Mixing at low Reynolds numbers can be achieved by stretching and folding motions in static mixers. Simple shear flow can achieve a stretching motion that grows approximately linearly with time. Extensional flows, in contrast, are potentially more effective due to exponential stretching of fluid elements in time. The split and recombine (SAR) static mixer employs primarily extensional flow through expansions and contractions that results in the number of lamination layers duplicating through each mixing unit. A SAR mixer was constructed by aligning a series of thin plates to achieve six mixing sections. Shear thinning carbopol solutions, one with and one without a paramagnetic tracer, were pumped through the mixer. Using a magnetic resonance imaging technique, the mixing of the minor component in the major component was evaluated qualitatively and quantitatively. The multi-lamination pattern was established; molecular diffusion of the paramagnetic tracer was observed and compared with an analytical model. (C) 2013 Elsevier Ltd. All rights reserved.