We consider a Hamiltonian for a complex fluid. The linear excess bulk viscosity due to a spatially varying order parameter is investigated. We calculate the dependence of the excess viscosity Dgr;eegr;, on the fundamental parametersdand xgr;, everywhere within the disordered phase (dbeing the domain size of water or oil and xgr; being the correlation length between domains). It is found that there is a crossover region in the isotropic disordered phase of this model. More specifically, in the structured region of the isotropic disordered phase Dgr;eegr; grows as xgr;3in the vicinity of disordered‐lamellar transition. This is quite distinct from the behavior of the excess viscosity in the unstructured region where Dgr;eegr; grows as xgr; in the vicinity of the order‐disorder transition.
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