In plastic industry, a large variety of polymeric materials are obtained by liquid-liquid mixing. Recently, a new type of mixer (later called RMX)-based on a convergent/divergent unit which favors elongational flow-was developed with improved efficiency in polymer blending compared to traditional shear flow-based devices such as internal mixers or extruders. The present study aims at identifying the possible origins of the higher dispersive and distributive mixing capability of this device. An experimental study, including flow visualization, and numerical simulations of creeping flows induced by this original device was carried out for Newtonian, shear-thinning and viscoelastic fluids. The predictedmixing capability is based on length stretch and Lyapunov exponent methods. Calculation results show a good agreement with experiments and highlight some fundamental mixing mechanisms, such as striation, for viscoelatic fluids. (C) 2013 Society of Plastics Engineers