Induced-charge Electrophoresis Research Articles

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.

Characterizing the Janus colloidal particles in AC electric field and a step towards label-free cargo manipulation

In recent years, researchers have been exploring Janus particles, a unique type of colloidal system, which has gained enormous attention across diverse fields, from soft matter physics to biology. These particles, possessing an asymmetric surface, can be manipulated in the fluid using external energy sources. This versatility makes them suitable for mimicking biological systems and applications like cargo transportation, drug delivery, biosensing, and environmental cleanup. The study focuses on the response of metal-dielectric (Ti-PMMA) Janus spheres in the AC field, created through the Physical Vapor Deposition (PVD) technique. When subjected to a uniform AC electric field, these particles exhibit different dynamic behaviors at various frequency ranges. The particles show diverse responses from induced-charge electrophoresis (ICEP: 500 Hz-80 kHz) to reversed motion (r: ICEP:100 kHz-1 MHz) and linear chain formation (1 MHz). Additionally, they randomly cluster near the lower characteristic frequency (800 Hz) of the ICEP, whereas they exhibit the 3D motion at frequencies (100 Hz) below this lower characteristic value. The mechanisms behind these dynamic phenomena could be explained by considering the concept of electric double layer (EDL), self-dielectrophoresis (sDEP), dipolar interactions, hydrodynamic interactions, and electrothermal effects. These active Janus Colloids are further leveraged to manipulate 1 µm diameter PMMA passive microparticles and E. coli bacteria at 1 kHz, showcasing their potential applications in cargo delivery under various microfluidic research areas. The manipulation of payloads could be explained based on dielectrophoretic trapping due to locally generated field gradients around the surface of the Janus sphere. Therefore, the present work offers an unprecedented opportunity to study such out-of-equilibrium complex active matter systems and create novel functional materials. Moreover, it could also be extended as a fuel-free microrobotics system for various biomedical applications such as sensing and on-demand drug or cargo delivery inside the microfluidics chamber.

Enabling rotation over electrophoretic motion in Janus particles under applied electric field

Janus particles (JPs) comprising metal and dielectric halves are capable of propulsion perpendicular to an applied electric field, with promising applications in targeted drug delivery and microrobotics. Yet, not all JP geometries behave the same when activated. We explore hemispherical, snowman, and matchstick-like particle dynamics and evaluate threshold values for which electric torque can overcome induced charge electrophoresis. We define conditions where JPs are more likely to undergo rotation in place rather than linear motion, highlighting design features which should be considered when using these particles as materials.

Dynamics and phase behavior of metallo-dielectric rod-shaped microswimmers driven by alternating current electric field.

The ability to move and self-organize in response to external stimuli is a fascinating feature of living active matter. Here, the metallo-dielectric rod-shaped microswimmers are shown to have a similar behavior in the presence of an AC electric field. The silica-copper Janus microrods were fabricated using the physical vapor deposition-based glancing angle deposition technique (GLAD). When the aqueous solution of the microrods was under the influence of an external AC electric field, they were found to exhibit different phases such as clustering, swimming, and vertical standing in response to variation of the applied frequency. The swimming behavior (5-90 kHz) of the rods is attributed to the induced-charge electrophoresis (ICEP) phenomenon, whereas the dynamic clustering (<5 kHz) could be explained in terms of the electrohydrodynamic (EHD) interaction. Interestingly, the rods flip to attain the vertically standing position when responding to the applied electric field above 90 kHz. The reorientation and switching of the major axis of the rod along the field direction is attributed to the electro-orientation phenomenon. This is basically due to the dominance of the electric torque above the upper limit of the characteristic frequency, where the strength of slip flows around the microrods is predicted to be poor. The present study not only offers insight into the fundamental aspects of the dynamics and the phase behavior of rod-shaped microswimmers, but also opens an avenue to design reconfigurable active matter systems with features inspired by biological systems.

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.

