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The authors present a numerical study of chaotic mixing of super-paramagnetic microparticles into a biofluid in order to evaluate the efficiency of the mixer in labelling the target biological cells with particles. Lagrangian trajectories of the particles and bio-cells in the mixing domain were obtained and percentage of the cells being labelled in the mixing process was used as the primary index to quantitatively characterize the system. Moreover, largest Lyapunov exponent was computed as a supplemental index in order to examine the incident of chaos in the particles' motion in the mixer, and investigate the relationship between chaotic strength and labelling of cells. Both indices were calculated for a range of driving parameters and were shown to be highly dependent on the condition of the system. It was found that while in some ranges of operating conditions both indices were in good agreement, there are some ranges where they appeared to yield contradicting results. It is found that the optimum operating conditions of the system is obtained when the Strouhal number is about 0.4, which corresponds to the efficiency of about 70%.