Microfluidics and Nanofluidics最新文献

Numerical investigations of high-speed rarefied gas outflow into a vacuum through channels with a forward- or backward-facing step have been conducted using the direct simulation Monte Carlo method. Calculations have been performed for various free-stream Mach numbers, covering transonic, supersonic, and hypersonic flow regimes, and over a wide range of gas rarefaction from free molecular to near hydrodynamic conditions. Mass flow rates through the channel and the gas flow field have been accurately calculated both inside the channel and in the regions upstream and downstream. It has been established that channel geometry, the free-stream velocity, and gas rarefaction strongly influence the gas flow. In the flow field, in front of the channel, a phenomenon known as a detached shock occurs, while inside the channel, a gas recirculation zone may form.

Microfluidic flow control systems are critical components for on-chip biomedical applications. This study introduces a new micropump for on-chip sample preparation and analysis by using an acoustic nozzle diffuser mechanism. The micropump implements a commercially available transducer and control board kit with 3D-printed fluid reservoirs. In this micropump, conic-shaped micro-holes on the metal sheet cover of the transducer are employed as oscillating nozzle diffuser micro arrays to achieve directional flow control. The micropump is shown to efficiently pump water and particle mixtures exceeding flow rates of 515 µl/min at a 12-volt input voltage. In addition, owing to the small size of the nozzle hole opening, larger particles can also be filtered out from a sample solution during fluid pumping enabling a new function. Importantly, the micropump can be fabricated and assembled without needing a cleanroom, making it more accessible. This feature is advantageous for researchers and practitioners, eliminating a significant barrier to entry. By combining commercially available components with 3D printing technology, this micropump presents a cost-effective and versatile solution for on-chip applications in biomedical research and analysis.

A method is proposed to deterministically obtain steady lubrication pressure for the Stokes flow in a channel bounded by a flat wall and a surface with roughness represented by sinusoidal waves. A streamline sufficiently far away from the rough surface is used to formulate a streamline-based lubrication equation with the velocity on the streamline, and the velocities on the streamline is imposed as a boundary condition. In the solution of the lubrication equation, by virtually moving the streamline towards the flat wall, the pressure on the flat wall is obtained, and then the wall-normal variation of the pressure is recovered from the wall pressure by a lubrication model that considers higher order terms. The proposed method is applied to lubrication flows in channels with roughness represented by a single sinusoidal wave and a superposition of several sinusoidal waves. Through comparison with analytical solutions, the validity of the proposed method is established, and the applicable range of superposition of waves is explained that lowest-wavenumber component in surface profile is sufficiently isolated from higher-wavenumber components. Although the problem setting intrinsically prohibits the application of the conventional Reynolds lubrication equation, this study provides new understandings for the pressure obeying the Reynolds lubrication equation and the role of the higher-order terms.

This study introduces an innovative method aimed at achieving exceptional stability in emulsions. The primary focus is on re-emulsifying precisely controlled and uniform initial single emulsions, generated by microfluidic devices, to produce single-core double emulsions and core–shell microparticles. Departing from traditional approaches, our method employs a unique combination of advanced Two-level fractional factorial design and numerical simulation. These tools are utilized to discern and optimize critical parameters necessary for the formation of highly monodispersed stable single emulsions and their subsequent transformation into double emulsions. Correlations are established to estimate the size and stability of the primary single emulsion based on immiscible phase flow rate ratio and surfactant concentration. These correlations provide a comprehensive understanding that facilitates the intentional development of desired water-in-oil emulsions. The proposed microfluidic paradigm shows promise for the controlled and efficient production of single-core double emulsions, with broad applications in Pharmaceuticals, Food, and Cosmetics.

We present the control of liquid flow through arrayed micron-sized pores in a macroporous silicon membrane. The pores are coated with about 150 nm polymer N-isopropylacrylamide (pNIPAAm) hydrogel brushes and 200 nm polypyrrole layer, which works as photothermal actuator. The size of pore openings is controlled by utilizing the swelling and de-swelling behavior of temperature-sensitive pNIPAAm brushes, and the temperature on pNIPAAm brushes is changed by 815 nm near infra-red (NIR) illumination to polypyrrole photothermal element layer. The dimension change of the pore openings is investigated by observing the transmitted light and fluorescence signal intensity through the pores in the membrane while changing the ambient temperature. It has shown that the intensity of transmitted light can be controlled by adjusting the ambient temperature across the low critical solution temperature (LCST) of the hydrogel brushes. The localized control of liquid flow through the pores is demonstrated by the diffusion of fluorescein dye from the bottom of the membrane to the surface of the membrane using pulsed NIR light illumination. Fast dynamic response of fluorescein dye diffusion upon the illumination of NIR light suggests that the presented photothermal actuation approach could be applied to diverse biomedical applications such as a localized drug release system.

