Particles In Microfluidic Devices Research Articles

Charge-Based Separation of Microparticles Using AC Insulator-Based Dielectrophoresis.

Surface charge is an important property of particles. It has been utilized to separate particles in microfluidic devices, where dielectrophoresis (DEP) is often the driving force. However, current DEP-based particle separations based on the charge differences work only for particles of similar sizes. They become less effective and may even fail for a mixture of particles differing in both charge and size. We demonstrate that our recently developed AC insulator-based dielectrophoresis (AC iDEP) technique can direct microparticles toward charge-dependent equilibrium positions in a ratchet microchannel. Such charge-based particle separation is controlled by the imposed AC voltage frequency and amplitude but is nearly unaffected by the size of either type of particle in the mixture except for the time required to achieve an effective separation. This AC iDEP technique may potentially be used to focus and separate submicron or even nanoparticles because of its virtually "infinite" channel length.

Dielectrophoretic Particle Chromatography: From Batch Processing to Semi-Continuous High-Throughput Separation

The development of highly selective separation processes is a focus of current research. In 2016, the German Science Foundation funded a priority program SPP 2045 “MehrDimPart—highly specific multidimensional fractionation of fine particles with technical relevance” that aims to develop new or enhance existing approaches for the separation of nano- and micrometer-sized particles. Dielectrophoretic separators achieve highly selective separations of (bio-)particles in microfluidic devices or can handle large quantities when non-selective separation is sufficient. Recently, separator designs were developed that aim to combine a high throughput and high selectivity. Here, we summarize the development from a microfluidic fast chromatographic separation via frequency modulated dielectrophoretic particle chromatography (DPC) toward a macrofluidic high throughput separation. Further, we provide a starting point for future work by providing new experimental data demonstrating for the first time the trapping of 200 nm polystyrene particles in a dielectrophoretic high-throughput separator that uses printed circuit boards as alternatives for expensive electrode arrays.

Designing a new microchannel to collect microparticles using dielectrophoretic forces: Numerical and experimental investigation

Dielectrophoresis (DEP) is an effective technique for manipulating particles in microfluidic devices. The DEP force depends on the frequency and square gradient of the electric field, as well as the fluid and particle dielectric properties. An efficient system for manipulating particles can be designed by adjusting these factors. This study aims to develop an efficient microsystem for particle trapping using dual-frequency DEP force. The microfluidic system is divided into two parts of focusing and attracting. The negative DEP (nDEP) force in the focusing part concentrates particles near the microchannel axis. The positive DEP (pDEP) force in the attractive area then absorbs particles into the internal chamber via electrodes. In general, the main advantage of the present design is the maximum trapping of incoming particles (with a trapping rate of over 95%) regardless of their initial location. In this study, numerical modeling was first done in three dimensions to sort and trap the microparticles. Then, a microchip was designed, built, and tested in a laboratory to validate the results and confirm the microfluidic system behavior. Finally, a parametric study was conducted to figure out the best voltage range of the electric fields in the microfluidic system.

Numerical simulations of the dynamics of a deformable particle in a viscoelastic liquid subjected to Poiseuille flow in a cylindrical microcapillary at non-negligible inertia

The manipulation and control of particles in microfluidic devices through non-intrusive methods is pivotal in many application fields, e.g., cell focusing and sorting. Inertial microfluidics is rapidly gaining attention in the scientific community because of the considerable advantages in terms of throughput. In addition to inertia, other factors can trigger the cross-stream migration of particles in liquids undergoing pressure-driven channel flows, such as the deformability of the particles themselves and/or the viscoelasticity of the carrier fluid. For this reason, the dynamics of an initially spherical elastic particle suspended in a viscoelastic liquid subjected to pressure-driven flow in a cylindrical channel at non-negligible inertia is studied through three-dimensional arbitrary Lagrangian–Eulerian finite-element numerical simulations. The mechanical behavior of the particle is described through the neo-Hookean hyper-elastic constitutive equation, whereas the rheological behavior of the carrier liquid is described through the Giesekus model. The Reynolds number Re, measuring the relative importance of inertial and viscous forces in the tube, the elastic capillary number Cae, measuring the relative importance of liquid viscous stress and solid elastic stress, and the Deborah number De, measuring the ratio of the liquid relaxation time and the flow characteristic time, are varied. The particle migrates transversally to the flow direction until reaching a radial equilibrium position depending on Re, Cae, and De. Different dynamics are observed depending on the interplay among inertia and elasticity of both the liquid and the solid phase: one, two, or even three stable equilibrium positions can be found along the tube radius.

