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Two-dimensional hydrodynamic flow focusing is demonstrated through a microfluidic device featuring a monolithic integrated glass micronozzle inside a flow-focusing geometry. Such a coaxial configuration allows simple one-step focusing of a sample fluid stream, jetted from the micronozzle tip, in both in-plane and out-of-plane directions. The width of the focused filament can be precisely controlled and further scaled down to the submicrometer regime to facilitate rapid hydrodynamic mixing. Fluorescence quenching experiments reveal ultra-fast microsecond mixing of the denaturant into the focused filament. This device offers new possibilities to a set of applications such as the study of protein folding kinetics.

Microfluidic devices have shown remarkable success in generating emulsions with precise control over their size. Yet, highly sensitive nature of generation mechanism to surface wettability requires such devices to be built out of specific materials showing homogenous wettability that favors the continuous phase rather than the dispersed phase. Moreover, the need to switch the continuous phase and the dispersed phase requires switching the device wettability by applying a suitable surface treatment. Here, we demonstrate a microfluidic device that can generate water-in-oil and oil-in-water emulsions without the necessity of surface treatment. The device features a suspended glass micronozzle integrated inside a flow-focusing geometry formed by silicon and poly(dimethylsiloxane) channels where drops of the dispersed phase can be sheared off at the micronozzle tip without touching channel walls in a coflow of the continuous phase. The micronozzle structure is a partially released segment of a self-enclosed capillary entirely built in phosphosilicate glass and with a cylindrical lumen ∼1.5 μm in diameter. Owing to high fluidic resistance of such fine capillary, emulsion generation in the device takes place in a dripping process and no noticeable jet formation of the dispersed phase has been observed throughout the tested flow rates. The effect of the flow rates on the diameter of the emulsions and their rate of generation has been experimentally investigated and found to show a similar trend to that of a simple physical model based on the critical Capillary number.

We report a force field on a particle in a concentration (conductivity) gradient under an externally applied oscillating electric field. The conductivity gradient was established through integrated microcapillaries bridging high- and low-conductivity streams in dedicated microchannels. Particles in low-conductivity electrolyte were observed to experience a strong force with the application of an oscillating field and pulled to the microcapillary openings where they were held against the flow. Particle trapping was accompanied by a concurrent electrolyte injection from high- to low-conductivity channel, triggered with the externally applied field and further contributed to the conductivity gradient near the trapping sites. We experimentally evaluated the force dependence on the magnitude and frequency of the excitation field for 10 μm polystyrene particles immersed at various conductivity levels. The experiments suggest that the observed force cannot be simply explained by dielectrophoresis or diffusiophoresis alone and further requires the consideration of a so-called concentration polarization force. This force has been rather recently postulated based on a theoretical treatment and yet to be experimentally validated. Using the theoretical treatment of this force, together with fluidic drag and diffusiophoresis, we correctly predicted trapping trajectories of particles based on a simultaneous solution of Poisson-Nernst-Planck and Stokes equations. The predicted and measured trapping velocities were found in reasonable agreement (within a factor of

Precise dispensing of microdroplets is an important process for droplet-based microfluidics. The droplet formation by shear force between two immiscible fluids depends on their flow rates, the viscosities, and the interfacial tension. In this letter, the authors report the use of integrated microheater and temperature sensor for controlling the droplet formation process. The technique exploits the dependency on temperature of viscosities and interfacial tension. Using a relatively low heating temperature ranging from 25 to 70 °C, the droplet diameter can be adjusted to over two times of its original value. The relatively low temperature range makes sure that this concept is applicable for droplets containing biological samples Published version

The authors present microfluidic logic gates based on two-phase flows at low Reynold’s number. The presence and the absence of a dispersed phase liquid (slug) in a continuous phase liquid represent 1 and 0, respectively. The working principle of these devices is based on the change in hydrodynamic resistance for a channel containing droplets. Logical operations including AND, OR, and NOT are demonstrated, and may pave the way for microfludic system automation and computation.

The authors present a microfluidic device towards an integrated patch-clamping assay. The device replaces conventional glass patch pipette with a buried microfluidic channel on silicon substrate. The microchannel fabrication involves reforming doped glass under heat and pressure, a process, in principle, analogous to the heat-pulling/polishing of glass patch pipettes. Unlike etching substrate, this process leaves a smooth glass surface for seal formation with cell membrane. The microchannel is evolved from a trapped void inside the trench during nonconformal deposition of the doped glass. The results of seal formation with mammalian cells captured at such microchannel opening are presented.