Direct calculation of the critical Casimir force in a binary fluid

We show that critical Casimir effects can be accessed through direct simulation of a model binary fluid passing through the demixing transition. We work in the semi-grand-canonical ensemble, in slab geometry, in which the Casimir force appears as the excess of the generalized pressure, P_{⊥}-nμ. The excesses of the perpendicular pressure, P_{⊥}, and of nμ, are individually of much larger amplitude. A critical pressure anisotropy is observed between forces parallel and perpendicular to the confinement direction, which collapses onto a universal scaling function closely related to that of the critical Casimir force.