As shown in Fig. (a), the VG variation gradient is created as A approaches B, where B is still. As shown in Fig. (a), A's VG decreases with the decrease of L. Since there is only one particle, 𝑉 = 𝑁 𝜋𝐿 . Here, it should be noted that the unit of L is dm because the unit of VG is L/mol. Therefore,