A concept of "pinched flow fractionation" for the continuous size separation and analysis of particles in microfabricated devices has been proposed and demonstrated. In this method, particles suspended in liquid were continuously introduced into a microchannel having a pinched segment and were aligned to one sidewall in the pinched segment by another liquid flow without particles. The particles were then separated perpendicularly to the flow direction according to their sizes by the spreading flow profile inside the microchannel. Polymer microbeads were successfully separated, and the effects of the flow rate and channel shapes on the separation performance were examined. Also, separated particles were collected independently by making branches at the end of the pinched segment. Since this method utilizes only the laminar flow profile inside a microchannel, complicated outer field control could be eliminated, which is usually required for other kinds of particle separation methods such as field flow fractionation. Also, this method can be applied both for particle size analysis and for preparation of monodispersed particles, since separation can be rapidly and continuously performed.
We propose here a new method for continuous concentration and classification of particles in microfluidic devices, named hydrodynamic filtration. When a particle is flowing in a microchannel, the center position of the particle cannot be present in a certain distance from sidewalls, which is equal to the particle radius. The proposed method utilizes this fact, and is performed using a microchannel having multiple side branch channels. By withdrawing a small amount of liquid repeatedly from the main stream through the side channels, particles are concentrated and aligned onto the sidewalls. Then the concentrated and aligned particles can be collected according to size through other side channels (selection channels) in the downstream of the microchannel. Therefore, continuous introduction of a particle suspension into the microchannel enables both particle concentration and classification at the same time. In this method, the flow profile inside a precisely fabricated microchannel determines the size limit of the filtered substances. So the filtration can be performed even when the channel widths are much larger than the particle size, without the problem of channel clogging. In this study, concentrations of polymer microspheres with diameters of 1-3 microm were increased 20-50-fold, and they were collected independently according to size. In addition, selective enrichment of leukocytes from blood was successfully performed.
Microspheres (MS), such as emulsion droplets, multiple emulsions, microparticles, microcapsules, and liposomes, have been utilized in various industries. However, size control of MS is not so easy. Recently, we proposed a novel method for preparing monodispersed emulsion droplets with a coefficient of variation less than 5% from a microfabricated channel (MC) array. In this study, we analyzed a droplet-formation mechanism from a MC using a microscope high-speed camera system. During droplet formation, the dispersed phase passed through the channel inflated on the terrace in a disklike shape, and the droplets were formed in 0.01 s. A droplet-formation mechanism was suggested in which the distorted dispersed phase on the terrace is cut off spontaneously into spherical droplets by interfacial tension. The mechanism is shown to be an adequate model from the viewpoint of interfacial free energy. This emulsification technique exploits the interfacial tension, which is the dominating force on a micrometer scale. It is a promising technique for producing MS requiring extreme monodispersity because of its simplicity.
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