A brief introduction to slippage, droplets and mixing in microfluidic systems

Optical measurement of the picosecond fluid mechanics in simple liquids generated by vibrating nanoparticles: a review

Standard continuum assumptions commonly used to describe the fluid mechanics of simple liquids have the potential to break down when considering flows at the nanometer scale. Two common assumptions for simple molecular liquids are that (1) they exhibit a Newtonian response, where the viscosity uniquely specifies the linear relationship between the stress and strain rate, and (2) the liquid moves in tandem with the solid at any solid-liquid interface, known as the no-slip condition. However, even simple molecular liquids can exhibit a non-Newtonian, viscoelastic response at the picosecond time scales that are characteristic of the motion of many nanoscale objects; this viscoelasticity arises because these time scales can be comparable to those of molecular relaxation in the liquid. In addition, even liquids that wet solid surfaces can exhibit nanometer-scale slip at those surfaces. It has recently become possible to interrogate the viscoelastic response of simple liquids and associated nanoscale slip using optical measurements of the mechanical vibrations of metal nanoparticles. Plasmon resonances in metal nanoparticles provide strong optical signals that can be accessed by several spectroscopies, most notably ultrafast transient-absorption spectroscopy. These spectroscopies have been used to measure the frequency and damping rate of acoustic oscillations in the nanoparticles, providing quantitative information about mechanical coupling and exchange of mechanical energy between the solid particle and its surrounding liquid. This information, in turn, has been used to elucidate the rheology of viscoelastic simple liquids at the nanoscale in terms of their constitutive relations, taking into account separate viscoelastic responses for both shear and compressible flows. The nanoparticle vibrations have also been used to provide quantitative measurements of slip lengths on the single-nanometer scale. Viscoelasticity has been shown to amplify nanoscale slip, illustrating the interplay between different aspects of the unconventional fluid dynamics of simple liquids at nanometer length scales and picosecond time scales.

Viscoelasticity Enhances Nanometer-Scale Slip in Gigahertz-Frequency Liquid Flows

The interaction between flowing liquids and solid surfaces underpins many physical phenomena and technologies, such as the ability of an airfoil to generate lift and the mixing of liquids for industrial applications. These phenomena are often described using the Navier-Stokes equations and the no-slip boundary condition: the assumption that the liquid immediately adjacent to a solid surface does not move relative to the surface. Herein, we observe violation of the no-slip condition with strong enhancement of slip due to intrinsic viscoelasticity of the bulk liquid. This is achieved by measuring the 20 GHz acoustic vibrations of gold nanoparticles in glycerol/water mixtures, for which the underlying physics is explored using rigorous, theoretical models. The reported enhancement of slip revises current understanding of ultrafast liquid flows, with implications for technologies ranging from membrane filtration to nanofluidic devices and biomolecular sensing.

Quantifying Frictional Drag Reduction Properties of Superhydrophobic Metal Oxide Nanostructures

We measure the frictional drag-reducing property of various superhydrophobic metal oxide nanostructures by quantifying their effective slip length. Scalable chemical methods tailored to each metal substrate are applied to grow oxide nanostructures on copper (Cu), aluminum (Al), and titanium (Ti), respectively. In particular, three different types of oxide nanostructures are grown on the titanium substrate by changing the chemical composition to investigate the morphological influence on the slip length. Microchannels containing metal oxide nanostructures are fabricated based on the microfluidic sticker method, while the slip length is unambiguously determined by measuring the ratio of the volume flow rate over the superhydrophobic surface to that over the flat surface simultaneously. The slip length is measured to be 6.8 ± 1.4 μm on Cu nanostructures, while it is measured to be 2.5 ± 0.6 μm on Al nanostructures. For Ti nanostructures, the measured slip lengths range from 1 to 2.5 ± 0.5 μm, where they increase proportionally with the structural pitch of the nanostructures, agreeing with the theoretical predictions. We believe that our results will be useful in applying scalable low-cost metal oxide nanostructures to underwater applications by providing their frictional characteristics.

