First, let's focus on the region where r < 10 μm in Fig. . In this region, as r increases, the magnitude of thermophoretic force decreases from above 220 fN to 5 fN, and thermophoresis is prominently observed. This region approximately corresponds to the depletion zone of particles within the first 30 seconds, as shown in Fig. and. Next, consider the region where 10 μm < r < 20 μm. In this region, Fig. shows a variation in the thermophoretic force, decreasing from 5 fN to around 1 fN as r increases. This 20 μm distance corresponds to the upper bound of the depletion area of PSNPs due to thermophoresis, as depicted in Fig. . Finally, in the region where r > 20 μm, the thermophoretic force falls below 1 fN and thermophoretic behavior becomes insignificant as seen in Fig. to -e. In other words, it can be concluded that a thermophoretic force of at least 1 fN is required to observe thermophoretic behavior.