Sorting algal cells by morphology in spiral microchannels using inertial microfluidics

Sub-millimetre phytoplankton (here referred to as algae) exist in a wide variety of shapes and sizes. Measuring algae morphology can be a useful tool for understanding the species dynamics in a body of water, and size-sorting in general is a valuable first step in automated species identification. Here, we demonstrate the sorting of algae by shape and size in a spiral microchannel, in which lift forces and Dean flow drag forces combine to position the cells in a shape-dependent location in the channel cross section. Three species were used for experiments: the high-aspect-ratio cylindrical Monoraphidium griffithii, the prolate spheroidal Cyanothece aeruginosa, and the small spherical Chlorella vulgaris. These results are compared with the sorting of similarly sized polystyrene latex microspheres in the same device over the same range of flow rates. Tests were done at conditions which yielded average Dean numbers over the channel length of 3 < De < 30. At 1.6 mL/min, the 10-and 20-mu m microspheres could be separated with an efficiency of 96 %. The best sorting results for the algae were obtained at a flow rate of 3.2 mL/ min, which yielded an average Dean number of De = 25 over the channel length. These conditions led to the separation of the Monoraphidium from the differently shaped Cyanothece; these two species could be sorted with a 77 % separation efficiency despite the relatively high polydispersity in cell sizes within each species. The elegance and simplicity of inertial microfluidics make it appropriate for the high-throughput pre-sorting of algae cells upstream of other integrated sensing modalities in a field-deployable device.