Non-uniform Surface Charge Distribution Research Articles

Non-uniform surface charge distribution of tribodielectric material by edge effect

The surface effect of dielectric material is one of the important cornerstones for the performance parameters. Here, an “edge effect” model of the surface charge of tribodielectric material in the natural environment based on the semiconductor interface and surface theory is established for the first time, and a theoretical exploration of the surface charge distribution is carried out as well. The result indicates that the surface charge density at the center of the tribodielectric material is higher and gradually decreases towards the edges. By exploring the functional relationship between the surface charge density and the edge distance, the interrelation between the inner edge width, outer edge width, and nick distance and the output charge on the tribodielectric material can be achieved. Meanwhile, the output performance is a nearly 3-fold higher between the fully covered triboelectric nanogenerator (FC-TENG) and non-fully covered triboelectric nanogenerator (NFC-TENG) due to the “edge effect” of the tribodielectric material. Finally, the distribution of surface charge is intuitively demonstrated by the visual experiment. This work provides a deeper understanding of the charge distribution on the surface of tribodielectric materials, holding significant implications for enhancing the fabrication process of TENG and its commercialization and mass production.

Utilizing the Complexity of Transition Metal Phosphide Surfaces to Tune Nitrate Reduction Selectivity

Electrocatalytic nitrate reduction (NO3RR) is one exciting method of upcycling nitrate in wastewater by conversion to ammonia, an essential component in fertilizers and a potential liquid fuel. Nitrate reduction to NH3 is impeded by the formation of other products such as NO2 - or H2 via the hydrogen evolution reaction (HER). Transition metal phosphide (TMP) nanocrystals have a non-uniform surface charge distribution due to their binary compositions, which leads to a variety of active sites with different binding affinities. We hypothesize that the diversity of active sites on Ni2P can be leveraged to perform complex electrocatalytic reactions such as the NO3RR. Specifically, a diversity of surface sites can create local active site motifs that promote the adsorption of different intermediates, e.g., *NO3 - and *H, that can readily react for the NO3RR. We prepare 5 nm Ni2P and Ni nanocrystals using colloidal syntheses and deposit them onto a carbon support to compare the simpler catalytic surface of Ni with the more complex Ni2P surface. We compare their NO3RR activities with cyclic voltammetry in neutral, buffered aqueous conditions. We also investigate the selectivity between the NO3RR and HER with bulk electrolysis coupled with a colorimetric assay, where Ni2P on carbon shows higher selectivity toward NH3 over NO2 - and H2. With these experiments, we evaluate our hypothesis that the surface complexity of Ni2P will lead to enhanced NO3RR selectivity relative to Ni. This work sets the stage for developing selective electrocatalysts by utilizing ensembles of active sites to drive electrocatalytic selectivity.

Mechanism of silicon-nanowire-diode orientation in DC electric fields

Doped semiconductor nanowires are emerging as next-generation electronic colloidal materials, and the efficient manipulation of such nanostructures is crucial for technological applications. In fluid suspension, pn nanowires (pn NWs), unlike homogeneous nanowires, have a permanent dipole, and thus, experience a torque under an external DC field that orients the nanowire with its n-type end in the direction of the field. Here, we quantitatively measure the permanent dipoles of various Si nanowire pn diodes and investigate their origin. By comparing the dipoles of pn NWs of different lengths and radii, we show that the permanent dipole originates from non-uniform surface-charge distributions, rather than the internal charges at the p–n junction as was previously proposed. This understanding of the mechanism for pn NWs orientation has relevance to the manipulation, assembly, characterization, and separation of nanowire electronics by electric fields.