Rheology of dense suspensions of ideally conductive particles in an electric field

The rheological behaviour of dense suspensions of ideally conductive particles in the presence of both electric field and shear flow is studied using large-scale numerical simulations. Under the action of an electric field, these particles are known to undergo dipolophoresis (DIP), which is the combination of two nonlinear electrokinetic phenomena: induced-charge electrophoresis (ICEP) and dielectrophoresis (DEP). For ideally conductive particles, ICEP is predominant over DEP, resulting in transient pairing dynamics. The shear viscosity and first and second normal stress differences $N_1$ and $N_2$ of such suspensions are examined over a range of volume fractions $15\,\% \leq \phi \leq 50\,\%$ as a function of Mason number $Mn$ , which measures the relative importance of viscous shear stress over electrokinetic-driven stress. For $Mn &lt; 1$ or low shear rates, the DIP is shown to dominate the dynamics, resulting in a relatively low-viscosity state. The positive $N_1$ and negative $N_2$ are observed at $\phi &lt; 30\,\%$ , which is similar to Brownian suspensions, while their signs are reversed at $\phi \ge 30\,\%$ . For $Mn \ge 1$ , the shear thickening starts to arise at $\phi \ge 30\,\%$ , and an almost five-fold increase in viscosity occurs at $\phi = 50\,\%$ . Both $N_1$ and $N_2$ are negative for $Mn \gg 1$ at all volume fractions considered. We illuminate the transition in rheological behaviours from DIP to shear dominance around $Mn = 1$ in connection to suspension microstructure and dynamics. Lastly, our findings reveal the potential use of nonlinear electrokinetics as a means of active rheology control for such suspensions.

Arrested-motility states in populations of shape-anisotropic active Janus particles.

The emergence of large-scale collective phenomena from simple interactions between individual units is a hallmark of active matter systems. Active colloids with alignment-dominated interparticle interactions tend to develop orientational order and form motile coherent states, such as flocks and swarms. Alternatively, a combination of self-propulsion and excluded-volume interactions results in self-trapping and active phase separation into dense clusters. Here, we reveal unconventional arrested-motility states in ensembles of active discoidal particles powered by induced-charge electrophoresis. Combining experiments and computational modeling, we demonstrate that the shape asymmetry of the particles promotes the hydrodynamically assisted formation of active particles' bound states in a certain range of excitation parameters, ultimately leading to a spontaneous collective state with arrested motility. Unlike the jammed clusters obtained through self-trapping, the arrested-motility phase remains sparse, dynamic, and reconfigurable. The demonstrated mechanism of phase separation seeded by bound state formation in ensembles of oblate active particles is generic and should be applicable to other active colloidal systems.

The role of particle-electrode wall interactions in mobility of active Janus particles driven by electric fields

HypothesisThe interaction of active particles with walls can explain discrepancies between experiments and theory derived for particles in the bulk. For an electric field driven metallodielectric Janus particle (JP) adjacent to an electrode, interaction between the asymmetric particle and the partially screened electrode yields a net electrostatic force – termed self-dielectrophoresis (sDEP) - that competes with induced-charge electrophoresis (ICEP) to reverse particle direction. ExperimentsThe potential contribution of hydrodynamic flow to the reversal is evaluated by visualizing flow around a translating particle via micro-particle image velocimetry and chemically suppressing ICEP with poly(l-lysine)-g-poly(ethylene glycol) (PLL-PEG). Mobility of Polystyrene-Gold JPs is measured in KCl electrolytes of varying concentration and with a capacitive SiO2 coating at the metallic JP surface or electrode. Results are compared with theory and numerical simulations accounting for electrode screening. FindingsPLL-PEG predominantly suppresses low-frequency mobility where propulsive electro-hydrodynamic jetting is observed; supporting the hypothesis of an electrostatic driving force at high frequencies. Simulations and theory show the magnitude, direction and frequency dispersion of JP mobility are obtained by superposition of ICEP and sDEP using the JP height and capacitance as fitting parameters. Wall proximity enhances ICEP and sDEP and manifests a secondary ICEP charge relaxation time dominating in the contact limit.

