Microfluidics and Nanofluidics

Optically induced dielectrophoretic (ODEP) chip is to combine their own advantages of optical tweezers and electrodynamics manipulation technologies, which can trap single particles in high resolution as well as enrich much of micro-/nanoparticles in high throughput. The paper analyzed the structure of optoelectronic tweezers (OET) chip, moreover, the frequency response of multi-membrane eukaryotic cells about 103–109 Hz. The Clausius–Mositti (CM) frequency factor in terms of cell membrane, cell cytoplasm, nuclear envelope thickness changes, and volume ratio was illustrated. In the end, the paper presented 3D numeric model of cells in OET chip. The dielectrophoresis force acting on the dipole of 11.8-μm cells subjected to a non-uniform electric field under 60-μm Gaussian-distributed beam spot could be simulated in the enrichment process. The separation of cells that were two different types of CM values was calculated. Furthermore, it was proved to be feasible to achieve the efficient separation of cells using ODEP technology in the biological numerical model. Comparing with the literature of experiment, the results in cell dielectric spectroscopy and numeric model findings were in general agreement. The simplified structure and numeric model of nucleated cell provide a theoretical basis for research of biosensor and complex life.

This paper provides an overview of the electrokinetic phenomena associated with particles and cells in microchannel systems. The most important phenomena covered include electrophoresis, dielectrophoresis, and induced-charge electrokinetics. The latest development of these electrokinetic techniques for particle or cell manipulations in microfluidic systems is reviewed, in terms of the basic theories, mathematical models, numerical and experimental methods, and the key results/findings from the published literatures in the most recent decades. Some of the limitations associated with the negative field effects are discussed and the perspectives for the future investigations are summarized.

Localized microfluidic rotational flow (Reynolds number < 1) confined to a cylindrical volume element (1,600 µm diameter, 620 µm high) with velocity magnitude ≤14 µm/s was achieved in a small cell (14.3 mm wide × 27.0 mm long × 620 µm high) contained over a chip by redox-magnetohydrodynamics (MHD) pumping. The chip consisted of an insulated silicon substrate patterned with pairs of concentric disk and ring gold microelectrodes. The MHD force (F B  = j × B) was generated with ionic current density, j, from electrochemistry of 0.095 M K3Fe(CN)6 and 0.095 M K4Fe(CN)6 in a supporting electrolyte of 0.095 M KCl, and with an external magnetic field, B, from a 0.36 T NdFeB permanent magnet beneath the chip. Fluid flow was monitored between the edge of the disk (radius 80 µm) and the inner radius (800 µm) of the ring, using video microscopy to track 10-µm polystyrene latex beads in the redox solution. Data analysis was performed by particle image velocimetry software. Fluid speeds decreased approximately proportionally with radial position across the disk-ring gap, consistent with the decline of ionic current density. This behavior was visualized when half the solution over the disk (160 µm radius) and ring (1,600 µm inner radius) contained a red dye, where faster circulation near the disk caused the two solutions to pass through each other to form a spiral pattern, increasing the interfacial length, and improving opportunities for diffusional interchange. These results suggest a means for local mixing without the need for moving parts or sidewalls to guide the flow.

In this paper, a Boolean lattice-gas model based on field mediators proposed by Santos and Philippi (Phys Rev E 65:046305, 2002) is used for the simulation of fluid–fluid interface displacement inside two-dimensional simplified porous media. A new procedure is introduced to allow the simulation of different viscosity ratios on the framework of lattice-gas models. The model is verified by simulating the spreading of a liquid drop on a solid surface and by comparing the simulation results with experimental spreading data. Some important basic physical mechanisms occurring at the pore scale are simulated and compared qualitatively with experimental visualizations. The break-off phenomenon of the fluid–fluid interface is observed in bifurcations, when a wetting (or non-wetting) fluid is displacing a non-wetting (or wetting) fluid. The role of break-off is shown to be different in imbibition and drainage processes in agreement with experimental results. Finally, the influence of wettability on the displacement efficiency is investigated in two-dimensional random arrays of disks.

Swimming micro-robots have great potential in biomedical applications such as targeted drug delivery, medical diagnosis, and destroying blood clots in arteries. Inspired by swimming microorganisms, micro-robots can move in biofluids with helical tails attached to their bodies. In order to design and navigate micro-robots, hydrodynamic characteristics of the flow field must be understood well. This work presents computational fluid dynamics modeling and analysis of the flow due to the motion of micro-robots that consist of magnetic heads and helical tails inside fluid-filled channels akin to bodily conduits; special emphasis is on the effects of the radial position of the robot. Time-averaged velocities, forces, torques, and efficiency of the micro-robots placed in the channels are analyzed as functions of rotation frequency, helical pitch (wavelength) and helical radius (amplitude) of the tail. Results indicate that robots move faster and more efficiently near the wall than at the center of the channel. Forces acting on micro-robots are asymmetrical due to the chirality of the robot’s tail and its motion. Moreover, robots placed near the wall have a different flow pattern around the head when compared to in-center and unbounded swimmers. According to simulation results, time-averaged forward velocity of the robot agrees well with the experimental values measured previously for a robot with almost the same dimensions.