Slow motion of a sphere near a sinusoidal surface

Particle motion near non-plane surfaces can exhibit intricate hydrodynamics, making it an attractive tool for manipulating particles in microfluidic devices. To understand the underlying physics, this work investigates the Stokesian dynamics of a sphere near a sinusoidal surface, using a combination of perturbation analysis and boundary element simulation. The Lorentz reciprocal theorem is employed to solve the particle mobility near a small-amplitude surface. Compared with a plane wall, the curved topography induces additional translation and rotation velocity components, with the direction depending on the location of the sphere and the wavelength of the surface. At a fixed distance from the surface, the longitudinal and vertical mobilities of the sphere are strongly affected by the wavelength and amplitude of the surface, whereas its transverse mobility is only mildly influenced. When a sphere settles perpendicular to a sinusoidal surface, the far-field hydrodynamic effect drives the particle towards the local hill, while the near-field effect attracts the particle to the valley. These results provide valuable insights into the particle motion near surfaces with complex geometry.

Rigid spheroid migration in square channel flow of power-law fluids

The characteristics of rigid spheroid migration in square channel flow of power-law fluids are numerically investigated by the three-dimensional lattice Boltzmann method. The effects of aspect ratio (α), blockage ratio of the particle (k), power-law index (n) and Reynolds number (Re) of the fluid on the inertial migration of rigid prolate and oblate spheroids are explored, respectively. The results show that the shear-thinning fluid with large fluid inertia is beneficial for fast focusing rigid particles to the equilibrium positions in square channel. There are two stages of particle migration and four stable channel face equilibrium positions for migration of spheroid; the channel corner equilibrium positions only exist for the oblate spheroid under a low inertia effect. The prolate spheroid exhibits tumbling and log-rolling (LR) rotational modes, and the LR mode is conditionally stable. For the oblate spheroid, only the LR mode exists when Re is large while two new rotational modes are captured when Re decreases. The equilibrium position of the prolate and oblate spheroids increase when α < 1.0 then decrease at larger α values, escaping the channel centreline with decreasing n and k and increasing Re. The changes of inertial focusing length, angular velocity, and rotational period of the spheroid are systematically analysed to study mechanism of spheroid migration. The current results enrich our understanding of rigid particle accumulation behaviour in channel flow of non-Newtonian fluids and may also shed light on how to efficiently focus and control particles in microfluidic devices.

Quantifying the dielectrophoretic force on colloidal particles in microfluidic devices

Quantifying the dielectrophoretic force on colloidal particles in microfluidic devices

Wall Repulsion of Charged Colloidal Particles during Electrophoresis in Microfluidic Channels.

Electrophoresis describes the motion of charged particles suspended in electrolytes when subjected to an external electric field. Previous experiments have shown that particles undergoing electrophoresis are repelled from nearby channel walls, contrary to the standard description of electrophoresis that predicts no hydrodynamic repulsion. Dielectrophoretic (DEP) repulsive forces have been commonly invoked as the cause of this wall repulsion. We show that DEP forces can only account for this wall repulsion at high frequencies of applied electric field. In the presence of a low-frequency field, quadrupolar electro-osmotic flows are observed around the particles. We experimentally demonstrate that these hydrodynamic flows are the cause of the widely observed particle-wall interaction. This hydrodynamic wall repulsion should be considered in the design and application of electric-field-driven manipulation of particles in microfluidic devices.

Deterministic Lateral Displacement Using Hexagonally Arranged, Bottom-Up-Inspired Micropost Arrays.

