Creation of single-particle environment by positive dielectrophoresis and liquid dielectrophoresis

Two electrical mechanisms for manipulating particles and fluids, dielectrophoresis (DEP) and liquid dielectrophoresis (LDEP), are integrated in a microfluidic chip for creating the single-particle environment. The fluid is activated by LDEP with a 100-kHz/240-V_pp signal. When the single polystyrene bead approaches the trapping area, positive DEP force is utilized to capture and immobilize the bead. After trapping the bead, the process of liquid cutting and droplet creation is employed to create a droplet containing a single bead by LDEP with a 100-kHz/320-V_pp signal.     

Droplet creation using liquid dielectrophoresis

Dielectrophoresis (DEP) is defined as polarizable particles moving into regions of higher electric field intensity. In liquid DEP (LDEP), a dielectric liquid tends to flow toward regions of high electric field intensity under a non-uniform electric field. This work presents a theoretical model of LDEP based on parallel electrodes. The LDEP force is derived using the lump parameter electromechanical method. The relationship between the minimum actuation voltage and the electrode width is investigated experimentally and theoretically. We also propose a method for creating a 20 nl droplet of deionized water using LDEP. The creation of a water droplet containing 15 μm polystyrene beads is placed at the desired location from a continuous flow driven by LDEP using the developed method.     

Purification of a droplet using negative dielectrophoresis traps in digital microfluidics

The development of techniques for manipulating particles and integrating them into the digital microfluidic (DMF) devices has been the subject of several studies in recent years. This paper presents a dielectrophoretic-based method that uses triangular traps to manipulate particles and purify a droplet in DMF platforms. Numerical and experimental studies are conducted to show the effectiveness of the proposed trap geometry which is also compatible with the other operators in the DMF platform. The triangular trap geometry is used to move the polystyrene particles to one side of the trap using negative dielectrophoresis (nDEP). The droplet is then split into two smaller droplets with very low and high concentrations of particles using the electrowetting on dielectric technique. The average velocity of the particles (as they move along the trap) as a function of the vertex angle of the triangular trap and the gap between the top and bottom plate is examined. It is observed that the vertex angle of the trap plays more important role on the motion of the particles than the gap. Thus, to enhance the motion of the particles and minimize the effect of splitting on the purification process, the vertex angle and the slope of the side arms of the triangular trap are modified based on the results of the numerical model simulating the dielectrophoretic force on the particle. The enhanced geometry is fabricated and tested experimentally to show the effectiveness and ease-of-use of the proposed technique in purifying (or concentrating) a droplet in DMF. The results show that using the proposed nDEP electrode geometry purification (or concentration) can be performed with the efficiency of 90 %.     

Performances of a broad range of dielectric stacks for liquid dielectrophoresis transduction

Among digital microfluidic techniques, liquid dielectrophoresis (LDEP) is well adapted to displace insulating liquids. One of the current challenges for LDEP concerns the robustness of both the dielectric and hydrophobic coatings (deposited atop the driving electrodes). Indeed, such layers may be exposed to high electric field, during operation. There is a need to optimize this stack of insulating layers to first prevent from their dielectric breakdown, secondly reduce the actuation voltage, and lastly ensure a reproducible and well-controlled droplet-generation process. For the first time, an extensive study is presented in that paper, comparing the performances of more than twenty different dielectric stacks (including SiN, High-K materials, hydrophobic coatings) from micro–nanoelectronics know-how and implemented onto a given LDEP design. This generic design features lateral bumps regularly spaced across coplanar electrodes to generate an array of 30 pL DI water droplets in a single open-plate architecture. The experiments have been carefully analyzed to identify which are the best stacks in terms of efficiency and quality for the LDEP transduction. As a result to that study, we propose a guideline to adjust the dielectric coating properties (thickness, material) depending on the liquids to displace and targeted applications.     

