Dean flow focusing and separation of small microspheres within a narrow size range

Rapid, selective particle separation and concentration within the bacterial size range (1–3 lm) in clinical or environmental samples promises significant improvements in detection of pathogenic microorganisms in areas including diagnostics and bio-defence. It has been proposed that microfluidic Dean flow-based separation might offer simple, efficient sample clean-up: separation of larger, bioassay contaminants to prepare bioassay targets including spores, viruses and proteins. However, reports are limited to focusing spherical particles with diameters of 5 lm or above. To evaluate Dean flow separation for (1–3 lm) range samples, we employ a 20 lm width and depth, spiral microchannel. We demonstrate focusing, separation and concentration of particles with closely spaced diameters of 2.1 and 3.2 lm, significantly smaller than previously reported as separated in Dean flow devices. The smallest target, represented by 1.0 lm particles, is not focused due to the high pressures associated with focussing particles of this size; however, it is cleaned of 93 % of 3.2 lm and 87 % of 2.1 lm microparticles. Concentration increases approaching 3.5 times, close to the maximum, were obtained for 3.2 lm particles at a flow rate of 10 ll min. Increasing concentration degraded separation, commencing at significantly lower concentrations than previously predicted, particularly for particles on the limit of being focused. It was demonstrated that flow separation specificity can be fine-tuned by adjustment of output pressure differentials, improving separation of closely spaced particle sizes. We conclude that Dean flow separation techniques can be effectively applied to sample clean-up within this significant microorganism size range.