Size-based separation of particles in microfluidic devices can be achieved using arrays of micro- or nanoscale posts using a technique known as deterministic lateral displacement (DLD). To date, DLD arrays have been limited to parallelogram or rotated-square arrangements of posts, with various post shapes having been explored in these two principal arrangements. This work examines a new DLD geometry based on patterning obtainable through self-assembly of single-layer nanospheres, which we call hexagonally arranged triangle (HAT) geometry. Finite element simulations are used to characterize the DLD separation properties of the HAT geometry. The relationship between the array angle, the gap spacing, and the critical diameter for separation is derived for the HAT geometry and expressed in a similar mathematical form as conventional parallelogram and rotated-square DLD arrays. At array angles <7°, HAT structures demonstrate smaller particle sorting capability (smaller critical diameter-to-gap spacing ratio) compared to published experimental results for parallelogram-type DLD arrays with circular posts. Experimental validation of DLD separation confirms the separation ability of the HAT array geometry. It is envisioned that this work will provide the first step toward future implementation of nanoscale DLD arrays fabricated by low-cost, bottom-up self-assembly approaches.

A single pixel tracking system for microfluidic device monitoring without image processing

Currently image-based optical measurement methods are the main approaches for detecting and tracking the movements of droplets or particles in microfluidic devices. However, the massive image data throughput and high computational consumption of image processing have become challenges to achieve real-time and long-term monitoring. Here a single-pixel tracking system is proposed to monitor the microfluidic device on the basis of single-pixel imaging and microscope technology without image reconstruction. The experimental results of tracking droplet in microfluidic device demonstrate that the proposed system have unique potential advantages in dynamic tracking for microfluidic devices. The data throughput of the proposed method is 1/792 of image-based methods and can reach an equivalent frame rate of 312.5 Hz with a low computational consumption of 89.3 μs per frame. The proposed image-free tracking method provides a welcomed boost to the further development of microfluidic device monitoring and indicates a potential way to real-time and long-term monitoring without image processing.

Structures and topological defects in pressure-driven lyotropic chromonic liquid crystals

Lyotropic chromonic liquid crystals are water-based materials composed of self-assembled cylindrical aggregates. Their behavior under flow is poorly understood, and quantitatively resolving the optical retardance of the flowing liquid crystal has so far been limited by the imaging speed of current polarization-resolved imaging techniques. Here, we employ a single-shot quantitative polarization imaging method, termed polarized shearing interference microscopy, to quantify the spatial distribution and the dynamics of the structures emerging in nematic disodium cromoglycate solutions in a microfluidic channel. We show that pure-twist disclination loops nucleate in the bulk flow over a range of shear rates. These loops are elongated in the flow direction and exhibit a constant aspect ratio that is governed by the nonnegligible splay-bend anisotropy at the loop boundary. The size of the loops is set by the balance between nucleation forces and annihilation forces acting on the disclination. The fluctuations of the pure-twist disclination loops reflect the tumbling character of nematic disodium cromoglycate. Our study, including experiment, simulation, and scaling analysis, provides a comprehensive understanding of the structure and dynamics of pressure-driven lyotropic chromonic liquid crystals and might open new routes for using these materials to control assembly and flow of biological systems or particles in microfluidic devices.

Electrophoretic ratcheting of spherical particles in well/channel microfluidic devices: Making particles move against the net field.

We examine the electrophoresis of spherical particles in microfluidic devices made of alternating wells and narrow channels, including a system previously used to separate DNA molecules. Our computer simulations predict that such systems can be used to separate spherical particles of different sizes that share the same free-solution mobility. Interestingly, the electrophoretic velocity shows an inversion as the field intensity is increased: while small particles have higher velocities at low field, the situation is reversed at high fields with the larger particles then moving faster. The resulting nonlinearity suggests that asymmetric pulsed electric fields could be used to build separation ratchets: particles then have a net size-dependent velocity in the presence of a zero-mean external field. Exploiting the inversion mentioned above, we show how to design pulsed field sequences that make particles move against the mean field (an example of negative mobility). Finally, we demonstrate that it is possible to use pulsed fields to make particles of different sizes move in opposite directions, even though their charge have the samesign.

Gain scheduling PID control for directed self‐assembly of colloidal particles in microfluidic devices

AbstractDirected self‐assembly provides a promising route to fabricate small‐scale structures for various engineering applications. The use of the external fields of variable strength and frequency allows for active controls to be implemented. For assembling specific structural features, precise control over a local particle density is often needed, which can be achieved using feedback control. However, the strong nonlinear behavior of directed self‐assembly complicates such control. In this work, a gain‐scheduling feedback control strategy for directed self‐assembly of colloidal particles is presented. The process gain is described by an empirical steady‐state model, which is used for scheduling a proportional‐integral feedback controller. The automated controller is implemented experimentally in a microfluidic device for directed self‐assembly of colloidal particles. The gain‐scheduled controller shows a significantly improved dynamic performance compared with a conventional proportional‐integral controller over a broad range of operating parameters.