Fibrin Nanoparticles Coupled with Keratinocyte Growth Factor Enhance the Dermal Wound-Healing Rate

Expediting the wound-healing process is critical for patients chronically ill from nonhealing wounds and diseases such as hemophilia or diabetes or who have suffered trauma including easily infected open wounds. FDA-approved external tissue sealants include the topical application of fibrin gels, which can be 500 times denser than natural fibrin clots. With lower clot porosity and higher polymerization rates than physiologically formed fibrin clots, the commercial gels quickly stop blood loss but impede the later clot degradation kinetics and thus retard tissue-healing rates. The fibrin nanoparticles (FBNs) described here are constructed from physiologically relevant fibrin concentrations that support new tissue and dermal wound scaffold formation when coupled with growth factors. The FBNs, synthesized in a microfluidic droplet generator, support cell adhesion and traction generation, and when coupled to keratinocyte growth factor (KGF), support cell migration and in vivo wound healing. The FBN-KGF particles enhance cell migration in vitro greater than FBN alone or free KGF and also improve healing outcomes in a murine full thickness injury model compared to saline, bulk fibrin sealant, free KGF, or bulk fibrin mixed with KGF treatments. Furthermore, FBN can be potentially administered with other tissue-healing factors and inflammatory mediators to improve wound-healing outcomes.

A droplet microfluidic platform for efficient enzymatic chromatin digestion enables robust determination of nucleosome positioning

The first step in chromatin-based epigenetic assays involves the fragmentation of chromatin to facilitate precise genomic localization of the associated DNA. Here, we report the development of a droplet microfluidic device that can rapidly and efficiently digest chromatin into single nucleosomes starting from whole-cell input material offering simplified and automated processing compared to conventional manual preparation. We demonstrate the digestion of chromatin from 2500-125 000 Jurkat cells using micrococcal nuclease for enzymatic processing. We show that the yield of mononucleosomal DNA can be optimized by controlling enzyme concentration and incubation time, with resulting mononucleosome yields exceeding 80%. Bioinformatic analysis of sequenced mononucleosomal DNA (MNase-seq) indicated a high degree of reproducibility and concordance (97-99%) compared with conventionally processed preparations. Our results demonstrate the feasibility of robust and automated nucleosome preparation using a droplet microfluidic platform for nucleosome positioning and downstream epigenomic assays.

Accelerated Transport of Particles in Confined Channels with a High Roughness Amplitude

We investigate the pressure-driven transport of particles 200 or 300 nm in diameter in shallow microfluidic channels ∼1 μm in height with a bottom wall characterized by a high roughness amplitude of ∼100 nm. This study starts with the description of an assay to generate cracks in hydrophilic thin polymer films together with a structural characterization of these corrugations. Microfluidic chips of variable height are then assembled on top of these rough surfaces, and the transport of particles is assessed by measuring the velocity distribution function for a set of pressure drops. We specifically detect anomalous transport properties for rough surfaces. The maximum particle velocity at the centerline of the channel is comparable to that obtained with smooth surfaces, but the average particle velocity increases nonlinearly with the flow rate. We suggest that the change in the boundary condition at the rough wall is not sufficient to account for our data and that the occurrence of contacts between the particle and the surface transports the particle away from the wall and speeds up its motion. We finally draw perspectives for the separation by field-flow fractionation.

Controlling states of water droplets on nanostructured surfaces by design

Surfaces that exhibit both superhydrophobic and superoleophobic properties have recently been demonstrated. Specifically, remarkable designs based on overhanging/inverse-trapezoidal microstructures enable water droplets to contact these surfaces only at the tips of the micro-pillars, in a state known as the Cassie state. However, the Cassie state may transition into the undesirable Wenzel state under certain conditions. Herein, we show from large-scale molecular dynamics simulations that the transition between the Cassie and Wenzel states can be controlled via precisely designed trapezoidal nanostructures on a surface. Both the base angle of the trapezoids and the intrinsic contact angle of the surface can be exploited to control the transition. For a given base angle, three regimes can be achieved: the Wenzel regime, in which water droplets can exist only in the Wenzel state when the intrinsic contact angle is less than a certain critical value; the Cassie regime, in which water droplets can exist only in the Cassie state when the intrinsic contact angle is greater than another critical value; and the bistable Wenzel-Cassie regime, in which both the Wenzel and Cassie states can exist when the intrinsic contact angle is between the two critical values. A strong base-angle dependence of the first critical value is revealed, whereas the second critical value shows much less dependence on the base angle. The stability of the Cassie state for various base angles (and intrinsic contact angles) is quantitatively evaluated by computing the free-energy barrier for the Cassie-to-Wenzel state transition.