Time-resolved electrochemical measurement device for microscopic liquid interfaces during droplet formation

An electrochemical measurement system with a high-speed camera for observation of dynamic behavior of ionic molecules at a water-in-oil interface during microfluidic droplet formation is described. In order to demonstrate the usefulness of the system, a liquid interface between 1 M sodium chloride aqueous solution and 0.02 M tetrabutylammonium tetraphenylborate 1,2-dichloroethane solution was investigated. During aqueous droplet formation in a microfluidic device, averaged and dynamic currents between the two phases were measured under potential control. The measured current corresponded to the transport of electrolyte ions to form the electrical double layer at the liquid interface. When an 18-μm-sized droplet was formed in each 1.2 ms, the amount of charge on each droplet was measured to be 20 pC at 0.4 V and negligible at the potential of zero charge (0.19 V). In addition, the high-speed camera observations revealed that the charge affects the stability of the droplet during and/or just after the generation process. This measurement system is expected to facilitate a fuller understanding of the droplet formation process.     

System-level modeling and simulation of biochemical assays in lab-on-a-chip devices

We present a “mixed-methodology” based system-level modeling and simulation for biochemical assays in lab-on-a-chip (LoC) devices. The methodology uses a combination of numerical schemes and analytical approaches to simulate biological and physicochemical processes, specifically, an integral approach for fluid flow and electric field, method of lines (MOL) and two-compartment models for biochemical reactions, and Fourier series-based model for analyte mixing. The solution procedure begins with decomposing the LoC device into a system of inter-connected components (e.g., channels and junctions) and the models are solved in a network fashion. Models are developed to accurately capture the multi-physics (e.g., flow, mixing, and reaction) behavior of individual components. The assembly of the components is facilitated via exchange of fluid flux and Fourier series coefficients (or average concentration) of analytes between various components, which enables network solution of the models. The system models are validated against both experimental and numerical models on various biochemical assays (e.g. immunoassays and enzymatic reactions), showing significant computational speedup (100–10,000-fold depending on the assay) without appreciably compromising accuracy (<10% error relative to numerical analysis).     

Development of temperature-control system for liquid droplet using surface Acoustic wave devices

In this paper, we present a liquid-droplet-heating system using a surface acoustic wave (SAW) device. When liquid is placed on a Rayleigh-SAW-propagating surface, a longitudinal wave is radiated into the liquid. If the SAW amplitude increases, the liquid shows non-linear dynamics, such as vibrating, streaming, small droplet flying, and atomizing. This phenomenon is well known as SAW streaming. The liquid temperature is measured during the longitudinal wave radiation and found to increase. First, the mechanism of the liquid-heating effect is discussed on the basis of experimental results. The surface electrical condition is changed to investigate the effect of dielectric heating. The obtained results indicate that the radiated longitudinal wave causes liquid heating and the dielectric heating effect does not. Second, the fundamental properties of the liquid temperature are measured by varying the applied voltage, duty factor, and liquid viscosity. The liquid temperature is found to be proportional to the duty factor and the square of the applied voltage. Therefore, the liquid temperature can be controlled by these applied signals. Also, by using highly viscous solutions, the liquid temperature is increased to more than 100 °C. Moreover, for chemical applications, the possibility of periodic temperature control is tested by varying the duty factor. The obtained results strongly suggest that an efficient thermal cycler is realized. A novel application of the SAW device is proposed on the basis of SAW streaming.     

Numerical simulation of droplet impacting liquid surface by SPH

Droplet impacting liquid surface is not only the extremely prevalent phenomenon in the nature and industrial production but also the extremely complicated problem of strong non-linear transient impact and free-surface flow. On the basis of the two-dimensional viscous incompressible N-S equations, this paper conducts a study of numerical simulation on the problem of droplet impacting liquid surface (water beads) of water container in certain initial velocity by the method of SPH (smoothed particle hydrodynam...     

Modelling droplet motion and interfacial tension in filters collecting liquid aerosols

Extensive experimental investigations have shown some of the differences between the behaviours of the barrel and the clamshell shapes of droplets on filter fibres in flow fields. Realistic flow velocities (such as those used in many industrial filter systems) were utilised. The forces acting are air drag, interfacial tension and gravity. The properties of the interfacial restoring force are modelled, and show agreement with the experimental results, at least in the linear extension region before the onset of oscillatory behaviour of the droplets (induced by instability of the flow field). The model for the oscillatory behaviour is explored, and the natural frequencies of oscillation in the radial and transverse directions are shown to be the same, for the barrel shape. The clamshell shape also has the same natural frequencies, but they are different to those of the barrel shape. The coupling of the radial and transverse oscillation modes is explored for both the barrel and clamshell shape. Some contact angle results are given, both without airflow acting on the droplet and with increasing airflow.     

