Sorting via injection of particle streams into an optical lattice

The growth of research into microfluidics, especially towards micro-Total Analysis Systems (μTAS), is leading to a demand for highly efficient and accurate methods for analyte delivery, sorting, mixing and analysis. Optical techniques, due to their non-invasive, non-contact properties are ideally suited to integration in to microfluidic systems. One of the key abilities in a μTAS device is the ability to sort microscopic matter. When done optically this typically involves fluorescence detection, management of the information detected and subsequent action such as the actuation of an electric field or electro-mechanical valve. We present here a method whereby the detection of a micro-particle's properties is done passively, with simultaneous separation of those particles. To do this particle streams are injected into a three-dimensional crystal-like lattice of optical intensity maxima. A particle's response to the three-dimensional optical potential landscape formed by the lattice depends on its polarisability. This leads to a sensitivity to size, refractive index and shape. More strongly interacting particles are deflected away from the main flow whilst those that interact weakly are washed straight through the lattice without little or no net deflection. We present analysis of both injection and subsequent re-routing/sorting of particle streams, using body-centred tetragonal and three-dimensional "log-pile" optical lattices to separate both inert colloid and blood cells by refractive index or size. Sorting with an efficiency as high as 96% has been achieved with particle deflections in excess of 45 degrees.