Integrated microfluidic system with simultaneous emulsion generation and concentration.

Because the size, size distribution, and concentration of emulsions play an important role in most of the applications, controlled emulsion generation and effective concentration are of great interest in fundamental and applied studies. While microfluidics has been demonstrated to be able to produce emulsion drops with controlled size, size distribution, and hierarchical structures, progress of controlled generation of concentrated emulsions is limited. Here, we present an effective microfluidic emulsion generation system integrated with an orifice structure to separate aqueous droplets from the continuous oil phase, resulting in concentrated emulsion drops in situ. Both experimental and simulation results show that the efficiency of separation is determined by a balance between pressure drop and droplet accumulation near the orifice. By manipulating this balance via changing flow rates and microfluidic geometry, we can achieve monodisperse droplets on chip that have a concentration as high as 80,000 drops per microliter (volume fraction of 66%). The present approach thus provides insights to the design of microfluidic device that can be used to concentrate emulsions (drops and bubbles), colloidal particles (drug delivery polymer particles), and biological particles (cells and bacteria) when volume fractions as high as 66% are necessary.

Effect of insulating posts geometry on particle manipulation in insulator based dielectrophoretic devices.

In this study, the effect of the geometry of insulating posts on microparticle trapping in insulator based dielectrophoresis (iDEP) was analyzed. The motivation for this research was to study how to improve particle trapping and enrichment by modifying the shape of insulating posts used in iDEP microdevices, while keeping post spacing constant. Mixtures of inert polystyrene particles were employed for demonstrating the effects of insulator shape on particle capture and enrichment. A series of experiments were carried out using an array of devices with different insulating post shapes. All the different post shapes employed had a width of 200 µm and were arranged in a square array of 250 µm center-to-center, thus, the spacing between posts was 50 µm in all cases. Mathematical modeling with COMSOL Multiphysics was employed to assess the magnitude of electric field gradients achieved with each one of the geometries tested. The results showed that the electric potential required to obtain effective particle trapping and enrichment can be significantly reduced by modifying the geometry of the insulating posts, without having to modify the separation distance between posts, thus, preserving the porosity of the microchannels. The separation of a mixture of 1-µm and 2-µm diameter particles was achieved in the form of dielectropherograms employing two different insulating post geometries (circle and diamond). Concentrated particles were released as peaks from the insulating post arrays where higher peak resolution separation was obtained with the sharper diamond geometry. Concentration enrichment above one order or magnitude was obtained for both particle types in both dielectropherograms. The results demonstrate that more efficient iDEP separations can be achieved at lower applied electric potentials by carefully selecting the geometry of the insulating structures.

Dynamic microparticle manipulation with an electroosmotic flow gradient in low-frequency alternating current dielectrophoresis.

In this study, the potential of low-frequency AC insulator-based DEP (iDEP) was explored for the separation of polystyrene microparticles and yeast cells. An EOF gradient was generated by employing an asymmetrical, 20 Hz AC electrical signal in an iDEP device consisting of a microchannel with diamond-shaped insulating posts. Two types of samples were analyzed, the first sample contained three types of polystyrene particles with different diameters (0.5, 1.0, and 2.0 µm) and the second sample contained two types of polystyrene particles (1.0 and 2 µm) and yeast cells (6.3 µm). This particular scheme uses a tapered AC signal that allows for all particles to be trapped and concentrated at the insulating post array, as the signal becomes asymmetrical (more positive), particles are selectively released. The smallest particles in each sample were released first, since they require greater dielectrophoretic forces to remain trapped. The largest particles in each sample were released last, when the applied signal became cyclical. A dielectropherogram, which is analogous to a chromatogram, was obtained for each sample, demonstrating successful separation of the particles by showing "peaks" of the released particles. These separations were achieved at lower applied potentials than those reported in previous studies that used solely direct current electrical voltages. Additionally, mathematical modeling with COMSOL Multiphysics was carried out to estimate the magnitude of the dielectrophoretic and EOF forces acting on the particles considering the low-frequency, asymmetrical AC signal used in the experiments. The results demonstrated the potential of low-frequency AC-iDEP systems for handling and separating complex mixtures of microparticles and biological cells.