Free-floating amphiphilic picoliter droplet carriers for multiplexed liquid loading in a microfluidic channel

We present free-floating amphiphilic picoliter microcarriers for multiplexed loading in a microfluidic device. The amphiphilic microcarrier is composed of encoded hydrophobic hexagonal outer structure and hydrophilic inner structure. We fabricate these free-floating droplet carriers and assemble them in a microfluidic device for a demonstration of multiplexed liquid loading. Picoliter loading is performed by serial solution exchange of aqueous and oil phase solution. We are able to precisely adjust the loaded volume by varying the diameter and depth of the microcarrier. We also fabricate arbitrary shaped microwells and load picoliter droplets into?them. A microbead suspension is also used to demonstrate mixing via continuous oil flow. Further development of this work may be applicable to high-throughput multiplexed assays using quantized liquid loading in a microfluidic environment.     

SU-8 microchannels for live cell dielectrophoresis improvements

In this work a novel highly precise SU-8 fabrication technology is employed to construct microfluidic devices for sensitive dielectrophoretic (DEP) manipulation of budding yeast cells. A benchmark microfluidic live cell sorting system is presented, and the effect of microchannel misalignment above electrode topologies on live cell DEP is discussed in detail. Simplified model of budding Saccharomyces cerevisiae yeast cell is presented and validated experimentally in fabricated microfluidic devices. A novel fabrication process enabling rapid prototyping of microfluidic devices with well-aligned integrated electrodes is presented and the process flow is described. Identical devices were produced with standard soft-lithography processes. In comparison to standard PDMS based soft-lithography, an SU-8 layer was used to construct the microchannel walls sealed by a flat sheet of PDMS to obtain the microfluidic channels. Direct bonding of PDMS to SU-8 surface was achieved by efficient wet chemical silanization combined with oxygen plasma treatment of the contact surface. The presented fabrication process significantly improved the alignment of the microstructures. While, according to the benchmark study, the standard PDMS procedure fell well outside the range required for reasonable cell sorting efficiency. In addition, PDMS delamination above electrode topologies was significantly decreased over standard soft-lithography devices. The fabrication time and costs of the proposed methodology were found to be roughly the same.

     

A centrifugal force-based serpentine micromixer (CSM) on a plastic lab-on-a-disk for biochemical assays

In this paper, a centrifugal force-based serpentine micromixer (CSM) on a plastic lab-on-a-disk (LOD) for biochemical assay was designed, fabricated, and fully characterized with numerical and experimental methods. The CSM comprised two inlets, an outlet, and a serpentine microchannel composed of five circumferential channels with connecting radial channels in one layer. The centrifugal force induced in the rotating disk thoroughly mixed the sample and reagent together throughout the serpentine microchannel of the CSM. Despite its simple geometry, effective mixing performance was achieved inside the CSM because of transverse secondary flows and the three-dimensional stirring effect in the microchannel. Numerical simulation showed that the interfaces of the two streams inside the circumferential microchannel were efficiently stirred by the induced transversal velocity field. The plastic LOD was fabricated by CNC-micromilling on one layer of the thermoplastic substrate, followed by thermal bonding with a cover plastic substrate. Mixing performance of the CSM was also investigated experimentally by means of colorimetric analysis using phenolphthalein. High levels of distributive mixings were obtained within a short required mixing length. As a proof-of-concept example, a biochemical assay of albumin level was successfully determined with the help of the LOD containing the CSM. Owing to the mass-producible simple geometry, excellent mixing performance, and convenience, the CSM can be applied to biochemical assays based on the centrifugal microfluidics.     

Bioparticle separation and manipulation using dielectrophoresis

Manipulation and separation of micro-sized particles, particularly biological particles, using the dielectrophoretic (DEP) force is an emerging technique in MEMS technology. This paper presents a DEP-based microsystem for the selective manipulation and separation of bioparticles using dielectrophoretic effects. The microfabricated DEP device consists of a sandwich structure, in which a microchannel with electrode array lining on its bottom is sandwiched between the substrate and the glass lid. Dielectrophoretic behavior of polystyrene particles with diameter of 4.3 μm was studied. Both positive DEP and negative DEP were observed. Particles under positive DEP were attracted to the edges of the electrodes, while those under negative DEP were repelled away from the electrodes and levitated at certain height above the electrodes (within a proper range of frequencies of the electric field). Levitation height of the particles was measured. It was demonstrated that the levitation height of a specific particle strongly depends on the combined contributions of a number of parameters, such as the frequency of the electric field, dielectric properties of the particles and the surrounding medium. Different particles can be separated and manipulated on the basis of their difference in these parameters.     