Double emulsion drops are well-suited templates to produce capsules whose dimensions can be conveniently tuned by adjusting those of the drops. To closely control the release kinetics of encapsulants, the composition and thickness of the capsule shell must be precisely tuned; this is greatly facilitated if the shell is homogeneous in its composition and thickness. However, the densities of the two drops that form the double emulsion are often different, resulting in an offset of the two drop centers and therefore in an inhomogeneous shell thickness. This difficulty can be overcome if the shell is made very thin. Unfortunately, a controlled fabrication of double emulsions with thin shells is difficult. In this paper, we present a microfluidic squeezing device that removes up to 93 vol% of the oil from the shell of water–oil–water double emulsions. This is achieved by strongly deforming drops; this deformation increases their interfacial energy to sufficiently high values to make splitting of double emulsions into double emulsions with a much thinner shell and a single emulsion oil drop energetically favorable. Therefore, we can reduce the shell thickness of the double emulsion down to 330 nm. Because this method does not rely on solvent evaporation, any type of oil can be removed. Therefore, it constitutes a new way to produce double emulsions with very thin shells that can be converted into thin-shell capsules made of a broad range of materials.

In this work, for the first time, we demonstrate nanoscale droplet generation from a continuous electrowetting microchannel using a simple and precise image-based droplet volume metering technique. One of the most popular ways of droplet generation in electrowetting devices is to split a droplet from a preloaded volume as a fluid reservoir. This method is effective, but lowers volume consistency after multiple droplets are generated. Impedance- and capacitance-based methods of volume metering have been successfully used in digital microfluidics, but require complex circuitry and feedback signal processing. In this work, we demonstrate nanoliter droplet generation from a continuous electrowetting channel used as a replenishable fluid reservoir which compensates for the loss of reservoir volume as droplets are sequentially split. This improves volume consistency especially for applications requiring multi-droplet generation. Based on the area of the electrode, the volume of each droplet split from the electrowetting channel can be obtained by a simple and precise image processing technique with no need for additional hardware and measurement errors of ±0.05 %. This simple technique can be used in a wide range of applications that require precise volume metering, such as immunoassay.

This paper reports the experimental and numerical analysis of time-dependent rarefied gas flows through a long metallic micro-tube. The experimental methodology was conceived on the basis of the constant volume technique and adapted to measure the evolution with time of a transient mass flow rate through a micro-tube. Furthermore, the characteristic time of each experiment, extracted from the pressure measurements in each reservoir, offered a clear indication on the dynamics of the transient flow as a function of the gas molecular mass and its rarefaction level. The measured pressure evolution with time at the inlet and outlet of the micro-tube was compared to numerical results obtained with the BGK linearized kinetic equation model. Finally, we present an original methodology to extract stationary mass flow rates by using the tube conductance, which can be associated with the characteristic time of the experiment, measured for different mean pressures between two tanks. The results were obtained in a wide range of rarefaction conditions for nitrogen ( $$N_2$$ ). A brief comparison is offered with respect to R134a (CH2FCF3), too, a heavy polyatomic gas which is typically used in the refrigeration industry.

The oscillatory Couette flow is important for further advancement of microengineering. In practice the size of the microfluidics can be so small that it can be compared with the molecular mean free path. Moreover, the oscillation frequency can be close to that of the intermolecular collisions. Under such conditions the problem must be solved on the kinetic level. In the present work, the oscillatory Couette flow is considered on the basis of the non-stationary kinetic equation. The solution to the problem is determined by two parameters: the Knudsen number and the ratio of collision frequency to oscillation frequency. The kinetic equation is solved by the discrete velocity method over the wide range of both parameters.

Gas–liquid slug flow is characterized by complex and intermittent hydrodynamic features that offer an efficient alternative to promote biofilm control. In the present work, the mechanism of transferring a gaseous solute into a co-current liquid in a micro-scale slug flow system was inspected in detail. Specifically, the gas–liquid mass transfer from an individual Taylor bubble filled with oxygen was numerically studied using CFD techniques. To accurately describe the referred phenomenon, the hydrodynamic and concentration fields were simultaneously solved. Furthermore, the interface capturing based on the VOF methodology was also coupled to this solution approach. Three sub-categories within slug flow pattern were identified based on the flow behavior in the liquid phase: no liquid in recirculation (Case A); closed wake below the bubble tail (Case B); and recirculation ahead and below bubble (Case C). Regarding the solute distribution, in Case A the solute is dispersed only backwards, it accumulates in the closed wake structure in Case B, and it reaches the wall within the film region in Case C. Local and average mass transfer coefficients were also estimated for the different cases. The influence of the two most relevant dimensionless groups (Reynolds and Capillary numbers) was also briefly analyzed. Global mass transfer coefficients results confirmed that the penetration theory can provide reasonable estimations for systems like Case C.