Tunability of the Casimir interaction has always been a challenging issue, especially in small-scale applications. Different mechanisms have recently been proposed to control the Casimir force between solid substrates through the manipulation of material properties using external stimuli such as temperature, external fields, and optical excitations. We show that the unique properties of ferrofluids (colloidal suspensions of superparamagnetic nanoparticles in a nonmagnetic host liquid) provide robust routes for the control of the Casimir interaction between solid (specifically, gold and glass) substrates separated by a thin ferrofluid layer. The tunability here arises due to the self-assembly of suspended magnetic nanoparticles into short magnetic chains; a reversible effect pertinent to equilibrium phases of semidilute ferrofluids subjected to external static magnetic fields, as observed in simulations and experiments. The chain formation makes the ferrofluid an anisotropic medium in its response to electromagnetic field fluctuations that produce the Casimir interaction between the bounding surfaces. Our calculations based on the Lifshitz formalism show that the resulting Casimir force can suitably be varied in magnitude and sign by adjusting the ambient temperature, strength and direction of an externally applied magnetic field, and volume fraction of the constituent nanoparticles.
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