Surface-resolved dynamic simulation of charged non-spherical particles

We present a numerical method for simulating the electrostatic interaction of a cluster of charged non-spherical particles as they collide with each other. The boundary element method (BEM) is employed to resolve the highly nonuniform surface charge distribution on individual particles, based on which their electrostatic interactions can be computed. The method of generalized minimum residual (GMRES) incorporated with the fast multipole method (FMM) is adopted to accelerate the electrostatic calculation. The framework is validated through four different cases and is proven to capture the induced higher-order multipole interaction as two particles become very close to each other. This interaction is also shown to be sensitive to the particle geometry, in particular the local curvatures. In addition to the electrostatic interactions, the contact forces and torques are included using the Hertzian contact model to capture the particle collision. Finally, this comprehensive framework is demonstrated by reproducing typical collision outcomes, e.g., sticking and fragmentation, among several non-spherical charged particles.

Numerical simulation of dielectric barrier discharge with asymmetrical electrode in atmospheric helium

The characteristics of the dielectric barrier discharge (DBD) equipped with asymmetrical electrode (ring electrode on the upper and disk electrode on the lower) in atmospheric helium are investigated by a two-dimensional self-consistent fluid model. Simulation results show that as the applied voltage increases, the discharge enhances and the onset of discharge advances, which is similar to the results of traditional DBD. However, with the applied voltage increasing, the symmetry of the discharge current pulses in the positive and negative half cycles disappears because of the asymmetric electrode configuration. In addition, only the spatial distribution of the electron density at the peak moments of the first and second current pulses satisfies the complementary characteristics, while the spatial distribution at other peak moments does not meet the complementary characteristics. Moreover, the electric field, near the upper dielectric barrier surface, presents a curtain-like distribution with considerable radial electric field components, which results from the non-uniform radial surface charge distribution and the ring electrode configuration. The relative variation of the radial distribution of surface charge density is largely determined by the geometry of the opposite electrode.

Microconfined electroosmotic flow of a complex fluid with asymmetric charges: Interplay of fluid rheology and physicochemical heterogeneity

Abstract We report a study on the micro-confined electroosmotic flow of an Oldroyd-B fluid over a surface having asymmetric charge modulation. We have employed a combination of regular and matched asymptotic expansion to obtain the analytical solution. We have also carried out a full numerical simulation of the physicochemical problem in order to assess the full parameter space of the problem. The analytical solution is shown to be in excellent agreement with the full numerical solution in the limiting conditions of thin electrical double layers and weakly viscoelastic fluids. We have used these solutions to describe the complex fluid flow pattern and the modified electroosmotic slip velocity. We have also highlighted the alteration in the net volumetric throughput and ionic current. We have subsequently presented the numerical solutions for the velocity field, flow rates, and streaming current by varying the physicochemical conditions at the surface and viscoelastic properties of the fluid. We report an augmentation in the electroosmotic slip velocity with the increase in relative strength. The symmetry breaking of flow and stress distributions are shown to affect the net-throughput and streaming current, which is indicative of the combined effect of phase angle, magnitude of charge distribution, and the fluid viscoelasticity. We have shown that the thickness of electrical double layer considerably affects the net-throughput and streaming current generation in the microchannel. Our results hold the key towards understanding the complex bio-fluid transport in a microchannel utilizing the electric field in the presence of non-uniform surface charge distribution.

Numerical investigation of the periodical complementary dielectric-barrier discharge equipped with double-ring electrodes in atmospheric helium

A two-dimensional fluid model was used to investigate the characteristics of helium dielectric-barrier discharge (DBD) equipped with double-ring electrodes at atmospheric pressure. Simulation results show that although the temporal evolutions of discharge current and current density at different radial positions exhibit the same or similar characteristics to those in traditional DBD, a distinctive spatial discharge structure is observed during the discharge process. The spatial distribution of electron density at current peak moments exhibits a periodical complementary feature between the center-advantage and the periphery-advantage with the inner ring electrode covering the radial axis from 0.5 to 1.0 mm. When the inner ring electrode covers the radial axis from 1.0 to 1.5 mm, the spatial distribution of electron density satisfies another periodical complementary behavior between the electrode-center-advantage and the coexistence of periphery-advantage and center-advantage. These complementary discharge features mainly result from non-uniform electric field and surface charge distribution at the end of the previous discharge. The difference of the discharge structure between the two cases is attributed to the fact that Laplacian field distributions are largely dependent on the arrangement of double-ring electrodes. Our observations show that electrode geometry plays an important role in the formation of the discharge structure.

