We investigated the formation of plugs with four combinations of aqueous solution and carrier fluid and found that plugs could be formed reliably, regardless of the viscosity of the fluids. The combinations of fluids were (1) NV-A and NV-CF, (2) V-A and V-CF, (3) NV-A and V-CF, and (4)V-A and NV-CF (abbreviations are defined in Section 2).