The influence of frequency and gravity on the orientation of active metallo-dielectric Janus particles translating under a uniform applied alternating-current electric field.

Theoretical and numerical models of active Janus particles commonly assume that the metallo-dielectric interface is parallel to the driving applied electric field. However, our experimental observations indicate that the equilibrium angle of orientation of electrokinetically driven Janus particles varies as a function of the frequency and voltage of the applied electric field. Here, we quantify the variation of the orientation with respect to the electric field and demonstrate that the equilibrium position represents the interplay between gravitational, electrostatic and electrohydrodynamic torques. The latter two categories are functions of the applied field (frequency, voltage) as well as the height of the particle above the substrate. Maximum departure from the alignment with the electric field occurs at low frequencies characteristic of induced-charge electrophoresis and at low voltages where gravity dominates the electrostatic and electrohydrodynamic torques. The departure of the interface from alignment with the electric field is shown to decrease particle mobility through comparison of freely suspended Janus particles subject only to electrical forcing and magnetized Janus particles in which magnetic torque is used to align the interface with the electric field. Consideration of the role of gravitational torque and particle-wall interactions could account for some discrepancies between theory, numerics and experiment in active matter systems.

Short communication: A simple and accurate method of measuring the zeta-potential of microfluidic channels.

We describe an improved method for determining the electroosmotic mobility and zeta potential of surfaces based on a current-monitoring method. This technique eliminates the requirement for measurements of channel dimensions and sample conductivities, leading to a simple high precision measurement. The zeta potential of PDMS is measured for native surfaces and surfaces treated with a nonionic surfactant in low-conductivity electrolytes.

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.

Concentration-Polarization Electroosmosis near Insulating Constrictions within Microfluidic Channels.

Electric fields are commonly used to trap and separate micro- and nanoparticles near channel constrictions in microfluidic devices. The trapping mechanism is attributed to the electrical forces arising from the nonhomogeneous electric field caused by the constrictions, and the phenomenon is known as insulator-based-dielectrophoresis (iDEP). In this paper, we describe stationary electroosmotic flows of electrolytes around insulating constrictions induced by low frequency AC electric fields (below 10 kHz). Experimental characterization of the flows is described for two different channel heights (50 and 10 µm), together with numerical simulations based on an electrokinetic model that considers the modification of the local ionic concentration due to surface conductance on charged insulating walls. We term this phenomenon concentration-polarization electroosmosis (CPEO). The observed flow characteristics are in qualitative agreement with the predictions of this model. However, for shallow channels (10 µm), trapping of the particles on both sides of the constrictions is also observed. This particle and fluid behavior could play a major role in iDEP and could be easily misinterpreted as a dielectrophoretic force.

Electrorotation and Electroorientation of Semiconductor Nanowires.

A number of experimental studies have shown electric-field manipulation of nanowires dispersed in liquids. We demonstrate from first-principles that the electrical response of semiconductor nanowires in liquids must be described by considering, at least, two mechanisms for interfacial polarization: the classical Maxwell-Wagner interfacial polarization and the formation of an electrical double layer. We compare the theoretical predictions with our experimental data and with data published elsewhere and show that both mechanisms play an important role.

Induced-charge electrophoresis of a tilted metal nanowire near an insulating wall.

Electric fields are commonly used to control the orientation and motion of microscopic metal particles in aqueous suspensions. For example, metallodielectric Janus spheres are propelled by the induced-charge electro-osmotic flow occurring on their metallic side, the most common case in electrokinetics of exploiting symmetry breaking of surface properties for achieving net particle motion. In this work, we demonstrate that a homogeneous metal rod can translate parallel to a dielectric wall as a result of the hydrodynamic wall-particle interaction arising from the induced-charge electro-osmosis on the rod surface. The applied electric field could be either dc or low-frequency ac. The only requirement for a nonvanishing particle velocity is that the axis of the rod be inclined with respect to the wall, i.e., it cannot be neither parallel nor perpendicular. We show numerical results of the rod velocity as a function of rod orientation and distance to the wall. The maximum particle velocity is found for an orientation of between ∼30^{∘} and ∼50^{∘}, depending on the position and aspect ratio of the cylinder. Particle velocities of up to tens of µm/s are predicted for typical conditions in electrokinetic experiments.

