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We report on electrokinetic manipulation of submicron particles in a device with microfabricated Si electrodes featuring segmented sidewalls acting as tracks guiding particles under hydrodynamic drag. Electrohydrodynamic drag, both AC electroosmosis in low-conductive medium and electrothermal flow in high-conductive medium, gives rise to fluid rolls that deliver particles to tracks where they are held under the influence of dielectrophoretic forces. Particles are being railed along tracks under pressure-driven flow to a downstream junction where tracks act as bridges, keeping particles on course within the main channel causing their post-junction enrichment. This outcome, however, is strongly coupled to electrode sidewall contours as demonstrated here. We also report on particle oscillations under strong AC electroosmosis. Specifically, particles in deionized water can be seen bouncing off tracks or circulating with convective rolls in and out of space beneath tracks at an oscillation frequency and amplitude depending on the activation frequency. These results collectively draw attention to the functional use of electrode sidewall contours for continuous-flow manipulation of particles, which are rarely explored in microfluidics and yet could lead to more effective designs for particle separation and enrichment.

The determination of the physical properties of fluids—such as the thermal characteristics, which include heat transfer time (Δt)—is becoming more challenging as system sizes shrink to micro- and nanometer scales. Hence, knowledge of these properties is crucial for the operation of devices requiring precise temperature (T) control, such as polymerase chain reactions, melting curve analysis (MCA), and differential scanning fluorimetry. In this paper, we introduced a flow-through microfluidic system to analyze thermal properties such as Δt among samples and the sidewall of a silicon chip using microscopic image analysis. We performed a spatial MCA with double-stranded deoxyribonucleic acid (dsDNA) and EvaGreen intercalator, using a flow-through microfluidic chip, and achieved a T gradient of ≈ 2.23 K mm−1. We calculated the mean value of Δt as ≈ 33.9 ms from a melting temperature (TM) location shift along the microchannel for a variable flow rate. Our system had a T resolution of ≈ 1.2 mK pixel−1 to distinguish different dsDNA molecules—based on the TM location within the chip—providing an option to use it as a high-throughput device for rapid DNA or protein analysis.

Microchannel plate (MCP), a porous glass membrane commonly used as an electron multiplier in particle detectors, has been experimentally investigated here for electro-osmotic pumping characteristics. MCP consists of millions of high-aspect ratio precision identical glass microcapillary tubes fused together. Uniform and straight microchannels in a relatively thick membrane can achieve a maximum flow rate per unit area and voltage ~0.2 mL min−1 cm−2 V−1 and a maximum pressure per unit voltage ~80 Pa V−1. MCP also shows a unique characteristic of directional preference in pumping with net flow consistently induced toward the nearest electrode regardless of polarity of the voltage potential applied.

This article reports a design that reliably adds reagents into droplets by exploiting the physics of fluid flow at a T-junction in the microchannel. An expanded section right after the T-junction enhances merging of a stream with a droplet, eliminates the drawbacks such as extra droplet formation and long mixing time. The expanded section reduces the pressure buildup at the T-junction and minimizes the tendency to form extra droplets; plays the role in creating low Laplace pressure jump across the interface of the droplet forming from the T-junction which reduces the probability of forming extra droplet in the merging process; provides space for droplet coalescence if there is an extra droplet due to droplet break-up before merging. In this design, after merging, the reactants are in axial arrangement inside the droplets which lead to faster mixing. Reliable addition of reagent to the droplets happens for the combination of flow rates in a broad range from 25 to 250 μl/h, for both DI water (Q DI) and fluorescent (Q fluo) streams.

This paper reports experimental investigations on the droplet formation and size manipulation of deionized water (DIW) and nanofluids in a microfluidic T-junction at different temperatures. Investigations of the effect of microchannel depths on the droplet formation process showed that the smaller the depth of the channel the larger the increase of droplet size with temperature. Sample nanofluids were prepared by dispersing 0.1 volume percentage of titanium dioxide (TiO2) nanoparticles of 15 nm and 10 nm × 40 nm in DIW for their droplet formation experiments. The heater temperature also affects the droplet formation process. Present results demonstrate that nanofluids exhibit different characteristics in droplet formation with the temperature. Addition of spherical-shaped TiO2 (15 nm) nanoparticles in DIW results in much smaller droplet size compared to the cylindrical-shaped TiO2 (10 nm × 40 nm) nanoparticles. Besides changing the interfacial properties of based fluid, nanoparticles can influence the droplet formation of nanofluids by introducing interfacial slip at the interface. Other than nanofluid with cylindrical-shaped nanoparticles, the droplet size was found to increase with increasing temperature. Accepted version