Some anticancer treatments may cause Multidrug Resistance (MDR). In these cases, cells pump the drug out of the intracellular environment, thereby preventing drug effects. Several strategies have been used to avoid MDR, including using two or more drugs at low concentrations to increase the sensitivity of cells to treatment. We present an effective, cheap, fast microfluidic alternative to test two drugs simultaneously using a reversible sealing and reusable device to determine the optimal concentration. We used the rugs doxorubicin (DOX) and paclitaxel (PXT) as proof of concept. The microdevice allows the testing of two drugs in real time. Furthermore, running two experiments in sextuplicates and control in the same microchip is possible. We used two combinations of drugs, varying the drug concentration (C₁ = 0.010 mg.mL^− 1 DOX and 0.002.mL^− 1 mg PXT, C₂ = 0.010 mg.mL^− 1 DOX and 0.004 mg.mL^− 1 PXT), and evaluated cell death over time. The intermediate drug concentrations were more efficient, reducing the time required to decrease the viability of breast tumor cells, MCF-7 (C₁ = 180 and C₂ = 120). In further analysis, the microdevice also allowed characterization of the effects of the drugs (antagonist, synergic, or additive). This microdevice is a reliable tool for estimating the different combinations of two drug concentrations in a single assay simply and quickly.

Despite several decades of development, microfluidics lacks a sealing material that can be readily fabricated, leak-tight under high liquid water pressure, stable over a long time, and vacuum compatible. In this paper, we report the performances of a micro-scale processable sealing material for nanofluidic/microfluidics chip fabrication, which enables us to achieve all these requirements. We observed that micrometric walls made of SU-8 photoresist, whose thickness range from 35 to 135 µm, are at least leak-tight to 1.5 bars and up to 5.5 bars, exhibit no water porosity even after 2 months of aging, and are able to sustain under (10^{-5}) mbar vacuum. This sealing material is therefore reliable and versatile for building microchips, part of which must be isolated from liquid water under pressure or vacuum. Moreover, the fabrication process we propose does not require the use of either aggressive chemicals or high-temperature or high-energy plasma treatment. It thus opens a new perspective to seal microchips with sensitive surfaces containing nanomaterials.

Snap-off events are one of the most common and essential phenomena in two-phase flow in porous media. This paper uses the scanning results of a siltstone slice to construct a two-dimensional heterogeneous pore network structure to visualise microscopic snap-off phenomena and displacement processes accurately. The relationship between snap-off events and the non-wetting phase saturation was studied using two-phase flow displacement experiments. Results show that although the non-wetting phase snap-off events benefit freeing the trapped non-wetting phase in the microchannels, high-frequency snap-off events are the main reason for trapping the non-wetting phase during the displacement process, eventually leading to residuals. The frequency of non-wetting phase snap-off events in the pore network structure can be reduced to lower the non-wetting phase saturation and reduce the non-wetting phase residuals by increasing the displacement fluid viscosity, reducing the surface tension coefficient between the phases and increasing the flow rate.

In droplet microfluidic devices with suction-based flow control, the microchannel geometry and suction pressure at the outlet govern the dynamic properties of the two phases that influence the droplet generation. Therefore, it is critical to understand the role of geometry along with suction pressure in the dynamics of droplet generation to develop a predictive model. We conducted a comprehensive characterization of droplet generation in a flow focusing device with varying control parameters. We used these results to formulate a scaling argument and propose a governing parameter, called as modified capillary number (Ca_L), that combines normalized droplet volume with geometrical parameters (length of dispersed and continuous phase channels) and flow parameters (interfacial tension, phase viscosity and velocity) in a power law relationship. Ca_L effectively captures the transition from squeezing to dripping regimes of droplet generation, providing essential insights into the design requirements for suction-driven droplet generation. These findings are key to standardize microfluidic flow-focusing devices that can achieve the desired droplet generation behavior with optimal pressure consumption.