Abstract: A nonlinear diffusion theory of two-phase mixture fluids has been discussed under the constitutive principles of continuum mechanics, where one phase is micropolar fluid, another is classical fluid. Both the two components are supposed to be rate-type simple material with second-order complexity for the translation and first-order for the micro-rotation. The general form of constitutive equations for the mixture motion is derived. As special cases, the second-order nonlinear constitutive equations for each ...