Alternating Current Electric Field Driven Topologically Defective Micro/nanomotors

Design and synthesis of electric-driven micro/nanomotors (EDMNMs) have attracted tremendous attention because of their unique advantages, such as non-necessity of fuel, high mobility, and flexible controllability. However, existing EDMNMs are mostly Janus structures based on two hemispheres made from heterogeneous materials. Here, we propose a new type of defective electric-driven micro/nanomotor with an asymmetric shape. It is found, for the first time, that the defective golden micro/nanomotors (DGMs) can generate a locally asymmetric field gradient through the topological defect, and can achieve controllable self-dielectrophoresis (sDEP) and induced-charge electrophoresis (ICEP) motion behaviors in different frequency ranges. In particular, the DGMs can achieve three-dimensional helical motion behavior through designing their defective structure. Furthermore, we demonstrate that the DGMs can attract or repel cargos under different electric frequencies. These new DGMs and their attractive performance pave the way for fabricating highly controllable EDMNMs via structural design and can be widely applied in biomedicine, micro/nanosensors, and micro/nanomechanical systems.

Fabrication and Electric Field-Driven Active Propulsion of Patchy Microellipsoids.

Active colloids are a synthetic analogue of biological microorganisms that consume external energy to swim through viscous fluids. Such motion requires breaking the symmetry of the fluid flow in the vicinity of a particle; however, it is challenging to understand how surface and shape anisotropies of the colloid lead to a particular trajectory. Here, we attempt to deconvolute the effects of particle shape and surface anisotropy on the propulsion of model ellipsoids in alternating current (AC) electric fields. We first introduce a simple process for depositing metal patches of various shapes on the surfaces of ellipsoidal particles. We show that the shape of the metal patch is governed by the assembled structure of the ellipsoids on the substrate used for physical vapor deposition. Under high-frequency AC electric field, ellipsoids dispersed in water show linear, circular, and helical trajectories which depend on the shapes of the surface patches. We demonstrate that features of the helical trajectories such as the pitch and diameter can be tuned by varying the degree of patch asymmetry along the two primary axes of the ellipsoids, namely longitudinal and transverse. Our study reveals the role of patch shape on the trajectory of ellipsoidal particles propelled by induced charge electrophoresis. We develop heuristics based on patch asymmetries that can be used to design patchy particles with specified nonlinear trajectories.

The zero-shear-rate limiting rheological behaviors of ideally conductive particles suspended in concentrated dispersions under an electric field

The rheological behaviors of suspension of ideally conductive particles in an electric field are studied using large-scale numerical simulations in the limit of zero-shear-rate flow. Under the action of an electric field, the particles undergo the nonlinear electrokinetic phenomenon termed as dipolophoresis, which is the combination of dielectrophoresis and induced-charge electrophoresis. For ideally conductive particles, the dynamics of the suspension are primarily controlled by induced-charge electrophoresis. To characterize the rheological properties of the suspension, the particle stress tensor and particle pressure are calculated in a range of volume fraction up to almost random close packing. The particle normal stress and particle pressure are shown to behave non-monotonically with volume fraction, especially in concentrated regimes. In particular, the particle pressure is positive for volume fraction up to 30\%, after which it becomes negative, indicating a change in the nature of the particle pressure. The microstructure expressed by pair distribution function and suspension entropy is also evaluated. Visible variations in the local microstructure seem to correlate with the non-monotonic variation in the particle normal stresses and particle pressure. These non-monotonic behaviors are also correlated with the change in the dominant mechanism of particle pairing dynamics observed in our recent study [Mirfendereski \& Park, J. Fluid Mech. \textbf{875}, R3 (2019)]. Lastly, the effects of confinement on the particle stress and particle pressure are investigated. It is found that the particle pressure changes its nature very quickly at high volume fractions as the level of confinement increases. This study should motivate control strategies to fully exploit the distinct changing nature of the pressure for rheological manipulation of such suspension system.