Processing code-multiplexed Coulter signals via deep convolutional neural networks.

Beyond their conventional use of counting and sizing particles, Coulter sensors can be used to spatially track suspended particles, with multiple sensors distributed over a microfluidic chip. Code-multiplexing of Coulter sensors allows such integration to be implemented with simple hardware but requires advanced signal processing to extract multi-dimensional information from the output waveform. In this work, we couple deep learning-based signal analysis with microfluidic code-multiplexed Coulter sensor networks. Specifically, we train convolutional neural networks to analyze Coulter waveforms not only to recognize certain sensor waveform patterns but also to resolve interferences among them. Our technology predicts the size, speed, and location of each detected particle. We show that the algorithm yields a >90% pattern recognition accuracy for distinguishing non-correlated waveform patterns at a processing speed that can potentially enable real-time microfluidic assays. Furthermore, once trained, the algorithm can readily be applied for processing electrical data from other microfluidic devices integrated with the same Coulter sensor network.

Feedback control for shaping density distributions of colloidal particles in microfluidic devices.

Directed self-assembly has great potential for the precise manufacture of structured materials at the micro/nano-scale. A local particle density often has to be controlled to make the assembly of complicated structures with no defects attainable. However, the control of spatial particle density distributions is challenged by the need for multiple actuators, kinetic trapping and the stochastic nature of self-assembly systems. In this paper, a novel feedback control approach for shaping spatial density distributions of colloidal particles is presented. The control objective is to maintain the ratio of the particle densities of two adjacent regions close to a desired value. A microfluidic device with a triple-parallel microelectrode is fabricated to provide multiple actuators for particle manipulation. The multiple-electrode actuators can be operated flexibly to either direct particles between two adjacent regions or to maintain particles within regions by preventing undesired particle movements. A feedback control scheme is implemented to control the density ratio over a broad range of tested set points. The method is generic and can be extended to include additional parallel electrodes for the control of density distributions at higher resolutions due to a modular design.

Tunable, Sheathless Focusing of Diamagnetic Particles in Ferrofluid Microflows with a Single Set of Overhead Permanent Magnets.

There has been increasing interest in the use of magnetic fluids to manipulate diamagnetic particles in microfluidic devices. Current methods for diamagnetic-particle focusing in magnetic fluids require either a pair of repulsive magnets or a diamagnetic sheath flow. We demonstrate herein a tunable, sheathless focusing of diamagnetic particles in a microchannel ferrofluid flow with a single set of overhead permanent magnets. Particles are focused into a single stream near the bottom wall of a straight rectangular microchannel, where a magnetic-field minimum is formed as a result of the magnetization of the ferrofluid. This focusing can be readily switched off and on by removing and replacing the permanent magnets. More importantly, the particle-focusing position can be tuned by shifting the magnets with respect to the microchannel. We perform a systematic experimental study of the parametric effects of the fluid-particle-channel system on diamagnetic-particle focusing in terms of a defined particle-focusing effectiveness.

Acoustic Manipulation of Bio-Particles at High Frequencies: An Analytical and Simulation Approach.

Manipulation of micro and nano particles in microfluidic devices with high resolution is a challenge especially in bioengineering applications where bio-particles (BPs) are separated or patterned. While acoustic forces have been used to control the position of BPs, its theoretical aspects need further investigation particularly for high-resolution manipulation where the wavelength and particle size are comparable. In this study, we used a finite element method (FEM) to amend analytical calculations of acoustic radiation force (ARF) arising from an imposed standing ultrasound field. First, an acoustic solid interaction (ASI) approach was implemented to calculate the ARF exerted on BPs and resultant deformation induced to them. The results were then used to derive a revised expression for the ARF beyond the small particle assumption. The expression was further assessed in numerical simulations of one- and multi-directional standing acoustic waves (SAWs). Furthermore, a particle tracing scheme was used to investigate the effect of actual ARF on separation and patterning applications under experimentally-relevant conditions. The results demonstrated a significant mismatch between the actual force and previous analytical predictions especially for high frequencies of manipulation. This deviation found to be not only because of the shifted ARF values but also due to the variation in force maps in multidirectional wave propagation. Findings of this work can tackle the simulation limitations for spatiotemporal control of BPs using a high resolution acoustic actuation.