Effective electrostatic forces between two neutral surfaces with atomic scale strip shape surface charge separation

By using classical density functional theory we investigate influences of atomic scale strip shape surface charge separation on the effective electrostatic force (EEF) between two overall neutral surfaces immersed in 1:1 electrolyte solution. Main novel findings are confirmed, and summarized as follows: (i) At lower bulk concentration, the EEF is always attractive for asymmetrical surface charge pattern and repulsive for symmetrical surface charge pattern. Strength of the EEF is positively correlated with the strip shape domain width and surface charge density intensity, and generally smaller than that of the EEF between two equally and oppositely or similarly charged planar surfaces with same surface charge intensity. (ii) With increasing of the bulk concentration, the asymmetrical strip shape surface charge pattern induces two repulsion peaks, strengths of the repulsion peaks around one times and double the ion diameter surface separation are negatively and positively correlated with the strip shape domain width, respectively; the second repulsion peak position reduces with the domain charge strength. (iii) At higher bulk concentration, the symmetrical strip shape charge pattern induces two attraction wells of the EEF curve; one positioned at wall separation larger than one times and smaller than double the ion size, the other approximately 2.75 times the ion size. (iv) Considering that systems favor low-energy configurations, the present calculations provide clear evidence that effective repulsion between colloids with non-uniform surface charge distribution or charge fluctuation is less than that with uniform surface charge distribution or with fixed charges, respectively. All of the observations can be explained self-consistently by using concepts such as ion saturation adsorption capacity, arrangement compactness of the adsorption layer, formation of quasi-hydrogen-bonding, and charge reversal.

Interpretation of Electrostatic Self-Potential Measurements Using Interface-Trapped Microspheres with Surface Heterogeneity

Electrostatic self-potentials of individual particles trapped at an oil–water interface were determined, and the effects of surface chemical nonuniformity on heterogeneous self-potentials and equilibrium microstructures were investigated. Direct measurement of the pair interactions and the self-potentials of polystyrene microspheres were performed using optical laser tweezers. The individual particles had different self-potentials even when they possessed the same surface functionalities. Atomic force microscopy measurements elucidated the relationship between nonuniform surface charge distribution and heterogeneity and magnitude of self-potentials. Monte Carlo simulations demonstrated that self-potential heterogeneity led to the formation of more melted microstructures that showed excellent consistency with experiments.

Parallel-plate fracture transport experiments of nanoparticulate illite in the ultra-trace concentration range investigated by Laser-Induced Breakdown Detection (LIBD)

This study investigates the appropriateness of monodisperse carboxylated polystyrene spheres as clay colloid analogues. Colloid transport experiments using Na-illite are conducted within a parallel-plate fracture flow cell with an aperture of 0.75mm at pH 5 under low ionic strength (1mM NaCl) and laminar flow (7mL/h) conditions. The effects of collector surface material (Grimsel granodiorite or acrylic glass), fracture orientation and residence time on colloid retention are examined. In order to avoid full surface coverage effects, ultra-trace (30ppb) Na-illite colloid concentrations are used. Laser-induced breakdown detection is applied in order to detect and quantify the Na-illite colloids. The results are directly compared to the experiments of Stoll, et al. [1] who investigated the transport and surface interaction of carboxylated polystyrene spheres (25 and 1000nm diameter) using the same experimental setup and conditions. It is concluded that polystyrene spheres are no ideal analogue to predict the mobility and transport behavior of a polydisperse clay colloid suspension. Despite the higher solid density, the Na-illite colloids show higher mobility compared to the polystyrene spheres without significant impact of surface roughness or gravity. The platelet shape, the non-uniform surface charge distribution and slower sedimentation may account for the observed differences.