Editorial for the Special Issue on AC Electrokinetics in Microfluidic Devices, Volume II.

The use of AC electric fields in manipulating and characterizing liquids and suspended particles in microfluidic systems continues to drive innovation in several fields, such as colloidal science, microelectronics, and biotechnology [...].

Concentration-polarization electroosmosis for particle fractionation.

Concentration-polarization electroosmosis (CPEO) refers to steady-state electroosmotic flows around charged dielectric micro-particles induced by low-frequency AC electric fields. Recently, these flows were shown to cause repulsion of colloidal particles from the wall of a microfluidic channel when an electric field is applied along the length of the channel. In this work, we exploit this mechanism to demonstrate fractionation of micron-sized polystyrene particles and bacteria in a flow-focusing device. The results are in agreement with predictions of the CPEO theory. The ease of implementation of CPEO-based fractionation in microfluidics makes it an ideal candidate for combining with current techniques commonly used to generate particle lift, such as inertial or viscoelastic focusing, requiring no extra fabrication steps other than inserting two electrodes.

Electrorotation of titanium microspheres.

Electrorotation (ROT) data for solid titanium micrometer-sized spheres in an electrolyte are presented for three different ionic conductivities, over the frequency range of 10 Hz to 100 kHz. The direction of rotation was found to be opposite to the direction of rotation of the electric field vector (counterfield electrorotation), with a single rotation peak. The maximum rotation rate occurs at a frequency of the order of the reciprocal RC time constant for charging the particle double layer capacitance through the resistor of the electrolyte bulk. A model for the electrical torque acting on a metallic sphere is presented, using a constant phase element impedance to describe the metal/electrolyte interface. The titanium spheres are much denser than the electrolyte and rest on the bottom substrate. Therefore, the electrical and viscous torques near a wall are considered in the analysis. Good agreement is found between the predicted and measured rotational speed as a function of frequency. Theory shows that there is no effect of induced charge electroosmotic flow on the ROT, as observed experimentally.

Insulating traveling-wave electrophoresis.

Traveling-wave electrophoresis (TWE) is a method for transporting charged colloidal particles used in many microfluidic techniques for particle manipulation and fractionation. This method exploits the traveling-wave components of the electric field generated by an array of electrodes subjected to ac voltages with a phase delay between neighboring electrodes. In this article, we propose an alternative way of generating traveling-wave electric fields in microchannels. We apply a rotating electric field around a cylindrical insulating micropillar and the resulting traveling-wave modes induce particle drift around the cylinder. We term this phenomenon insulating traveling-wave electrophoresis (i-TWE) to distinguish it from standard TWE performed with arrays of microelectrodes. We characterized the particle drift experimentally and show a quantitative comparison of the particle velocity with theoretical predictions. Excellent agreement is found when the influence of electro-osmosis on the channel walls is also considered.

Low-frequency electrokinetics in a periodic pillar array for particle separation.

Deterministic Lateral Displacement (DLD) exploits periodic arrays of pillars inside microfluidic channels for high-precision sorting of micro- and nano-particles. Previously we demonstrated how DLD separation can be significantly improved by the addition of AC electrokinetic forces, increasing the tunability of the technique and expanding the range of applications. At high frequencies of the electric field (>1 kHz) the behaviour of such systems is dominated by Dielectrophoresis (DEP), whereas at low frequencies the particle behaviour is much richer and more complex. In this article, we present a detailed numerical analysis of the mechanisms governing particle motion in a DLD micropillar array in the presence of a low-frequency AC electric field. We show how a combination of Electrophoresis (EP) and Concentration-Polarisation Electroosmosis (CPEO) driven wall-particle repulsion account for the observed experimental behaviour of particles, and demonstrate how this complete model can predict conditions that lead to electrically induced deviation of particles much smaller than the critical size of the DLD array.

Alternating current electrokinetic properties of gold-coated microspheres.

We present dielectrophoresis (DEP) and electrorotation (ROT) measurements of gold-coated polystyrene microspheres as a function of frequency and for several electrolyte conductivities. Particle rotation was counterfield with a maximum rotation rate observed at a single characteristic frequency. Negative DEP was observed for frequencies lower than this characteristic frequency and positive DEP for signal frequencies higher than this. These experimental observations are in agreement with predictions for the force and torque on the induced dipole of a perfectly polarizable metal sphere. We present a theoretical model for this case, and good agreement is found for both ROT and DEP measurements if we take into account the viscous friction for a spherical particle near a wall. From the characteristic frequency for rotation, we obtain the capacitance of the electrical double layer at the electrolyte-particle interface. Remarkably, no effect of induced charge electroosmosis around the particles can be inferred from DEP measurements.

