Electrostatic Adhesion Force Research Articles

Development of microscopic rheological model of cementitious suspension containing attapulgite based on water film thickness and bonding strength coefficient

Based on the theoretical framework of water film thickness (WFT) and bonding strength coefficient (ζ), the motion and collision laws of particles in shear flow field were analyzed by using continuum mechanics, particle collision mechanics, fluid mechanics, thermodynamics and hydration dynamics, and the microscopic rheological model of cementitious suspension containing attapulgite (BGA and GBA) was established to predict the rheological parameters. The results indicated that the error range of rheological parameters predicted by the microscopic rheological model based on WFT-ζ and the experimental values was 0.09%–8.66%. The model can well predict the influence of BGA and GBA on rheological parameters. In addition, ζ has been proved to be closely related to the net force of four microscopic forces in the cementitious suspension, including gravity, van der Waals force, electrostatic double repulsion force and adhesion force caused by hydration product formation, which can characterize the bonding strength between particles.

Strong Reliable Electrostatic Actuation Based on Self-Clearing Using a Thin Conductive Layer.

Electrostatic adhesion, as a promising actuation technique for soft robotics, severely suffers from the failure caused by the unpredictable electrical breakdown. This study proposes a novel self-clearing mechanism for electrostatic actuators, particularly for electrostatic adhesion. By simply employing an enough thin conductive layer (e.g., <7 μm for copper), this method can spontaneously clear the conductor around the breakdown sites effectively once breakdowns onset and survive the actuator shortly after the electrical damage. Compared with previous self-clearing methods, which typically rely on new specific materials, this mechanism is easy to operate and compatible with various materials and fabrication processes. In our tests, it can improve the maximum available voltage by 260% and the maximum electrostatic adhesive force by 276%. In addition, the robustness and repeatability of the self-clearing mechanism are validated by surviving consecutive breakdowns and self-clearing of 173 times during 65 min. This method is also demonstrated to be capable of recovering the electrostatic pad from severe physical damages such as punctures, penetrations, and cuttings successfully and enabling stable and reliable operation of the electrostatic clutch, or gripping, for example, even after the short-circuit takes place for hundreds of times. Therefore, the proposed self-clearing method sheds new light on high performance and more extensive practical applications of electrostatic actuators in the future.

An Electrostatic Locking Mechanism For Tendon-driven Surgical Robot

Variable stiffness surgical robots are capable of adapting to different surgical scenarios, providing enhanced stability during minimally-invasive surgeries. Prevalent methods for achieving variable stiffness, such as lockable mechanisms, granular jamming, and phase transition materials, have inherent weaknesses including difficulties in downsizing and issues related to excessive temperatures. In this paper, a novel lockable mechanism based on electrostatics is proposed, along with a tendon-driven surgical robot prototype that utilizes this locking mechanism to achieve variable stiffness. The proposed mechanism involves the installation of graphite electrode sheets at each joint, with silica gel being used as the dielectric material. By applying a high voltage to the electrode sheets, adjacent joints automatically absorb and restrict relative sliding, effectively preventing joint rotation and ensuring complete joint locking. An electrostatic adhesive braking force model for the robot is presented. Simulations and experiments conducted based on the robot prototype have been carried out to validate the locking capability of the proposed mechanism and the accuracy of the electrostatic adhesive braking force model.

Top-Down Preparation of Nanoquartz for Toxicological Investigations.

Occupational exposure to quartz dust is associated with fatal diseases. Quartz dusts generated by mechanical fracturing are characterized by a broad range of micrometric to nanometric particles. The contribution of this nanometric fraction to the overall toxicity of quartz is still largely unexplored, primarily because of the strong electrostatic adhesion forces that prevent isolation of the nanofraction. Furthermore, fractured silica dust exhibits special surface features, namely nearly free silanols (NFS), which impart a membranolytic activity to quartz. Nanoquartz can be synthetized via bottom-up methods, but the surface chemistry of such crystals strongly differs from that of nanoparticles resulting from fracturing. Here, we report a top-down milling procedure to obtain a nanometric quartz that shares the key surface properties relevant to toxicity with fractured quartz. The ball milling was optimized by coupling the dry and wet milling steps, using water as a dispersing agent, and varying the milling times and rotational speeds. Nanoquartz with a strong tendency to form submicrometric agglomerates was obtained. The deagglomeration with surfactants or simulated body fluids was negligible. Partial lattice amorphization and a bimodal crystallite domain size were observed. A moderate membranolytic activity, which correlated with the number of NFS, signaled coherence with the previous toxicological data. A membranolytic nanoquartz for toxicological investigations was obtained.

