept constant. Similarly, the variation of H\({}_{2}\)O\({}_{2}\) species over the six different exposed teeth for nozzle to tooth distances of 2 and 4 mm are shown in Figures 5 and 6, respectively. The concentrations of H\({}_{2}\)O\({}_{2}\) over the teeth decreased with the increasing nozzle to tooth distance, which was explained by the smaller amount of H\({}_{2}\)O\({}_{2}\) covering the surfaces of exposed teeth for larger distances. These snapshots were also captured at \(t=10\) ms (i.e., the final timestep). The normalized concentration of OH radicals over the tooth started to increase with time upon contact between the CAP plume carrying OH radicals and the tooth. The temporal variations of the normalized concentration of OH radicals averaged over the tooth varied for different teeth, due to their distinct geometries. For the nozzle to tooth distances of 2 and 4 mm, the concentrations of OH radicals started to increase for \(t>\) -1 ms (see Figure 7a) and for \(t>\) -2 ms (see Figure 7b), respectively. The OH radicals needed these onset time durations to travel from the nozzle to the tooth. The temporal variations of H\({}_{2}\)O\({}_{2}\) species were similar to those of OH radicals. The H\({}_{2}\)O\({}_{2}\) species were distributed over the surface of the exposed tooth and the normalized concentrations increased with time. The OH radicals had a comparatively larger diffusion coefficient compared to the H\({}_{2}\)O\({}_{2}\) species [51; 52], hence, the temporal variations of normalized concentrations of OH radicals were in general larger than those of the H\({}_{2}\)O\({}_{2}\) species. The two main mechanisms for the dispersion of OH species (such as OH and H\({}_{2}\)O\({}_{2}\)) on different types of teeth were diffusion and convection, both of which were included in  
Figure 3: 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 2 mm. (Note: the scale bar shows the normalized concentration of the OH radicals). Equation (2). More specifically, the reactive species diffused away from the plasma discharge through diffusion, and they were also carried to different types of teeth by the plasma carrier gas through convection. Figure 7a,b shows the temporal variation of normalized concentrations of OH and H\({}_{2}\)O\({}_{2}\) species, respectively, which were averaged over a treated tooth surface. These results quantitatively described the dispersion of reactive species over the surface of the tooth under treatment, which was in turn related to the surface morphology of the tooth.