Phase-change Janus particles with switchable dual properties

We developed switchable Janus particles (JPs) fabricated by coating one hemisphere of silica microspheres with a phase-change film. We used the chalcogenide GeSbTe (GST), which exhibits a reversible phase change between a metal-like crystalline phase (c-GST) and a dielectric-like amorphous phase (a-GST). As a driving force for self-propelling the JPs, a perpendicular alternating current electric field was applied; the frequency dependence of the motion of an individual JP and that of inter-JP interaction were investigated. At lower frequencies (2–20 kHz), a-GST JPs were propelled with their silica side facing forward, which is similar to the behavior of Au–silica JPs propelled by the well-known induced-charge electrophoresis mechanism, whereas c-GST JPs were immobile because they adhered to the indium tin oxide substrate. At higher frequencies (50–300 kHz), both a-GST and c-GST JPs were propelled with their GST side facing forward and a substantial difference in inter-JP interaction was observed: repulsive collision for c-GST JPs but attractive stacking for a-GST JPs.

Induced-charge electrophoresis of ideally polarizable particle pairs

Owing to nonlinearity of induced-charge electrophoresis, the relative two-dimensional motion of a pair of conducting cylinders under a uniform DC field comprises---in addition to the symmetric translation along their line of centers (LOC) associated with the separate contributions of the parallel- and perpendicular- external-field components---LOC rotation resulting from their interaction. Consequently, all particles are eventually moving apart in the general direction perpendicular to the field. When LOC is initially oriented sufficiently close to the field, this takes place through transient pairing events.

Frequency Response of Induced-Charge Electrophoretic Metallic Janus Particles.

The ability to manipulate and control active microparticles is essential for designing microrobots for applications. This paper describes the use of electric and magnetic fields to control the direction and speed of induced-charge electrophoresis (ICEP) driven metallic Janus microrobots. A direct current (DC) magnetic field applied in the direction perpendicular to the electric field maintains the linear movement of particles in a 2D plane. Phoretic force spectroscopy (PFS), a phase-sensitive detection method to detect the motions of phoretic particles, is used to characterize the frequency-dependent phoretic mobility and drag coefficient of the phoretic force. When the electric field is scanned over a frequency range of 1 kHz–1 MHz, the Janus particles exhibit an ICEP direction reversal at a crossover frequency at ~30 kH., Below this crossover frequency, the particle moves in a direction towards the dielectric side of the particle, and above this frequency, the particle moves towards the metallic side. The ICEP phoretic drag coefficient measured by PFS is found to be similar to that of the Stokes drag. Further investigation is required to study microscopic interpretations of the frequency at which ICEP mobility switched signs and the reason why the magnitudes of the forward and reversed modes of ICEP are so different.

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.

Dipolophoresis in concentrated suspensions of ideally polarizable spheres

The dynamics of ideally polarizable spherical particles in concentrated suspensions under the effects of nonlinear electrokinetic phenomena is analysed using large-scale numerical simulations. Particles are assumed to carry no net charge and considered to undergo the combination of dielectrophoresis and induced-charge electrophoresis termed dipolophoresis. Chaotic motion and resulting hydrodynamic diffusion are known to be driven by the induced-charge electrophoresis, which dominates the dielectrophoresis. Up to a volume fraction $\unicode[STIX]{x1D719}\approx 35\,\%$, the particle dynamics seems to be hindered by the increase in the magnitude of excluded volume interactions with concentration. However, a non-trivial suspension behaviour is observed in concentrated regimes, where the hydrodynamic diffusivity starts to increase with the volume fraction at $\unicode[STIX]{x1D719}\approx 35\,\%$, before reaching a local maximum, and then drastically decreases on approaching random close packing. Similar non-trivial behaviours are observed in the particle velocity and number-density fluctuations around volume fractions at which the non-trivial behaviour of the hydrodynamic diffusion is observed. We explain these non-trivial behaviours as a consequence of particle contacts, which are related to the dominant mechanism of particle pairings. The particle contacts are classified into attractive and repulsive classes by the nature of contacts, and in particular, the strong repulsive contact becomes predominant at $\unicode[STIX]{x1D719}&gt;20\,\%$. Moreover, this transition is visible in the pair distribution functions, which also reveal the change in the suspension microstructure in concentrated regimes. It appears that strong and massive repulsive contacts along the direction perpendicular to an electric field promote the non-trivial suspension behaviours observed in concentrated regimes.

