High-viscosity liquid mixing in a slug-flow micromixer: a numerical study

A New Contactless Cross-Correlation Velocity Measurement System for Gas-Liquid Two-Phase Flow

Based on the principle of Contactless Conductivity Detection (CCD), a new contactless cross-correlation velocity measurement system with a three-electrode construction is developed in this work and applied to the contactless velocity measurement of gas-liquid two-phase flow in small channels. To achieve a compact design and to reduce the influence of the slug/bubble deformation and the relative position change on the velocity measurement, an electrode of the upstream sensor is reused as an electrode of the downstream sensor. Meanwhile, a switching unit is introduced to ensure the independence and consistency of the upstream sensor and the downstream sensor. To further improve the synchronization of the upstream sensor and the downstream sensor, fast switching and time compensation are also introduced. Finally, with the obtained upstream and downstream conductance signals, the velocity measurement is achieved by the principle of cross-correlation velocity measurement. To test the measurement performance of the developed system, experiments are carried out on a prototype with a small channel of 2.5 mm. The experimental results show that the compact design (three-electrode construction) is successful, and its measurement performance is satisfactory. The velocity range for the bubble flow is 0.312-0.816 m/s, and the maximum relative error of the flow rate measurement is 4.54%. The velocity range for the slug flow is 0.161 m/s-1.250 m/s, and the maximum relative error of the flow rate measurement is 3.70%.

A microfluidic finger-actuated blood lysate preparation device enabled by rapid acoustofluidic mixing

For many blood-based diagnostic tests, including prophylactic drug analysis and malaria assays, red blood cells must be lysed effectively prior to their use in an analytical workflow. We report on a finger-actuated blood lysate preparation device, which utilises a previously reported acoustofluidic micromixer module. The integrated device includes a range of innovations from a sample interface, to the integration of blisters on a laser engraved surface and a large volume (130 μL) one-stroke manual pump which could be useful in other low-cost microfluidic-based point-of-care devices. The adaptability of the acoustic mixer is demonstrated on highly viscous fluids, including whole blood, with up to 65% percent volume fraction of red blood cells. Used in conjunction with a lysis buffer, the micromixer unit is also shown to lyse a finger-prick (approximately 20 μL) blood sample in 30 seconds and benchmarked across ten donor samples. Finally, we demonstrate the ease of use of the fully integrated device. Cheap, modular, but reliable, finger-actuated microfluidic functions could open up opportunities for the development of diagnostics with minimal resources.

Pressure-Driven Nitrogen Flow in Divergent Microchannels with Isothermal Walls

Gas flow and heat transfer in confined geometries at micro-and nanoscales differ considerably from those at macro-scales, mainly due to nonequilibrium effects such as velocity slip and temperature jump. Nonequilibrium effects increase with a decrease in the characteristic length-scale of the fluid flow or the gas density, leading to the failure of the standard Navier–Stokes–Fourier (NSF) equations in predicting thermal and fluid flow fields. The direct simulation Monte Carlo (DSMC) method is employed in the present work to investigate pressure-driven nitrogen flow in divergent microchannels with various divergence angles and isothermal walls. The thermal fields obtained from numerical simulations are analysed for different inlet-to-outlet pressure ratios (1.5≤Π≤2.5), tangential momentum accommodation coefficients, and Knudsen numbers (0.05≤Kn≤12.5), covering slip to free-molecular rarefaction regimes. The thermal field in the microchannel is predicted, heat-lines are visualised, and the physics of heat transfer in the microchannel is discussed. Due to the rarefaction effects, the direction of heat flow is largely opposite to that of the mass flow. However, the interplay between thermal and pressure gradients, which are affected by geometrical configurations of the microchannel and the applied boundary conditions, determines the net heat flow direction. Additionally, the occurrence of thermal separation and cold-to-hot heat transfer (also known as anti-Fourier heat transfer) in divergent microchannels is explained.

Microfluidic testing of flocculants for produced water treatment: Comparison with other methodologies

Flocculants are often added during produced water treatment to improve the crude oil droplet growth and separation from the water phase. Prior to use in the field, their performance is tested in laboratory conditions, typically with jar tests that require quite large volumes of sample. In this paper we present a microfluidic method as an alternative to study the efficiency of flocculants on enhancing coalescence between oil droplets. Two crude oil emulsions and four flocculants at different concentrations were tested. The new method is also compared to the more traditional techniques. An anionic flocculant showed the biggest improvement in separation for almost all systems. What is more, marked differences were observed between methods with static (bottle and turbidity tests) and dynamic test conditions (light scattering and microfluidics), where stabilization and dispersion effects were observed for the latter. The microfluidic methodology, with added benefits such as visualization, lower sample volumes and shorter measurement times, yielded similar trends as compared to other techniques. Overall, it was shown that microfluidics is a viable alternative to the standard tests.