On the mechanism of performance improvement of electroactive polyvinyl chloride (PVC) gel actuators via conductive fillers

The electromechanical actuation of transparent plasticized polyvinyl chloride (PVC) gels with conductive fillers were studied. The effects of functionalized carbon nanotubes (CNTs) and 1-butyl-3-methylimidazolium tetrafluoroborate ([Bmim]BF4) ionic liquid (IL) on both the electrical conduction and dielectric processes within PVC gels were investigated, and the differences between the two were clarified. Both CNTs and IL were shown to increase the conductivity of the gels and produce larger electromechanical transduction of a contraction actuator, but only CNTs were shown to increase the electrostatic adhesion force of the PVC gels. The addition of charge carriers to the gel via the inclusion of ILs was shown to significantly reduce the conductivity relaxation time, and the transient current upon voltage polarity reversal indicated multiple peaks corresponding to the introduction of carriers with different polarities and mobilities into the gel. This is believed to cause a screening effect, reducing the charge accumulation at the anode that is the foundational basis for PVC gels’ actuation mechanism. A recommendation for preferable conductive fillers for various applications is made.

A study of electroactive polyvinyl chloride (PVC) gel actuators through the use of the electric modulus formalism and cyclic linear voltage sweeps

The conductivity and dielectric properties are integral to the function of polyvinyl chloride (PVC) gel actuators. The frequency-dependent properties of PVC gel actuators are investigated here in terms of their impedance, permittivity, and for the first time, electric modulus. The data shows that PVC gels’ conductive properties are just as, if not more, important as their dielectric properties in electromechanical transduction applications. The electrode polarization (EP) and its impact on the impedance and dielectric spectra of PVC gels, as well as the developed asymmetric space charge at the anode, are discussed. The electric modulus and tan spectra are used for the fitting of Cole-Cole (CC) and Debye relaxation models for gels of varying plasticizer content. The electrostatic adhesive force for PVC gels of varying plasticizer is also measured, indicating large electrostatic adhesion (>2 N cm−2). A cyclic linear voltage sweep is used to clarify the dynamics of space charge within the gels. The peak current (associated with space charge development) is seen to be concurrent with the onset of mechanical deformation, showing the asymmetric charge as the origin of electromechanical transduction. Additionally, the maximum charge transferred (as measured by the integration of current over time) before space charge development is found to correlate with the electrostatic adhesive force measured for the gels, pointing to a new method of characterizing PVC gels for actuation.

Binding between an amine cationic surfactant and DNA and surface properties of the resultant aggregates

Binding between cetyltrimethylammonium bromide, a cationic surfactant, and a variety of lengths of single stranded DNA was measured using fluorescence polarization and a simple cooperative model was used to obtain dissociation constants on the order of 1 × 10−5 for the aggregates that formed. Aggregation depended on strand length where strands much shorter than 40 nucleotides (for example strands of 24-nucleotides) were too short to form the same size aggregates. Other factors such as salt concentration and temperature also affected aggregate formation: increasing either the salt concentration or performing binding at the highest temperature studied (60 °C) made it more difficult for aggregates to form. Both heating and dilution of aggregates caused the anisotropy signal to decrease, which suggested that the complexes fell apart under these conditions. Force spectroscopy of aggregate surfaces showed that both electrostatic and hydrophobic adhesive forces were present between aggregates and derivatized AFM tips. These findings can be used to better understand the stability of cationic surfactant-DNA aggregates and may provide guidance for lipid nanoparticle design used in vaccine development and therapeutics.

Isogeometric Analysis of Electrostatic Adhesive Forces in Two-Dimensional Curved Electrodes

Isogeometric Analysis of Electrostatic Adhesive Forces in Two-Dimensional Curved Electrodes

Theoretical Analysis and Experimental Study of Time-Varying Electric Field and Electrostatic Adhesion Force Generated by Interdigital Electrode Arrays

A theoretical model is presented for the analysis of the electric field and electrostatic adhesion force produced by interdigital electrode arrays. The electric field is derived by solving the Laplace equation for the electrical potential in each subregion. The electrostatic adhesion force is calculated using the Maxwell stress tensor formulation. The dynamic properties of the electric field and electrostatic adhesion force are assessed by evaluating the transient response of the field and force under a step in applied voltages. Experimental studies are carried out to evaluate the adhesion performance of an electrode panel on a glass pane, and the experimental results verify the correctness of the theoretical model.

