Isotachophoresis with Oscillating Sample Zones to Control the Spatial Overlap of Co-focused Species.

Microfluidic isotachophoresis (ITP) is a powerful technique that can significantly increase the reaction rate of homogeneous chemical reactions by cofocusing reactants in a narrow sample zone. Correspondingly, ITP has been utilized to reduce the reaction time in various bioanalytical assays. However, in conventional ITP, it is hardly possible to control the reaction rate in real time, i.e., speeding up or slowing down a reaction on demand. Here, we experimentally demonstrate a new mode of ITP that allows the spatial overlap of two ITP zones to be precisely controlled over time, which is a crucial first step toward controlling reaction rates. Two nonreactive samples are initially focused and separated by a spacer using a DC electric field. By superimposing an oscillating field component with sufficiently high amplitude on the DC field, the spatial overlap of their concentration profiles is temporarily increased due to electromigration dispersion. The time-average of this overlap can be precisely controlled by varying the frequency and amplitude of the oscillation. We suggest that this scheme can be transferred to chemical reactions between ionic species with sufficiently different electrophoretic mobilities. Tuning the parameters of the oscillatory electric field should allow direct control of the corresponding reaction rate.

Giant Thermoelectric Response of Confined Electrolytes with Thermally Activated Charge Carrier Generation.

The thermoelectric response of thermally activated electrolytes (TAEs) in a slit channel is studied theoretically and by numerical simulations. The term TAE refers to electrolytes whose charge carrier concentration is a function of temperature, as recently suggested for ionic liquids and highly concentrated aqueous electrolyte solutions. Two competing mechanisms driving charge transport by temperature gradients are identified. For suitable values of the activation energy that governs the generation of charge carriers, a giant thermoelectric response is found, which could help explain recent experimental results for nanoporous media infiltrated with TAEs.

How Charges Separate when Surfaces Are Dewetted.

Charge separation behind moving water drops occurs in nature and technology. Yet, the physical mechanism has remained obscure, as charge deposition is energetically unfavorable. Here, we analyze how a part of the electric double layer charge remains on the dewetted surface. At the contact line, the chemical equilibrium of bound surface charge and diffuse charge in the liquid is influenced by the contact angle and fluid flow. We summarize the mechanism in an analytical model that compares well with experiments and simulations. It correctly predicts that charge separation increases with increasing contact angle and decreases with increasing velocity.

Improved ionic current rectification utilizing cylindrical nanochannels coated with polyelectrolyte layers of non-uniform thickness.

Conical nanochannels employed to create ionic current rectification (ICR) in nanofluidic devices are prone to clogging due to the contraction at one end. As an alternative approach for creating ICR, a cylindrical nanochannel covered with a polyelectrolyte layer (PEL) of variable thickness is proposed in the present study. The efficacy of the proposed design is studied by numerically solving the governing equations including the Poisson, Nernst-Planck, and Stokes-Brinkman equations. Furthermore, the fundamental mechanism behind ICR is explained using a simplified one-dimensional model. The effects of the nanochannel radius, concentration of PEL fixed charges, and bulk ionic concentration on the rectification factor are then investigated in detail. It is shown that the proposed nanochannel provides larger rectification factors as compared to conical nanochannels over wide ranges of the fixed charge concentration and bulk ionic concentration. Such a performance can be achieved even at channel radii much larger than the tip radius of conical nanochannels, indicating not only the better performance of the proposed nanochannel but also its likely longer service life, because of reducing the probability of total ionic current blockage. This means that the proposed nanochannel could find widespread use in fluidic devices, as a replacement for conical nanofluidic diodes.

Detecting Isotachophoresis Zones Hidden in Noise Using Signal Processing.

Lowering the limit of detection in chemical or biochemical analysis is key to extending the application scope of sensing schemes. Usually, this is related to an increased instrumentation effort, which in turn precludes many commercial applications. We demonstrate that the signal-to-noise ratio of isotachophoresis-based microfluidic sensing schemes can be substantially increased merely by postprocessing of recorded signals. This becomes possible by exploiting knowledge about the physics of the underlying measurement process. The implementation of our method is based on microfluidic isotachophoresis and fluorescence detection, for which we take advantage of the physics of electrophoretic sample transport and the structure of noise in the imaging process. We demonstrate that by processing only 200 images, the detectable concentration, compared to the detection from a single image, is already lowered by 2 orders of magnitude without any additional instrumentation effort. Furthermore, we show that the signal-to-noise ratio is proportional to the square root of the number of fluorescence images, which leaves room for further lowering of the detection limit. In the future, our results could be relevant for various applications where the detection of minute sample amounts plays a role.