Electrostatic adsorption and orientational order of inhomogeneously charged particles on flat surfaces

Abstract Electrolyte solutions often contain globular macroions such as certain proteins, colloids, and lipid vesicles that carry an asymmetric, non-uniform surface charge distribution. For the electrostatic adsorption of these macroions onto an oppositely charged planar surface, orientational entropy contributions becomes relevant and thus influence the structure of the electric double layer. We consider a simple model electrolyte that contains besides small salt ions also spatially extended spherical particles with one single surface charge or two surface charges attached on opposite sides and employ both a modified Poisson-Boltzmann approach and Monte Carlo simulations to study how these particles adsorb on a flat surface. Our Poisson-Boltzmann approach accurately accounts for the shape of the spherical particles and their charge distribution but neglects correlations and steric interactions between different particles. We characterize the degree of adsorption and how that is influenced by the particles' charge distribution and their orientational entropy loss. We generally find our mean-field approach in good agreement with Monte Carlo simulation results.

Predicting electrostatic charge on single microparticles

Electrostatic charge significantly affects the adhesion of microparticles. Presently the nature of surface charge distribution on a single microparticle and the effect of electrostatic charge as a contributor to its adhesion are not well understood. As a result of the strong discrepancies between the experimental adhesion measurements data and theoretical predictions over the years, some questions regarding adhesion force contributors in charged microparticles still remain unresolved. One reason for the current state of knowledge is difficulties associated with accurate measurement of particle charge. In current work, a non-contact and non-destructive opto-ultrasonic method is presented and employed to predict the equivalent bulk charge on a single charged micro-scale toner particle and to provide insight into the nature of charge distribution on its surface. From the vibrational spectral response of the particle to the ultrasonic excitation of the substrate, irregular shifting patterns of the vibrational (in-plane rocking) resonance frequencies of the particle are observed for the applied levels of substrate surface voltage (0–1500V), implying non-uniform (patch-charges) surface charge distribution on the microparticle. For predicting the equivalent bulk charge on a single patch-charged toner particle from this resonance frequency shift, a mathematical model is developed and employed. In conclusion, it is demonstrated that, in a non-invasive/non-contact manner, the total charge on a single microparticle can be predicted using the presented experimental approach, and evidence for the non-uniform charge distribution on a single particle is observed and reported.

Charge contribution to patch-charged microparticle adhesion

Microparticle adhesion influenced by electrostatic charge has been a significant research interest for over past three decades or so in a wide spectrum of areas of interest from manufacturing (electrophotography, powder technology, metallurgy, and semi-conductor manufacturing) to natural phenomena (desert sandstorms and northern lights (auroras)). However, over the years, as a result of the strong discrepancies between the experimental adhesion measurements data and theoretical predictions, some key issues regarding the contributors of adhesion forces in charged microparticles and the nature of surface charge distribution still remain unresolved. In the current work, a non-contact ultrasonic approach is presented and employed for understanding the nature of charge distribution on a single microparticle and determining the effect of electrostatic charge on its adhesion in a non-invasive manner. From the vibrational spectra of the charged particle response to the ultrasonic substrate oscillations under various electrostatic loading conditions, three distinct shifting patterns of vibrational (rocking) resonance frequencies are observed for each level of applied substrate surface voltage, implying an un-symmetric force field on the particle, thus depicting non-uniform non-symmetric surface charge distribution on its surface. Also, a simple mathematical model was presented and employed for predicting the equivalent bulk charge on a single microparticle (toner) from resonance frequency shifts. In summary, it is found that the charge levels reported here are consistent with the previously published data, and it is demonstrated that, in a non-invasive manner, non-uniform charge distribution on a single microparticle can be observed and its total charge can be predicted.

Electrophoresis of pH-regulated, zwitterionic particles: Effect of self-induced nonuniform surface charge

The electrophoresis of a pH-regulated, zwittterionic nanoparticle, taking account of its self-induced nonuniform surface charge, the presence of multiple ionic species, and the effect of double-layer polarization, is modeled. This extends previous models to a case closer to reality because the charged properties of a particle can be influenced by both its physicochemical nature and the solution conditions. We show that the nonuniform surface charge distribution induces a local electric field having the direction either opposite to or the same as that of the applied electric field, thereby either reducing or raising the particle velocity. This effect is most important when both the surface potential and the degree of nonuniformality are high. The competition of the effects of the position dependent double layer thickness, the surface proton distribution, and the double-layer polarization yields complicated and interesting behaviors. Depending upon the level of pH, the influence of the types of background salt on the particle mobility can be significant.