Two-dimensional flow of driven particles: a microfluidic pathway to the non-equilibrium frontier

We discuss the basic physics of the flow of micron-scale droplets in 2D geometry.

The importance of chemical potential in the determination of water slip in nanochannels

We investigate the slip properties of water confined in graphite-like nanochannels by non-equilibrium molecular dynamics simulations, with the aim of identifying and analyze separately the influence of different physical quantities on the slip length. In a system under confinement but connected to a reservoir of fluid, the chemical potential is the natural control parameter: we show that two nanochannels characterized by the same macroscopic contact angle--but a different microscopic surface potential--do not exhibit the same slip length unless the chemical potential of water in the two channels is matched. Some methodological issues related to the preparation of samples for the comparative analysis in confined geometries are also discussed.

Mixing with herringbone-inspired microstructures: overcoming the diffusion limit in co-laminar microfluidic devices

Enhancing mixing is of uttermost importance in many laminar microfluidic devices, aiming at overcoming the severe performance limitation of species transport by diffusion alone. Here we focus on the significant category of microscale co-laminar flows encountered in membraneless redox flow cells for power delivery. The grand challenge is to achieve simultaneously convective mixing within each individual reactant, to thin the reaction depletion boundary layers, while maintaining separation of the co-flowing reactants, despite the absence of a membrane. The concept presented here achieves this goal with the help of optimized herringbone flow promoting microstructures with an integrated separation zone. Our electrochemical experiments using a model redox couple show that symmetric flow promoter designs exhibit laminar to turbulent flow behavior, the latter at elevated flow rates. This change in flow regime is accompanied by a significant change in scaling of the Sherwood number with respect to the Reynolds number from Sh ~ Re(0.29) to Sh ~ Re(0.58). The stabilized continuous laminar flow zone along the centerline of the channel allows operation in a co-laminar flow regime up to Re ~325 as we demonstrate by micro laser-induced fluorescence (μLIF) measurements. Micro particle image velocimetry (μPIV) proves the maintenance of a stratified flow along the centerline, mitigating reactant cross-over effectively. The present work paves the way toward improved performance in membraneless microfluidic flow cells for electrochemical energy conversion.

Droplet-based microfluidics at the femtolitre scale

We have built a toolbox of modules for droplet-based microfluidic operations on femtolitre volume droplets. We have demonstrated monodisperse production, sorting, coalescence, splitting, mixing, off-chip incubation and re-injection at high frequencies (up to 3 kHz). We describe the constraints and limitations under which satisfactory performances are obtained, and discuss the physics that controls each operation. For some operations, such as internal mixing, we obtained outstanding performances: for instance, in 75 fL droplets the mixing time was 45 μs, 35-fold faster than previously reported for a droplet microreactor. In practice, in all cases, a level of control comparable to nanolitre or picolitre droplet manipulation was obtained despite the 3 to 6 order of magnitude reduction in droplet volume. Remarkably, all the operations were performed using devices made using standard soft-lithography techniques and PDMS rapid prototyping. We show that femtolitre droplets can be used as microreactors for molecular biology with volumes one billion times smaller than conventional microtitre plate wells: in particular, the Polymerase Chain Reaction (PCR) was shown to work efficiently in 20 fL droplets.

Recent progress in the physics of microfluidics and related biotechnological applications

Since the mid-nineties, the physical understanding of microfluidic flows has reached a level sufficiently elaborate for envisaging applications in all sorts of domains. As the domain expanded, the existence of new situations where fluid dynamics at small or moderate Reynolds numbers combines with confinement, interfaces, transport, particles along with disordered substrates raised new challenges. The present review is restricted to three domains in which progress in the physical description has been made recently (droplet-based, inertial and paper-based microfluidics) and for which biotechnological applications are foreseeable.

