  Making use of the NJL model and the multiple reflection expansion
pproximation, we study the phase transition of the finite size droplet with u
and d quarks. We find that the dynamical masses of u, d quarks are different,
and the chiral symmetry can be restored at different critical radii for u, d
quark. It rovides a clue to understand the effective nucleon mass splitting in
nuclear matter. Meanwhile, it shows that the maximal isospin chemical potential
at zero temperature is much smaller than the mass of pion in free space.