Electrostatic adhesion of multiple non-uniformly charged dielectric particles

Charged particle adhesion measurements can be as much as one order of magnitude higher than theoretical calculations. This discrepancy has ignited a debate as to the nature of the physics occurring with microscopic particle adhesion. Attempts to bring calculations closer to measured results have produced models that include multiple particle interactions, dielectric polarization, and non-uniform charge. Individually, each of these models can only produce a 2× to 5× enhancement in predicted adhesion force over simple Coulombic attraction. In this correspondence, an analytical model of electrostatic particle forces is developed, which allows for independent assignment of dielectric constant and non-uniform surface charge distribution to an arbitrary number of particles. Because the model can include an image plane, it is ideal for electrostatic adhesion calculations. Application to a monolayer of printed toner particles predicts nearly an order of magnitude increase in adhesion force over Coulombic attraction. These results are the first analytical results to place predictions of charged particle adhesion on the same order of magnitude with measurements.

Double Charge Inversion in Polyethylenimine-Decorated Liposomes

The study of the interaction of a cationic polymer as PEI with phospholipids membranes is of special relevance for gene therapy because the PEI is a potential nonviral vector to transfer DNA in living cells. We used light scattering, zeta potential, and electron transmission microscopy to characterize the interaction between DMPG and DOPC liposomes with PEI as a function of the charge molar ratio, pH, temperature, initial size of the liposomes, and headgroup of the lipids. Unexpectedly, a double charge inversion and two different ranges of PEI-liposome concentrations where an aggregation occurs were found, when the proper pH and initial size of the liposomes were chosen. The interaction is analyzed in terms of the interaction potential proposed by Velegol and Thwar for colloidal particles with a nonuniform surface charge distribution. Results show a remarkable dependence of the stability on pH and the initial size of the liposomes, which explains the low reproducibility of the experiments if no special care is taken in preparing the samples. Comparatively small changes in the pH or in the liposomes size lead to a completely different stability behavior.

Adsorption of Transgenic Insecticidal Cry1Ab Protein to Silica Particles. Effects on Transport and Bioactivity

Bt crops are genetically modified to be resistant against insect pests by expressing insecticidal Cry proteins. The processes governing the fate and bioavailability of the expressed transgenic Cry proteins in soils are poorly understood. We studied adsorption of Cry1Ab to negatively charged silica (SiO(2)) particles, a major soil constituent and a model for negatively charged mineral surfaces, at pH 5 to 10 and ionic strengths I = 10 mM to 250 mM, both in solution depletion and saturated column transport experiments. Cry1Ab-SiO(2) interactions were dominated by patch-controlled electrostatic attraction (PCEA), as evident from increasing Cry1Ab attraction to SiO(2) with decreasing I at pH at which both Cry1Ab and SiO(2) were net negatively charged. Experimental and modeling evidence is provided that the surface heterogeneity of SiO(2) particles modulated PCEA, leading to a fraction of adsorption sites with slow Cry1Ab desorption kinetics. Desorption rates from these sites increased upon increasing the solution pH. In toxicity bioassays, we demonstrated that Cry1Ab retained insecticidal activity when adsorbed to SiO(2), suggesting high protein conformational stability during adsorption-desorption cycles. Models predicting Cry1A protein adsorption in soils therefore need to account for combined effects of the nonuniform protein surface charge distribution and of sorbent surface heterogeneity.