Long-lasting strong electrostatic attraction and adhesion forces of dust particles on photovoltaic modules

Photovoltaic (PV) energy-yield loss due to solar module soiling has become increasingly important as solar module deployment is now at the hundreds of gigawatts scale and continues to grow rapidly. The electrostatic attraction and adhesion force (Fes) of dust particles by the high voltages from solar panels have been reported to be 1 to 2 orders of magnitude stronger than the van der Waals and water capillary forces, which has been corroborated by observing the increase in system voltage-induced soiling rate by setting up an outdoor test. Here, we report another characteristic of Fes on soiling—long-lasting or slow decay after turning off the high voltage applied to solar panels. The Fes decay time varies across a wide time range of 1 to 10 h, depending on two factors: 1) whether the cell, the particle, or both were charged with high voltage before the voltages were turned off; and 2) how the cell was connected to the ground after the voltage was turned off—either connected through the power-supply electronics, directly connected to the ground, or electrically floated. The Fes decay is understood in terms of 1) net electrical charge dissipations in both particle and cell, 2) thermal disordering of dipole polarization in the module glass dielectrics, and 3) charge redistribution by the electrostatic interaction of particle and module glass. This long-lasting Fes for hours can affect the solar panel soiling after sunset, and it can have an even greater effect when combined with water condensation at night.

Force and current in a contact gap between single highly resistive particles: experimental observations

In case of electrical conduction through highly resistive dust layers, the generation of electrostatic adhesion force is strongly coupled to the mechanism of electrical (current) transport in the solid. High field strengths lead to a significant increase of the adhesive force. Here, more insight into the underlying mechanisms is given by experiments on the microscopic scale. An experimental arrangement is described which allows to study a particle pair subject to a strong electric field. Both the current and the force between the particles (150 μm) can be measured as a function of voltage and gap distance. The results show an extremely complex behaviour of the contact for the case of highly resistive particles. For current transport, both gas discharges and thermionic field emission are observed, depending on the width of the contact gap and the field strength. For both the force and the current across the gap, a strongly non-linear behaviour with pronounced time effects is observed.

Testing the application of Free Flapping Foils (FFF) as a method to improve adhesion in an electrostatic wall-climbing robot

In this paper, extensive testing on an electrostatic wall-climbing robot is carried out to examine the electrostatic adhesion force when using the Free Flapping Foils (FFF) method. This method is successfully applied to an electrostatic robot to enable it to stick more firmly, by about three times, on walls with different materials, including wood, plaster, granite, glass and climb wall with a continuous movement. The innovation in this paper is that successful tests were carried out, when using thin flexible electrodes made from aluminium foil and free movement of those electrodes. The FFF electrodes are flexible enough to get the shape and roughness of the wall and increases the contact surface. Thus, they increase the adhesion force and would also improve performance of the robot in rough surfaces. The experimental parameters including charging voltage, charging time, number of electrodes, area of electrodes, wall material (permittivity) and surface finishing of wall that affect the electrostatic carrying force were investigated. The robot could cross obstacles with different heights. The final robot was 1.27 Kg and able to convey 14 N extra load when sticking and climbing vertical walls.

A tunable dielectric to improve electrostatic adhesion in electrostatic/microstructured adhesives

This paper introduces a method to improve the electrostatic adhesion force of an electrostatic/microstructured (gecko-like) adhesive using a tunable dielectric layer doped with a highly polarizable material, Copper(II) Phthalocyanine. The dopant increases the relative permittivity of the base elastomer by more than two times. Numerical simulations demonstrate that increasing the dielectric's relative permittivity increases the electroadhesion force, and experimental results verify this by showing that adhesion of a flat (non-microstructured) sample can be improved up to four times. Adding microstructured adhesive elements on top of the doped layer yields a 2x improvement on a variety of materials.

Interdependent Roles of Electrostatics and Surface Functionalization on the Adhesion Strengths of Nanodiamonds to Gold in Aqueous Environments Revealed by Molecular Dynamics Simulations.

Molecular dynamics simulations demonstrate that adhesion strengths as a function of charge for aqueous nanodiamonds (NDs) interacting with a gold substrate result from an interdependence of electrostatics and surface functionalization. The simulations reveal a water layer containing Na+ counterions between a negative ND with surface -COO- functional groups that is not present for a positively charged ND with -NH3+ functional groups. The closer proximity of the positive ND to the gold surface and the lack of cancelation of electrostatic interactions due to counterions and the water layer lead to an electrostatic adhesion force for the positive ND that is nearly three times larger than that of the negative ND. Prior interpretations of experimental tribological studies of ND-gold systems suggested that electrostatics or surface functionalization could be responsible for observed adhesion strength differences. The present work demonstrates how these two effects work together in determining adhesion for this system.