Simultaneous Separation and Washing of Nonmagnetic Particles in an Inertial Ferrofluid/Water Coflow.

Magnetic fluids (e.g., paramagnetic solutions and ferrofluids) have been increasingly used for label-free separation of nonmagnetic particles in microfluidic devices. Their biocompatibility, however, becomes a concern in high-throughput or large-volume applications. One way to potentially resolve this issue is resuspending the particles that are separated in a magnetic fluid immediately into a biocompatible buffer. We demonstrate herein the proof-of-principle of the first integration of negative magnetophoresis and inertial focusing for a simultaneous separation and washing of nonmagnetic particles in coflowing ferrofluid and water streams. The two operations take place in parallel in a simple T-shaped rectangular microchannel with a nearby permanent magnet. We find that the larger and smaller particles' exiting positions (and hence their separation distance) in the sheath water and ferrofluid suspension, respectively, vary with the total flow rate or the flow rate ratio between the two streams.

Cell agglomeration in the wells of a 24-well plate using acoustic streaming

Cell agglomeration is essential both to the success of drug testing and to the development of tissue engineering. Here, a MHz-order acoustic wave is used to generate acoustic streaming in the wells of a 24-well plate to drive particle and cell agglomeration. Acoustic streaming is known to manipulate particles in microfluidic devices, and even provide concentration in sessile droplets, but concentration of particles or cells in individual wells has never been shown, principally due to the drag present along the periphery of the fluid in such a well. The agglomeration time for a range of particle sizes suggests that shear-induced migration plays an important role in the agglomeration process. Particles with a diameter of 45 μm agglomerated into a suspended pellet under exposure to 2.134 MHz acoustic waves at 1.5 W in 30 s. Additionally, BT-474 cells also agglomerated as adherent masses at the center bottom of the wells of tissue-culture treated 24-well plates. By switching to low cell binding 24-well plates, the BT-474 cells formed suspended agglomerations that appeared to be spheroids, fully fifteen times larger than any cell agglomerates without the acoustic streaming. In either case, the viability and proliferation of the cells were maintained despite acoustic irradiation and streaming. Intermittent excitation was effective in avoiding temperature excursions, consuming only 75 mW per well on average, presenting a convenient means to form fully three-dimensional cellular masses potentially useful for tissue, cancer, and drug research.

Rolling Spheres on Bioinspired Microstructured Surfaces.

Microstructured surfaces, such as those inspired by nature, mediate surface interactions and are actively sought after to control wetting, adhesion, and friction. In particular, the rolling motion of spheres on microstructured surfaces in fluid environments is important for the transport of particles in microfluidic devices or in tribology. Here, we characterize the motion of smooth silicon nitride spheres (diameters 3-5 mm) as they roll down inclined planes decorated with hexagonal arrays of microwells and micropillars. For both types of patterned surfaces, we vary the area fraction of the micropatterned features from 0.04 to 0.96. We measure directly and independently the rotational and translational velocities of the spheres as they roll down planes with inclination angles that vary between 5 and 30°. For a given area fraction, we find that spheres have a higher translational and rotational velocity on surfaces with microwells than on micropillars. We rely on the model of Smart and Leighton [Phys. Fluids A 5, 13 (1993)] to obtain an effective gap width and coefficient of friction for all microstructured surfaces investigated. We find that the coefficient of friction is significantly higher for a surface with micropillars than that for one with microwells, likely due to the presence of interconnected drainage channels that provide additional paths for the fluid flow and favor solid-solid contact on the surface with micropillars. We find that while the effective gap width at a very low solid fraction is equal to the height of the patterned features, the effective separation decreases exponentially as the surface coverage of microstructures increases, with little measured differences between the two geometries. Superposition of resistance functions is used to relate the rapid decrease in the effective gap height with increase in the surface coverage observed in experiments.