Surface Charge Deposition by Moving Drops Reduces Contact Angles.

Slide electrification-the spontaneous charge separation by sliding aqueous drops-can lead to an electrostatic potential in the order of 1 kV and change drop motion substantially. To find out how slide electrification influences the contact angles of moving drops, we analyzed the dynamic contact angles of aqueous drops sliding down tilted plates with insulated surfaces, grounded surfaces, and while grounding the drop. The observed decrease in dynamic contact angles at different salt concentrations is attributed to two effects: An electrocapillary reduction of contact angles caused by drop charging and a change in the free surface energy of the solid due to surface charging.

Resonant Nanopumps: ac Gate Voltages in Conical Nanopores Induce Directed Electrolyte Flow.

Inducing transport in electrolyte-filled nanopores with dc fields has led to influential applications ranging from nanosensors to DNA sequencing. Here we use the Poisson-Nernst-Planck and Navier-Stokes equations to show that unbiased ac fields can induce comparable directional flows in gated conical nanopores. This flow exclusively occurs at intermediate driving frequencies and hinges on the resonance of two competing timescales, representing space charge development at the ends and in the interior of the pore. We summarize the physics of resonant nanopumping in an analytical model that reproduces the results of numerical simulations. Our findings provide a generic route toward real-time controllable flow patterns, which might find applications in controlling the translocation of small molecules or nanocolloids.

Phase separation in evaporating all-aqueous sessile drops.

The phase transition and phase distribution in an all-aqueous sessile drop containing polyethylene glycol (PEG) and dextran is studied. Evaporation of water triggers the formation of dextran-rich droplets close to the contact line of the drop that subsequently migrate towards the drop center. The likely reason for the droplet migration is Marangoni convection due to stresses at the interface between the dextran-rich droplets and the surrounding liquid.

Microscale Hydrodynamic Cloaking and Shielding via Electro-Osmosis.

We demonstrate theoretically and experimentally that injection of momentum in a region surrounding an object in microscale flow can yield both "cloaking" conditions, where the flow field outside the cloaking region is unaffected by the object, and "shielding" conditions, where the hydrodynamic forces on the object are eliminated. Using field-effect electro-osmosis as a mechanism for injection of momentum, we present a theoretical framework and analytical solutions for a range of geometrical shapes, validate these both numerically and experimentally, and demonstrate the ability to dynamically switch between the different states.

Wetting of a liquid annulus in a capillary tube.

In this paper, we systematically investigate the static wetting behavior of a liquid ring in a cylindrical capillary tube. We obtain analytical solutions of the axisymmetric Young-Laplace equation for arbitrary contact angles. We find that, for specific values of the contact angle and the volume of the liquid ring, two solutions of the Young-Laplace equation exist, but only the one with the lower value of the total interfacial energy corresponds to a stable configuration. Based on a numerical scheme determining configurations with a local minimum of the interfacial energy, we also discuss the stability limit between axisymmetric rings and non-axisymmetric configurations. Beyond the stable regime, a liquid plug or a sessile droplet exists instead of a liquid ring, depending on the values of the liquid volume and the contact angle. The stability limit is characterized by specific critical parameters such as the liquid volume, throat diameter, etc. The results are presented in terms of a map showing the different stable liquid morphologies that are obtained from an axisymmetric ring as base state.

Interaction of proteins with phase boundaries in aqueous two-phase systems under electric fields.

