A spherical water droplet with radii R$1=1$ mm and R$2=2$ mm is placed at the two ends of a circular channel to produce a pressure$-$driven flow through the channel. The diameter and length of the circular channel are D$=100\backslash$mu m and L$=3$ cm$,$ respectively, and the surface tension of the water is $72$ mN$/$m$.$What is the pressure difference between the two ends A and B of the circular channel?