Scattering of Metal Colloids by a Circular Post under Electric Fields.

We consider the scattering of metal colloids in aqueous solutions by an insulating circular post under the action of an AC electric field. We analyze the effects on the particle of several forces of electrical origin: the repulsion between the induced dipole of the particle and its image dipole in the post, the hydrodynamic interaction with the post due to the induced-charge electroosmotic (ICEO) flow around the particle, and the dielectrophoresis arising from the distortion of the applied electric field around the post. The relative influence of these forces is discussed as a function of frequency of the AC field, particle size and distance to the post. We perform numerical simulations of the scattering of the metal colloid by the insulating circular post flowing in a microchannel and subjected to alternating current electric fields. Our simulation results show that the maximum particle deviation is found for an applied electric field parallel to the flow direction. The deviation is also greater at low electric field frequencies, corresponding to the regime in which the ICEO's interaction with the post is predominant over other mechanisms.

The Simpsons did it: Exploring the film trope space and its large scale structure.

Creating a story is a challenging task due to the the complex relations between the parts that make it up, which is why many new stories are built on those cohesive elements or patterns, called tropes that have been shown to work in the past. A trope is a recurring storytelling device or pattern, or sometimes a meta-element, used by the authors to express ideas that the audience can recognize or relate to, such as the Hero's Journey. Discovering tropes and how they cluster in popular works and doing it at scale to generate new plots may benefit writers; in this paper, we analyze them and use a principled procedure to identify trope combinations, or communities, that could possible be successful. The degree of development of these different communities can help us identify areas that are under-developed and, thus, susceptible to such a type of development. To detect these communities, with their associated degree of development and interest, we propose a methodology based on scientometric and complex network analysis techniques. As a secondary objective, we will obtain a general perspective in the trope and films network: the tropesphere. We have used a dataset of 10,766 movies and 25,776 tropes associated with them, together with rating, genres and popularity. Our analysis has shown that not only there are different trope communities associated with specific genres, and that there are significant differences between the rating and popularity of these communities but also there are differences on the level of development between them: emerging/declining, specific, transversal or motor.

Innate Muscle Patterns Reproduction During Afferent Somatosensory Input With Vojta Therapy in Healthy Adults. A Randomized Controlled Trial.

OBJECTIVE: Vojta therapy describes stereotypic widespread motor responses as a pattern of tonic muscle contractions during a peripherical pressure stimulation. The present work proposes to characterize the responses at muscles level to a specific tactile input based on Vojta therapy, assessed by sEMG, compared to a sham stimulation in healthy subjects. METHODS: Surface electromyography (sEMG) signal was acquired with dipolar electrodes placed at wrist extensors of both forearms, right tibialis anterior, and top part of rectus abdominus, ground channel placed over the right olecranon. It was amplified and digitized by a 4-channel hub Biosignalsplux device (Plux Wireless Biosignals S.A., Lisboa, Portugal), sampled at 1000 Hz with 16-bit per channel. A continuous 10-minute record of the sEMG signal from the four electrodes were registered. Resting EEG during the first minute before the stimulation period was recorded by 64 active electrodes. RESULTS: Statistically significant differences were showed between sham and experimental group. Experimental group participants were subjected to cluster analysis based on their muscle activation patterns, generating three different models of activation. Differences in the previous resting cortical activity in left superior frontal area were found between clusters that activated limb muscles and the cluster that did not. CONCLUSIONS: Vojta specific stimulation area activates innate muscle responses assessed by sEMG in healthy subjects, compared to a sham stimulation. SIGNIFICANCE: This characterization might be helpful to the prescription and application of Vojta therapy in an individual-basis for non-neurophysiologically damaged adult subjects.

Dipolophoresis and Travelling-Wave Dipolophoresis of Metal Microparticles.