The electric-field driven transport of proteins across the liquid-liquid interface in an aqueous two-phase system (ATPS) is studied in a microfluidic device using fluorescence microscopy. An ATPS containing polyethylene glycol (PEG) and dextran is employed, and bovine serum albumin (BSA) and bovine γ-globulins (BγG) are considered as model proteins. It is shown that both proteins, initially in the dextran-rich phase, accumulate at the liquid-liquid interface, preferably close to the three-phase contact line between the two liquid phases and the microchannel wall. It is in these regions where the proteins penetrate into the PEG-rich phase. The transport resistance of the liquid-liquid interface is higher for BγG than for BSA, such that a much larger molar flux of BSA into the PEG phase is observed. This opens up the opportunity of separating different protein species by utilizing differences in the transport resistance at the interface. A mathematical model is developed, accounting for adsorption and desorption processes at the liquid-liquid interface. The underlying theoretical concept is that of an electrostatic potential minimum formed by superposing the applied electric field and the field due to the Donnan potential at the interface. A fit of the model parameters to the experimental data results in good agreement between theory and experiments, thereby corroborating the underlying picture.

Electric-Field-Induced Pattern Formation in Layers of DNA Molecules at the Interface between Two Immiscible Liquids.

The concentration patterns of DNA molecules attached to the interface between two immiscible aqueous phases forming under an electric field are studied. The pattern formation is driven by hydrodynamic interactions between the molecules originating from the electro-osmotic flow due to the Debye layer around a molecule. A nonlinear integrodifferential equation is derived describing the time evolution of the concentration field at the liquid-liquid interface. A linear stability analysis of this equation shows that a mode of given wavelength is initially stable, but destabilizes after a critical time which is inversely proportional to the wavelength. The scaling behavior of the critical time with electric field strength and viscosity found in the experiments agrees with the predictions by the theoretical model.

Intermediate States of Wetting on Hierarchical Superhydrophobic Surfaces.

Wetting transition on superhydrophobic surfaces is commonly described as an abrupt jump between two stable states-either from Cassie to Wenzel for nonhierarchical surfaces or from Cassie to nano-Cassie on hierarchical surfaces. We here experimentally study the electrowetting of hierarchical superhydrophobic surfaces composed of multiple length scales by imaging the light reflections from the gas-liquid interface. We present the existence of a continuous set of intermediate states of wetting through which the gas-liquid interface transitions under a continuously increasing external forcing. This transition is partially reversible and is limited only by localized Cassie to Wenzel transitions at nanodefects in the structure. In addition, we show that even a surface containing many localized wetted regions can still exhibit extremely low contact angle hysteresis, thus remaining useful for many heat transfer and self-cleaning applications. Expanding the classical definition of the Cassie state in the context of hierarchical surfaces, from a single state to a continuum of metastable states ranging from the centimeter to the nanometer scale, is important for a better description of the slip properties of superhydrophobic surfaces and provides new considerations for their effective design.

On-Demand Production of Femtoliter Drops in Microchannels and Their Use as Biological Reaction Compartments.

We present a method allowing to produce monodisperse droplets with volumes in the femtoliter range in a microchannel on demand. The method utilizes pulsed electric fields deforming the interface between an aqueous and an oil phase and pinching off droplets. Water and xanthan gum solutions are considered as disperse-phase liquids, and it is shown that the method can be applied even to solutions with a zero-shear rate viscosity more than 104-times higher than that of water. The droplet formation regimes are explored by systematically varying the pulse amplitude and duration as well as the salt concentration. The dependence of the process on the pulse amplitude can be utilized to tune the droplet size. To demonstrate the applicability of the electric-field-driven droplet generator, it is shown that the droplets can be used as versatile biological reaction compartments. It is proven that droplets containing a cell-free transcription-translation system execute gene transcription and protein biosynthesis in a timely and programmable fashion. Moreover, it is verified that biomolecules inside the aqueous droplets such as small RNAs can be diffusionally activated from the outside to induce a ligand-driven biochemical switch.

Stability of Evaporating Droplets on Chemically Patterned Surfaces.

The stability of water droplets on striped surfaces exposing regions of different wettability is studied experimentally, numerically, and based on a scaling model. Different values of the stripe widths and different contact angle contrasts between the hydrophilic and hydrophobic stripes are considered. The boundary between the contact angle contrasts leaving the droplets intact and those leading to droplet breakup is computed numerically. The minimum contrast for which breakup occurs increases with increasing hydrophobic contact angle. The existence of an unstable and a stable regime is confirmed experimentally. In the unstable regime, when approching droplet breakup, a configuration with two liquid fingers on the hydrophilic stripes connected by a capillary bridge on the hydrophobic stripe is found. For decreasing volumes, the width of this capillary bridge decreases until a critical value is reached at which the droplet breaks up. The critical width depends on the ratio of the hydrophilic and the hydrophobic stripe width. A simple scaling model is presented with which the critical width can be predicted. According to the model, the droplet becomes unstable when the increasing Laplace pressure inside the bridge can no longer be balanced by the pressure inside the liquid fingers on the hydrophilic stripes.