Flow resistance of a liquid droplet confined between two hydrophobic surfaces

Flow resistance of a liquid droplet confined between two hydrophobic surfaces has been investigated experimentally and factors contributing to the flow resistance have been studied. A water droplet has been sheared between two hydrophobic surfaces and shear resistance has been measured. The experimental results show that the shear resistance at low shear velocities is primarily caused by asymmetrical surface tension due to the contact angle hysteresis. A droplet on a rough hydrophobic surface remains almost symmetrical under shear and exhibits extremely low friction. The shear resistance at high shear velocities is affected by viscous force. Furthermore, sliding angles of water droplets on micropatterned hydrophobic surfaces have been measured to clarify the effects of surface topography on flow resistance. Surfaces with many prismatic structures raised out of the plane tend to exhibit low sliding angles.     

Characterization and modeling of a microfluidic dielectrophoresis filter for biological species

Microfabricated interdigitated electrode array is a convenient form of electrode geometry for dielectrophoretic trapping of particles and biological entities such as cells and bacteria within microfluidic biochips. We present experimental results and finite element modeling of the holding forces for both positive and negative dielectrophoretic traps on microfabricated interdigitated electrodes within a microfluidic biochip fabricated in silicon with a 12-/spl mu/m-deep chamber. Anodic bonding was used to close the channels with a glass cover. An Experimental protocol was then used to measure the voltages necessary to capture different particles (polystyrene beads, yeast cells, spores and bacteria) against destabilizing fluid flows at a given frequency. The experimental results and those from modeling are found to be in close agreement, validating our ability to model the dielectrophoretic filter for bacteria, spores, yeast cells, and polystyrene beads. This knowledge can be very useful in designing and operating a dielectrophoretic barrier or filter to sort and select particles entering the microfluidic devices for further analysis.     

Monitoring of droplet growth with nano-litre resolution for liquid flow rate, level or surface tension measurement

The determination of small liquid volumes and their changes over a period of time is a measuring problem with increasing importance considering the growing market of micro-systems. In this paper, experimental investigations are described which aim to observe the growth of droplets at the end of a micro-channel, such as a cannula or capillary. The main focus of the work lies on the calculation of flow rates below 1 ml/h. Furthermore, the potential of the measuring equipment for liquid level and surface tension measurement is described.     

Numerical simulation of oscillations and rotations of a free liquid droplet using the level set method

Three-dimensional oscillations and rotations of a free liquid droplet are simulated numerically using the level set method. The oscillations of order 2, 3 and 4 are simulated, and the frequency and the damping for small-amplitude oscillations are shown to agree well with those by the linear theory. Flow fields in the inside and outside of the droplet are visualized, and three-dimensional vortex structures are found to appear around the droplet. It is also found that the number of vortices is the same as the order of oscillation. The effects of initial amplitude and rotation on the oscillation frequency are studied, and it is shown that the oscillation frequency decreases as the initial amplitude increases, while it increases as the rotation rate increases. These effects are found to be overestimated by theoretical predictions.     

Real-time droplet caliper for digital microfluidics

We developed an optical, microfabrication-free approach for performing real-time measurements of individual droplet characteristics (frequency of production, velocity, and length) flowing in a transparent microfluidic channel. Our approach consists in an interpretation of the differential signal produced by a pair of photodiodes connected head-to-tail due to the variations of illumination at the passage of a droplet. We checked the relevance of this zero-background method by comparing results to video measurements, and observed a very good agreement at rates up to the kHz range. Moreover, since the measured values are stored in a simple text file, flow characterization over very long times (several hours) becomes accessible. We applied this facility to perform three examples of long-term studies: stationary regimes, transient regimes, and the effect of an external forcing. Several unexpected features, like long-period fluctuations, can thus be evidenced.