Comparative Effects of pH and Ionic Strength on Protein–Protein Interactions, Unfolding, and Aggregation for IgG1 Antibodies

Changes in protein-protein interactions, protein unfolding, and nonnative aggregation were assessed for a series of human IgG1 antibodies as a function of pH and solution ionic strength (I). Unfolding transitions were characterized with differential scanning calorimetry. Protein-protein interactions were characterized with the apparent second virial coefficient (A(2)) from light scattering. Aggregation pathways were assessed using size-exclusion chromatography and multi-angle laser light scattering, aggregation kinetics, and structural changes monitored by circular dichroism spectroscopy and thioflavine T (ThT) binding. Ionic strength had relatively minor qualitative effects on unfolding, while pH had large effects for all four antibodies. A(2) was sensitive to both pH and I, and indicated that electrostatic interactions and nonuniform surface-charge distributions were important near neutral pH. Depending on solution pH and I, distinct aggregation pathways were found for each antibody, and these shared similar patterns versus pH, I, and A(2). Main differences observed across different antibodies included thermal unfolding transitions in DSC and the effects of pH and I on aggregation kinetics and pathways. These correlated strongly with whether aggregates of a given antibody bound ThT, suggesting possible differences with respect to conformational changes and/or regions of the proteins that are structurally involved in stabilizing the aggregates.

Adsorption of Transgenic Insecticidal Cry1Ab Protein to SiO2. 2. Patch-Controlled Electrostatic Attraction

Adsorption governs the fate of Cry proteins from genetically modified Bt crops in soils. The effect of ionic strength (I) on the adsorption of Cry1Ab (isoelectric point IEP(Cry1Ab) ≈ 6) to negatively charged quartz (SiO(2)) and positively charged poly-L-lysine (PLL) was investigated at pH 5 to 8, using quartz crystal microbalance with dissipation monitoring and optical waveguide lightmode spectroscopy. Cry1Ab adsorbed via positively and negatively charged surface patches to SiO(2) and PLL, respectively. This patch controlled electrostatic attraction (PCEA) explains the observed increase in Cry1Ab adsorption to sorbents that carried the same net charge as the protein (SiO(2) at pH > IEP(Cry1Ab) and PLL at pH < IEP(Cry1Ab)) with decreasing I. In contrast, the adsorption of two reference proteins, BSA and HEWL, with different adsorption mechanism, were little affected by similar changes of I. Consistent with PCEA, Cry1Ab desorption from SiO(2) at pH > IEP(Cry1Ab) increased with increasing I and pH. Weak Cry1Ab-SiO(2) PCEA above pH 7 resulted in reversible, concentration dependent adsorption. Solution depletion experiments showed that PCEA also governed Cry1Ab adsorption to SiO(2) particles at environmentally relevant concentrations (a few ng mL(-1)). These results imply that models describing Cry1Ab adsorption to charged surfaces in soils need to account for the nonuniform surface charge distribution of the protein.

Adsorption of Transgenic Insecticidal Cry1Ab Protein to SiO2. 1. Forces Driving Adsorption

Genetically modified Bt crops express insecticidal Cry proteins (Bt toxins) that may enter agricultural soils. A mechanistic understanding of Cry protein adsorption to soils is critical for risk assessment, as this process governs Cry protein fate and bioavailability. We used quartz crystal microbalance and optical waveguide lightmode spectroscopy to elucidate the driving forces of the adsorption of monomeric Cry1Ab to negatively charged quartz (SiO(2)) and positively charged poly-L-lysine (PLL) at pH 5-8 and constant ionic strength of 50 mM (NaCl). Bovine serum albumin and hen egg white lysozyme were used as reference proteins because of their known adsorption behavior. Electrostatics governed Cry1Ab adsorption; as pH increased above the isoelectric point of Cry1Ab, the initial rate and the extent of adsorption decreased on SiO(2) and increased on PLL. Reversible adsorption to SiO(2) suggested weak Cry1Ab-SiO(2) electrostatic interactions and no irreversible conformational changes of Cry1Ab at the surface. High conformational stability of Cry1Ab was further supported by supply rate-independent extent of adsorption of Cry1Ab to apolar gold. Some evidence is presented that the nonuniform surface charge distribution of Cry1Ab resulted in patch-controlled electrostatic attraction with sorbents that carried the same net charge as Cry1Ab. A more detailed discussion of this mechanism is given in a companion paper.