Dielectrophoresis based continuous-flow nano sorter: fast quality control of gene vaccines

We present a prototype nanofluidic device, developed for the continuous-flow dielectrophoretic (DEP) fractionation, purification, and quality control of sample suspensions for gene vaccine production. The device consists of a cross injector, two operation regions, and separate outlets where the analytes are collected. In each DEP operation region, an inhomogeneous electric field is generated at a channel spanning insulating ridge. The samples are driven by ac and dc voltages that generate a dielectrophoretic potential at the ridge as well as (linear) electrokinetics. Since the DEP potential differs at the two ridges, probes of three and more species can be iteratively fully fractionated. We demonstrate the fast and efficient separation of parental plasmid, miniplasmid, and minicircle DNA, where the latter is applicable as a gene vaccine. Since the present technique is virtually label-free, it offers a fast purification and in-process quality control with low consumption, in parallel, for the production of gene vaccines.

A study of the production and reversible stability of EGaIn liquid metal microspheres using flow focusing

This manuscript describes an experimental study of the production of micro-scale droplets of the room-temperature liquid alloy eutectic gallium indium (EGaIn) formed using a microfluidic flow-focusing device. The EGaIn surface oxidizes readily to form a passivating oxide "skin" that imparts some mechanical stability to the resulting microspheres, but does not appear to affect the dynamics of droplet formation. EGaIn has an interfacial tension nearly an order of magnitude larger than typical water-in-oil systems that are used to study droplet production in microfluidic flow-focusing devices. The size of the microdroplets increase as the ratio of the flow rates of the dispersed and continuous-phase increase for both EGaIn-in-glycerol and water-in-oil systems; however, these fluid pairs form droplets through different dispersing modes at otherwise identical flow conditions (i.e., flow rate ratios and capillary numbers). Consequently, the EGaIn droplets are larger than the water droplets. The difference in dispersing modes and droplet size are attributed to the relatively larger interfacial and inertial forces of the EGaIn system compared to the water-in-oil system. The addition of polyvinyl alcohol (PVA), which is known to bind to oxide surfaces, to the continuous phase yields stable, monodisperse emulsions of liquid metal. These emulsions can be destabilized on demand by changing the solution pH, allowing the liquid metal to be recovered. The ability of the PVA to bind to the liquid metal also influences droplet production by changing the shape of the liquid as it approaches the orifice of the flow focusing device, which results in droplets with smaller diameters relative to those formed without PVA.

Optimized preparation of pDNA/poly(ethylene imine) polyplexes using a microfluidic system

Poly(ethylene imine) (PEI) is an established non-viral vector system for the delivery of various nucleic acids in gene therapy applications. Polyelectrolyte complexes between both compounds, so called polyplexes, are formed by electrostatic interactions of oppositely charged macromolecules and are thought to facilitate uptake into cells. Such complexes form spontaneously and on lab scale they are usually prepared by mixing solutions through pipetting. Hence, an optimized preparation procedure allowing the scale-up of well-defined polyplexes would be of general interest. We developed a new method for microfluidic polyplex preparation on a chip. The mixing behaviour within the microfluidic channels was evaluated. Polyplexes with PEI and plasmid DNA were prepared using this method, in comparison to the standard pipetting procedure. Sizes and polydispersity indices of these complexes were examined. The influence of various parameters on the polyplex characteristics and the suitability of this production procedure for other PEI-based complexes were also evaluated. It was shown that polyplexes could easily be prepared by microfluidics. The ratio of PEI to DNA was most important for the formation of small polyplexes, whereas other parameters had minor influence. The size of polyplexes prepared with this new method was observed to be relatively constant between 140 nm and 160 nm over a wide range of complex concentrations. In comparison, the size of polyplexes prepared by pipetting (approximately 90 nm to 160 nm) varied considerably. The versatility of this system was demonstrated with different (targeted) PEI-based vectors for the formation of complexes with pDNA and siRNA. In conclusion, polyplex preparation using microfluidics could be a promising alternative to the standard pipetting method due to its suitability for preparation of well-defined complexes with different compositions over a wide range of concentrations.