We study theoretically and numerically the electrokinetic behavior of metal microparticles immersed in aqueous electrolytes. We consider small particles subjected to non-homogeneous ac electric fields and we describe their motion as arising from the combination of electrical forces (dielectrophoresis) and the electroosmotic flows on the particle surface (induced-charge electrophoresis). The net particle motion is known as dipolophoresis. We also study the particle motion induced by travelling electric fields. We find analytical expressions for the dielectrophoresis and induced-charge electrophoresis of metal spheres and we compare them with numerical solutions. This validates our numerical method, which we also use to study the dipolophoresis of metal cylinders.

Electrokinetic biased deterministic lateral displacement: scaling analysis and simulations.

Deterministic Lateral Displacement (DLD) is a microfluidic technique where arrays of micropillars within a microchannel deflect particles leading to size-based segregation. We recently demonstrated that applying AC electric fields orthogonal to the fluid flow increases the separation capabilities of these devices with a deflection angle that depends on the electric field magnitude and frequency. Particle deviation occurs in two distinct regimes depending on frequency. At high frequencies particles deviate due to negative dielectrophoresis (DEP). At low frequencies (below 1 kHz) particles oscillate perpendicular to the flow direction due to electrophoresis and are also deflected within the device. Significantly, the threshold electric field magnitude for the low frequency deviation is much lower than for deflection at high frequencies by DEP. In order to characterize the enhanced separation at low frequencies, the induced deviation was compared between the two frequency ranges. For high frequencies, we develop both theoretically and experimentally scaling laws for the dependence of particle deviation on several parameters, namely the amplitude of the applied voltage, particle size and liquid velocity where DEP forces compete with viscous drag. A novel theoretical framework is presented that enables simulation of particle trajectories subjected to DEP forces in DLD devices. Deviation angles predicted by simulations are in very good agreement with experimental data. At low frequencies (below 1 kHz), particles follow the same scaling law, but with much lower voltages. This indicates that electrokinetic phenomena other than DEP play an important role in driving particle behaviour. Experiments show that at low frequencies, particle motion is affected by quadrupolar electrohydrodynamic flows around the insulating pillars of the DLD array. We quantify the difference between the two frequency regimes and show that an electrokinetic model based only on DEP forces is limited to frequencies of 1 kHz and above.

Editorial for the Special Issue on AC Electrokinetics in Microfluidic Devices.

The use of AC electric fields for manipulating and/or characterizing liquids and small particles in suspension is well-known [...].

Modeling the AC Electrokinetic Behavior of Semiconducting Spheres.

We study theoretically the dielectrophoresis and electrorotation of a semiconducting microsphere immersed in an aqueous electrolyte. To this end, the particle polarizability is calculated from first principles for arbitrary thickness of the Debye layers in liquid and semiconductor. We show that the polarizability dispersion arises from the combination of two relaxation interfacial phenomena: charging of the electrical double layer and the Maxwell⁻Wagner relaxation. We also calculate the particle polarizability in the limit of thin electrical double layers, which greatly simplifies the analytical calculations. Finally, we show the model predictions for two relevant materials (ZnO and doped silicon) and discuss the limits of validity of the thin double layer approximation.

Electrokinetics of metal cylinders.

We study theoretically the rotation induced on an uncharged metal nanocylinder immersed in an electrolyte by AC electric fields. We consider the rotation of the cylinder when subjected to a rotating electric field (electrorotation) and the orientation of the cylinder in an AC field with constant direction (electro-orientation). The cylinder rotation is due to two mechanisms: the electric field interaction with the induced dipole on the particle and the hydrodynamic stress on the particle originated by the induced-charge electro-osmotic (ICEO) flow around the particle. The cylinder rotation induced by the ICEO mechanism can be calculated by using the Lorentz reciprocal theorem, while the rotation due to the induced dipole is calculated from the cylinder polarizability. We employ 3D numerical computations using finite elements for the general case as well as analytical methods for slender cylinders. Both calculations use the thin-double-layer approximation. We compare the results for slender cylinders of both methods showing good agreement. The electro-orientation (EOr) due to dipole torque aligns the axis of slender cylinders with the applied field, but aligns the axis of short cylinders perpendicularly to the field. The EOr due to ICEO torque always aligns the axis of cylinders with the field. The rotation induced by ICEO torque tends to disappear for frequencies of the applied field much greater than the characteristic frequency for charging the double-layer capacitance of the metal-electrolyte interface.