Supercolloidal Spinners: Complex Active Particles for Electrically Powered and Switchable Rotation

AbstractA class of supercolloidal particles that controllably spin about their central axis in AC electric fields is reported. The rational design of these “microspinners” enables their rotation in a switchable manner, which gives rise to several interesting and programmable behaviors. It is shown that due to their complex shape and discrete metallic patches on their surfaces, these microspinners convert electrical energy into active motion via the interplay of four mechanisms at different electric field frequency ranges. These mechanisms of rotation include (in order of increasing frequency): electrohydrodynamic flows, reversed electrohydrodynamic flows, induced charge electrophoresis, and self‐dielectrophoresis. As the primary mechanism powering their motion transitions from one phenomenon to the next, these microspinners display three directional spin inversions (i.e., from clockwise to anticlockwise, or vice versa). To understand the mechanisms involved, this experimental study is coupled with scaling analyses. Due to their frequency‐switchable rotation, these microspinners have potential for applications such as interlocking gears in colloidal micromachines. Moreover, the principles used to power their switchable motion can be extended to design other types of supercolloidal particles that harvest electrical energy for motion via multiple electrokinetic mechanisms.

Reconfigurable engineered motile semiconductor microparticles

Locally energized particles form the basis for emerging classes of active matter. The design of active particles has led to their controlled locomotion and assembly. The next generation of particles should demonstrate robust control over their active assembly, disassembly, and reconfiguration. Here we introduce a class of semiconductor microparticles that can be comprehensively designed (in size, shape, electric polarizability, and patterned coatings) using standard microfabrication tools. These custom silicon particles draw energy from external electric fields to actively propel, while interacting hydrodynamically, and sequentially assemble and disassemble on demand. We show that a number of electrokinetic effects, such as dielectrophoresis, induced charge electrophoresis, and diode propulsion, can selectively power the microparticle motions and interactions. The ability to achieve on-demand locomotion, tractable fluid flows, synchronized motility, and reversible assembly using engineered silicon microparticles may enable advanced applications that include remotely powered microsensors, artificial muscles, reconfigurable neural networks and computational systems.

Shape-directed dynamics of active colloids powered by induced-charge electrophoresis

The symmetry and shape of colloidal particles can direct complex particle motions through fluid environments powered by simple energy inputs. The ability to rationally design or "program" the dynamics of such active colloids is an important step toward the realization of colloidal machines, in which components assemble spontaneously in space and time to perform dynamic (dissipative) functions such as actuation and transport. Here, we systematically investigate the dynamics of polarizable particles of different shapes moving in an oscillating electric field via induced-charge electrophoresis (ICEP). We consider particles from each point group in three dimensions (3D) and identify the different rotational and translational motions allowed by symmetry. We describe how the 3D shape of rigid particles can be tailored to achieve desired dynamics including oscillatory motions, helical trajectories, and complex periodic orbits. The methodology we develop is generally applicable to the design of shape-directed particle motions powered by other energy inputs.

Two dimensional numerical study on mixing enhancement in micro-channel due to induced charge electrophoresis

Mixing different species to produce a homogeneous solution is of great interest, affecting the performance of lab-on-a-Chip (LOC) devices. Because of the laminar characteristic of the flow field in microchannels, the LOC devices usually suffer from poor mixing efficiency. This study numerically investigates the enhancement of mixing efficiency in a microchannel using a conductive micro-particle. For this, the governing equations of the induced-charged electrokinetics based on the thin EDL assumption are solved by the finite volume method to provide a comprehensive parametric study on the mixing process and its efficiency taking into account the physical parameters such as the particle size, applied electric field, electric potential and branching inlet.