Microfluidic sorting with a moving array of optical traps

Abstract

Optical sorting was demonstrated by selective trapping of a set of microspheres (having specific size or composition) from a flowing mixture and guiding these in the desired direction by a moving array of optical traps. The approach exploits the fact that whereas the fluid drag force varies linearly with particle size, the optical gradient force has a more complex dependence on the particle size and also on its optical properties. Therefore, the ratio of these two forces is unique for different types of flowing particles. Selective trapping of a particular type of particles can thus be achieved by ensuring that the ratio between fluid drag and optical gradient force on these is below unity whereas for others it exceeds unity. Thereafter, the trapped particles can be sorted using a motion of the trapping sites towards the output. Because in this method the trapping force seen by the selected fraction of particles can be suitably higher than the fluid drag force, the particles can be captured and sorted from a fast fluid flow (about 150 μm/s). Therefore, even when using a dilute particle suspension, where the colloidal trafficking issues are naturally minimized, due to high flow rate a good throughput (about 30 particles/s) can be obtained. Experiments were performed to demonstrate sorting between silica spheres of different sizes (2, 3, and 5 μm) and between 3 μm size silica and polystyrene spheres.

Escape velocity sorting in optical tweezers system using a home-made piezo mirror

“…Sorting micro-particles with different sizes and different refractive indices are investigated with 95% efficiency, which is better than the reported efficiencies in [8,20]. Further, the proposed method efficiency is comparable with the reported efficiencies in [6,18,19,27]. However, there are some methods with higher reported efficiencies [7,17] which need more complicated optical setups or procedures in the laboratory.…”

“…In that case, the optical force is no longer able to hold the particle and the particle escapes from the trap. The maximum force applied by the trap can be determined as ph = F r V 6 coefficient η according to the Faxenʼs law [18] should be considered:…”

“…Moreover, in optically passive methods, the intrinsic physical parameters of the samples, such as size, refractive index, and resonance wavelength, are commonly used as critical parameters in particle sorting [7,[15][16][17][18][19][20]. Most of these methods utilize optical traps like optical lattice [7,18,21] or acousto-optics assisted methods [19], which could be applied to sort high population particles.…”

Latest achievements in generalized Lorenz‐Mie theories: A commented reference database

“…, who carried out an experimental demonstration of optical transport, sorting and self‐arrangement using a 'tractor' beam (a beam in which objects are pulled from a far distance towards a light source in the absence of axial optical gradient forces), and also by Dasgupta et al . who demonstrated optical sorting by selective trapping of a set of microspheres (having specific size or composition) from a flowing mixture and guiding these in the desired direction by a moving array of optical traps. The approach exploited the fact that whereas the fluid drag force varies linearly with particle size, the optical gradient force has a more complex dependence on the particle size and also on its optical properties.…”

Optical sorting of nonspherical and living microobjects in moving interference structures

“…Some of these methods can be even applied in a static fluid. The sorting is then based on the size dependent radiation pressure applied in opposite directions [34,35], the principle of space-time symmetry breaking in optical ratchets [36][37][38][39][40][41][42][43] or the travelling interference pattern, usually periodic, that drags particles of different sizes or refractive indices with different strength [44][45][46][47][48]. The latter method is based on the nonlinear mobility of particles when these are forced to move in a sinusoidal energy potential [49].…”