Particle sorting in a mini step-split-flow thin channel: influence of hydrodynamic shear on transversal migration.

A mini splitterless-split-flow thin fractionation (SPLITT) device has been developed to achieve fast separations of micrometer-sized species. In this device, inlet and outlet steps have replaced the splitters, which are common to conventional SPLITT channels. By elimination of the splitters, it becomes straightforward to reduce channel dimensions while maintaining the classic method of fabrication. Reduced dimension channels allow high axial velocity at relatively low flow rate. These high axial velocities generate an enhancement of inertial lift forces and hydrodynamic shear-induced diffusion. Experiments carried out with particulate and biological species in a mini step-SPLITT channel demonstrate that these hydrodynamic effects yield highly enriched fractions of smaller species from binary mixtures.

Characterization of nonspecific crossover in split-flow thin channel fractionation.

Split-flow thin channel (SPLITT) fractionation is a technique for continuous separation of particles or macromolecules in a fluid stream into fractions according to the lateral migration induced by application of a field perpendicular to the direction of flow. Typical applications have involved isolation of different fractions from a polydisperse sample. Some specialized applications involve the separation of the fraction influenced by the transverse field from the fraction that is not. For example, immunomagnetically labeled biological cells may be separated from nonlabeled cells with the application of a transverse magnetic field gradient. In such cases, it may be critically important to minimize contamination of the labeled cells with nonlabeled cells while at the same time maximizing the throughput. Such contamination is known as nonspecific crossover (NSC) and refers to the real or apparent migration of nonmobile particles or cells across stream lines with the mobile material. The possible mechanisms for NSC are discussed, and experimental results interpreted in terms of shear-induced diffusion (SID) caused by viscous interactions between particles in a sheared flow. It is concluded that SID may contribute to NSC, but that further experiments and mathematical modeling are necessary to more fully explore the phenomenon.

Transient surface tension in miscible liquids.

Evidence of the existence of a transient surface tension between two miscible fluid phases is given. This is done by making use of a density matched free of gravity perturbations, binary liquid of isobutyric acid and water, which presents a miscibility gap and is studied by light scattering. The experiment is performed very near the critical point of the binary liquid, where the diffusion of phases is extremely slow. The surface tension is deduced from the evolution of the structure factor obtained from low angle light scattering. The latter evolution is successfully analyzed in terms of a local equilibrium diffusive approach that makes explicit how the surface tension decreases with time.

Digital holographic microscopy with reduced spatial coherence for three-dimensional particle flow analysis.

We investigate the use of a digital holographic microscope working in partially coherent illumination to study in three dimensions a micrometer-size particle flow. The phenomenon under investigation rapidly varies in such a way that it is necessary to record, for every camera frame, the complete holographic information for further processing. For this purpose, we implement the Fourier-transform method for optical amplitude extraction. The suspension of particles is flowing in a split-flow lateral-transport thin separation cell that is usually used to separate the species by their sizes. Details of the optical implementation are provided. Examples of reconstructed images of different particle sizes are shown, and a particle-velocity measurement technique that is based on the blurred holographic image is exploited.