Conventional level set-based approaches have an inherent difficulty in tracking miscible fluids due to its discrete treatment for interface. This paper proposes a unified framework to efficiently handle both miscible and immiscible fluid simulations. Based on the chemical potential energy, our method describes the evolution of multiple fluids as time-varying concentration fields. Handling of multiple fluids is straightforward and, unlike level set methods, ad hoc reinitialization or fictitious particle deployment is not necessary. For numerical computation of the Navier-Stokes equations, we adopt advanced lattice Boltzmann methods (LBMs) for computational efficiency. The experiments show that our approach works well with immiscible fluids, miscible fluids, and interaction with objects