Digital microfluidics: a versatile tool for applications in chemistry, biology and medicine

Digital microfluidics (DMF) has recently emerged as a popular technology for a wide range of applications. In DMF, nanoliter to microliter droplets containing samples and reagents can be manipulated to carry out a range of discrete fluidic operations simply by applying a series of electrical potentials to an array of patterned electrodes coated with a hydrophobic insulator. DMF is distinct from microchannel-based fluidics as it allows for precise control over multiple reagent phases (liquids and solids) in heterogeneous systems with no need for complex networks of connections, microvalves, or pumps. In this review, we discuss the most recent developments in this technology with particular attention to the potential benefits and outstanding challenges for applications in chemistry, biology, and medicine.

Combining molecular dynamics simulation and transition state theory to evaluate solid-liquid interfacial friction in carbon nanotube membranes

A molecular dynamics (MD) methodology based on Eyring theory of reaction rates is proposed for investigating solid-liquid interfacial properties crucial to the development of many nanotechnology applications. The method involves the calculation of activation energy required for the flow process directly from the MD trajectory information. We have applied this methodology to study the behavior of water in hydrophobic confinement in carbon nanotubes (CNTs) and also between graphene sheets. In the case of confined water molecules in CNTs and between graphene sheets the degree of confinement and curvature effects were found to have more influence on the solid-liquid interfacial friction, with almost negligible friction below a certain characteristic dimension in both the cases. This behavior of confined and unconfined water molecules is explained on the basis of molecular interactions and subsequent changes in the activation energy. Analysis based on this method also revealed that a finite amount of friction does exist at the channel entry and exit region. This could limit the flow of liquid molecules through the nanochannels and hence needs to be taken into account in the design of nanofluidic devices.

Droplet based microfluidics

Droplet based microfluidics is a rapidly growing interdisciplinary field of research combining soft matter physics, biochemistry and microsystems engineering. Its applications range from fast analytical systems or the synthesis of advanced materials to protein crystallization and biological assays for living cells. Precise control of droplet volumes and reliable manipulation of individual droplets such as coalescence, mixing of their contents, and sorting in combination with fast analysis tools allow us to perform chemical reactions inside the droplets under defined conditions. In this paper, we will review available drop generation and manipulation techniques. The main focus of this review is not to be comprehensive and explain all techniques in great detail but to identify and shed light on similarities and underlying physical principles. Since geometry and wetting properties of the microfluidic channels are crucial factors for droplet generation, we also briefly describe typical device fabrication methods in droplet based microfluidics. Examples of applications and reaction schemes which rely on the discussed manipulation techniques are also presented, such as the fabrication of special materials and biophysical experiments.

Biosensing with quantum dots: a microfluidic approach

Semiconductor quantum dots (QDs) have served as the basis for signal development in a variety of biosensing technologies and in applications using bioprobes. The use of QDs as physical platforms to develop biosensors and bioprobes has attracted considerable interest. This is largely due to the unique optical properties of QDs that make them excellent choices as donors in fluorescence resonance energy transfer (FRET) and well suited for optical multiplexing. The large majority of QD-based bioprobe and biosensing technologies that have been described operate in bulk solution environments, where selective binding events at the surface of QDs are often associated with relatively long periods to reach a steady-state signal. An alternative approach to the design of biosensor architectures may be provided by a microfluidic system (MFS). A MFS is able to integrate chemical and biological processes into a single platform and allows for manipulation of flow conditions to achieve, by sample transport and mixing, reaction rates that are not entirely diffusion controlled. Integrating assays in a MFS provides numerous additional advantages, which include the use of very small amounts of reagents and samples, possible sample processing before detection, ultra-high sensitivity, high throughput, short analysis time, and in situ monitoring. Herein, a comprehensive review is provided that addresses the key concepts and applications of QD-based microfluidic biosensors with an added emphasis on how this combination of technologies provides for innovations in bioassay designs. Examples from the literature are used to highlight the many advantages of biosensing in a MFS and illustrate the versatility that such a platform offers in the design strategy.

