We report the experimental discovery of "electrorheological (ER) complex plasmas," where the control of the interparticle interaction by an externally applied electric field is due to distortion of the Debye spheres that surround microparticles (dust) in a plasma. We show that interactions in ER plasmas under weak ac fields are mathematically equivalent to those in conventional ER fluids. Microgravity experiments, as well as molecular dynamics simulations, show a phase transition from an isotropic to an anisotropic (string) plasma state as the electric field is increased.

By employing a self-consistent kinetic approach, an analytical expression is derived for the potential of a test charge in a weakly ionized plasma with ion drift. The drift is assumed to be due to an external electric field, with the velocity being mobility-limited and much larger than the thermal velocity of neutrals. The derived expression is proven to be in excellent agreement with the measurements by Konopka et al. ͓Phys. Rev. Lett. 84, 891 ͑2000͔͒ performed in the sheath region of a rf discharge.

Investigations of shear flows in three-dimensional complex-plasma fluids produced in a dc discharge were carried out. The shear was induced either by an inhomogeneous gas flow or by a laser beam. The viscosity of complex plasmas was measured over a broad range of shear rates, up to the hydrodynamic limit when the discreteness becomes important. Analysis of the measurements reveals non-Newtonian behavior of complex plasmas accompanied by substantial shear thinning.