Controlling the Trajectories of Nano/Micro Particles Using Light-Actuated Marangoni Flow.

The ability to manipulate small objects and to produce patterns on the nano- and microscale is of great importance, both with respect to fundamentals and technological applications. The manipulation of particles with diameters of the order of 100 nm or below is a challenge because of their Brownian motion but also because of the scaling behavior of methods such as optical trapping. The unification of optical and hydrodynamic forces is a potential route toward the manipulation of tiny objects. Herein we demonstrate the trapping and manipulation of nano- and microparticles based on interfacial flows controlled by visible light, a method we denote as "Light-Actuated Marangoni Tweezer (LAMT)". We experimentally study the manipulation of particles having diameters ranging from 20 nm to 10 µm, including quantum dots and polystyrene nano/microparticles. The particles can be manipulated by scanning a light beam along a liquid surface. In this way, we are able to define almost arbitrary particle trajectories, for example, in the form of letters. In addition, we are able to handle a number of particles in parallel by creating an optical "landscape" consisting of a multitude of laser spots. The inherent advantages of LAMTs are the linear scaling of the trapping force with the particle diameter and the fact that the force is less dependent on particle properties than in the case of conventional methods.

Relaxation of surface-tethered polymers under moderate confinement.

We study the relaxation of surface-tethered polymers in microchannels under moderate confinement (i.e. h ∼ Rg, where h is the channel height and Rg is the radius of gyration of the polymer) by experiments with fluorescence-marked DNA molecules and coupled lattice-Boltzmann/molecular dynamics simulations. The determined scaling exponent suggests that the relaxation is dominated by Zimm-dynamics with significant intra-chain hydrodynamic interactions. The relaxation of the DNA molecules is slower in shallower channels, indicating a pronounced effect of confinement on the longest relaxation time. An experimental correlation is obtained for the longest relaxation time as a function of the molecular contour length and the channel height. Good agreement between the experimental and the simulation results is found.

Stretching of surface-tethered polymers in pressure-driven flow under confinement.

We study the effect of pressure-driven flow on a single surface-tethered DNA molecule confined between parallel surfaces. The influence of flow and channel parameters as well as the length of the molecules on their extension and orientation is explored. In the experiments the chain conformations are imaged by laser scanning confocal microscopy. We find that the fractional extension of the tethered DNA molecules mainly depends on the wall shear stress, with effects of confinement being very weak. Experiments performed with molecules of different contour length show that the fractional extension is a universal function of the product of the wall shear stress and the contour length, a result that can be obtained from a simple scaling relation. The experimental results are in good agreement with results from coarse-grained molecular dynamics/Lattice-Boltzmann simulations.

The stretching force on a tethered polymer in pressure-driven flow.

We use mesoscopic lattice-Boltzmann/molecular dynamics simulations to study the stretching behavior of a single tethered polymer in micro- and nanochannels. In particular, we are interested in the connection between fluid flow properties and the force on the polymer chain. An analytical expression for the stretching force is proposed, which linearly depends on the number of monomers and the boundary shear rate. In agreement with theory, the numerical findings reveal that the influence of hydrodynamic interactions can be ignored, which is also supported by results of additional Langevin dynamics simulations. Our simulation data coincide with the analytical expression for the fractional extension of the chain and further indicate that even weak Poiseuille flow profiles induce a strong alignment of the chain along the channel walls. The numerical results are in good agreement with experimental data obtained by microfluidic stretching of tethered λ-DNA.

Thermoelectricity in Confined Liquid Electrolytes.

The electric field in an extended phase of a liquid electrolyte exposed to a temperature gradient is attributed to different thermophoretic mobilities of the ion species. As shown herein, such Soret-type ion thermodiffusion is not required to induce thermoelectricity even in the simplest electrolyte if it is confined between charged walls. The space charge of the electric double layer leads to selective ion diffusion driven by a temperature-dependent electrophoretic ion mobility, which-for narrow channels-may cause thermovoltages larger in magnitude than for the classical Soret equilibrium.