Microbubble generation in a co-flow device operated in a new regime

A new regime of operation of PDMS-based flow-focusing microfluidic devices is presented. We show that monodisperse microbubbles with diameters below one-tenth of the channel width (here w = 50 μm) can be produced in low viscosity liquids thanks to a strong pressure gradient in the entrance region of the channel. In this new regime bubbles are generated at the tip of a long and stable gas ligament whose diameter, which can be varied by tuning appropriately the gas and liquid flow rates, is substantially smaller than the channel width. Through this procedure the volume of the bubbles formed at the tip of the gas ligament can be varied by more than two orders of magnitude. The experimental results for the bubble diameter d(b) as function of the control parameters are accounted for by a scaling theory, which predicts d(b)/w ∝ (μ(g)/μ(l))(1/12)(Q(g)/Q(l))(5/12), where μ(g) and μ(l) indicate, respectively, the gas and liquid viscosities and Q(g) and Q(l) are the gas and liquid flow rates. As a particularly important application of our results we produce monodisperse bubbles with the appropriate diameter for therapeutic applications (d(b) ≃ 5 μm) and a production rate exceeding 10(5) Hz.

Reliable measurements of interfacial slip by colloid probe atomic force microscopy. I. Mathematical modeling

We developed a stable spread-sheet algorithm for the calculation of the hydrodynamic forces measured by colloid probe atomic force microscopy to be used in investigations of interfacial slip. The algorithm quantifies the effect on the slip hydrodynamic force for factors commonly encountered in experimental measurements such as nanoparticle contamination, nonconstant drag force due to cantilever bending that varies with different cantilevers, flattening of the microsphere, and calibration at large separations. We found that all of these experimental factors significantly affect the fitted slip length, approximately in the order listed. Our modeling is applied to fit new experimental data reproducibly. Using this new algorithm, it is shown that the fitting of hydrodynamic theories to experimental data is reliable and the fitted slip length is accurate. A "blind test" protocol was developed that produces a reliable estimate of the fitting error in the determination of both the slip length and spring constant. By this blind test, we estimate that our modeling determines the fitted slip length with an average systematic error of 2 nm and the fitted spring constant with a 3% error. Our exact calculation of the drag force may explain previous reports that the fitted slip length depends upon the shape and spring constant of the cantilever used to perform the measurements.

Migration and fractionation of deformable particles in microchannel

The complexity of the coupling between soft particle deformation and fluid perturbation has limited studies of soft particle hydrodynamics to dilute suspensions. A hybrid Brownian dynamics-lattice Boltzmann method is presented that models nondilute soft spherical deformable particle (DP) suspensions in flow. Dependences on particle size and density are investigated for suspensions with over 100 DP. Multi-DP interactions lead to complex dependence of particle distributions on concentration and flow rate. Flow-induced DP migration toward channel center for DP in narrow channels is found. In wide channels, off-center peaks in the center of mass distribution for DP are found. The migration of DP leads to faster average speed of DP than the flow, which can be exploited for fractionating DPs of different sizes.

Analytical modeling and thermodynamic analysis of robust superhydrophobic surfaces with inverse-trapezoidal microstructures

A polydimethylsiloxane (PDMS) elastomer surface with perfectly ordered microstructures having an inverse-trapezoidal cross-sectional profile (simply PDMS trapezoids) showed superhydrophobic and transparent characteristics under visible light as reported in our previous work. The addition of a fluoropolymer (Teflon) coating enhances both features and provides oleophobicity. This paper focuses on the analytical modeling of the fabricated PDMS trapezoids structure and thermodynamic analysis based on the Gibbs free energy analysis. Additionally, the wetting characteristics of the fabricated PDMS trapezoids surface before and after the application of the Teflon coating are analytically explained. The Gibbs free energy analysis reveals that, due to the Teflon coating, the Cassie-Baxter state becomes energetically more favorable than the Wenzel state and the contact angle difference between the Cassie-Baxter state and the Wenzel state decreases. These two findings support the robustness of the superhydrophobicity of the fabricated Teflon-coated PDMS trapezoids. This is then verified via the impinging test of a water droplet at a high speed. The dependencies of the design parameters in the PDMS trapezoids on the hydrophobicity are also comprehensively studied through a thermodynamic analysis. Geometrical dependency on the hydrophobicity shows that overhang microstructures do not have a significant influence on the hydrophobicity. In contrast, the intrinsic contact angle of the structural material is most important in determining the apparent contact angle. On the other hand, the experimental results showed that the side angles of the overhangs are critical not for the hydrophobic but for the oleophobic property with liquids of a low surface tension. Understanding of design parameters in the PDMS trapezoids surface gives more information for implementation of superhydrophobic surfaces.