An analytical model for electrostatic adhesive dynamics on dielectric substrates

For electrostatic adhesion on a dielectric material, the electrostatic adhesion force gradually increases to a steady value after the voltage exerted onto the interdigital electrodes. However, little has been addressed to reveal governing mechanisms behind this dynamic phenomenon. In this paper, a theoretical model is presented for analysis of the dynamic properties of electrostatic adhesion on dielectric materials. Firstly, the electric field was derived by solving the Laplace equation of the electrical potential for each sub-area using general solution and boundary conditions. Then, the electrostatic adhesion force was obtained using the Maxwell stress tensor formulation. Finally, the dynamic properties of the electric field and electrostatic adhesion force were assessed by evaluating the transient response of the field and force under a step in applied voltages. Experimental studies for verification were conducted by evaluating the adhesion performance of an electrode panel on three different substrate plates: glass, wood and Polyvinylidene Fluoride (PVDF). Results from these experiments are highly consistent with the theoretical model. The overall results of this paper provide theoretical guidelines for systematic optimization of electrostatic adhesion technology in various application scenarios, such as electrostatic chucks, electrostatic suspension systems and electroadhesive wall-climbing robots.

Functional Wettability in Carbonate Reservoirs

Oil adsorbs to carbonate reservoirs indirectly through a relatively thick separating water layer, and directly to the surface through a relatively thin intervening water layer. Whereas directly sorbed oil desorbs slowly and incompletely in response to changes in reservoir conditions, indirectly sorbed oil can be rapidly desorbed by changing the chemistry of the separating water layer. The additional recovery might be as much as 30% original oil in place (OOIP) above the ∼30% OOIP recovered from carbonates through reservoir depressurization (primary production) and viscous displacement (waterflooding). Electrostatic adhesive forces are the dominant control over carbonate reservoir wettability. A surface complexation model that quantifies electrostatic adhesion accurately predicts oil recovery trends for carbonates. The approach should therefore be useful for estimating initial wettability and designing fluids that improve oil recovery.

Effective removal of emulsified oil from oily wastewater using in-situ generated metallic hydroxides from leaching solution of white mud

Leaching solution of white mud (LSWM), a form of industrial waste from alumina plants, was added to alkaline oily wastewater, and metallic hydroxides were in-situ generated (IGMHs). We evaluated the removal efficiency of emulsified oil by IGMHs from simulated alkali/surfactant/polymer flooding produced water. The influence of various factors, such as LSWM dosage, initial pH, temperature, and contact time on oil removal efficiency was investigated. It was found that approximately 95–99% of emulsified oil was successfully removed by the co-precipitation/adsorption process. In addition, highly basic conditions and lower temperatures were preferred for IGMHs to remove emulsified oil. Thermodynamic analysis showed that the co-precipitation/adsorption process was spontaneous, exothermic and physical in nature, involving weak interactions, such as electrostatic attraction, hydrogen bonding, and adhesive force between the emulsified oil droplets and the binding sites on the surface of IGMHs. It was observed that IGMHs lost approximately 18.9% adsorption capacity for emulsified oil after more than four cycles. Therefore, IGMHs could be used effectively for at least four cycles. Moreover, recovery of oil from oily wastewater could improve the economic viability of this treatment process.

Rapid and effective removal of sodium lignosulfonate from aqueous solutions by in-situ formed magnesium hydroxide

We investigated the efficiency of in-situ formed magnesium hydroxide (Mg(OH)2) for the removal of sodium lignosulfonate (SLSN) from aqueous solution. Adsorption experiments considered the effects of various conditions such as pH, MgCl2 concentration, contact time, and temperature on SLSN removal efficiency. It was found that approximately 93%-99% SLSN was removed by in-situ formed Mg(OH)2. The adsorption was rapid, and the contact time required to reach complete adsorption equilibrium was less than 2min. Moreover, that the Mg(OH)2 lost about 0.5–3.0% adsorption capacity for SLSN when NO 3 − , HCO 3 − , H2PO 4 − and SO 4 2− anions were simultaneously present with SLSN. The experimental data suggested that there was little competitive adsorption of SLSN with other coexisting anions on Mg(OH)2. The co-precipitation/adsorption process was exothermic and physical, involving weak interactions such as electrostatic attraction, hydrogen bonding, adhesive forces, and van der Waals forces between SLSN molecules and the binding sites on Mg(OH)2.

Effect of charge transfer on electrostatic adhesive force under different conditions of particle charge and external electric field

The electrostatics of charged particles are utilized for various applications. This paper presents an analysis of the electric field and electrostatic adhesive force on a charged dielectric particle lying on a conducting plane under an externally applied electric field. The purpose of the analysis is to quantitatively investigate the force variation when there is charge transfer between the particle and the conducting plane. We treat the distribution of charges as either uniform on the particle or partially on the lower half. The transferred charge density is assumed to be dependent on the applied electric field. The results show that the electric field is very strong near the contact point, where the charge transfer may occur. Without the charge transfer, the electrostatic adhesive force on a negatively charged particle increases when the applied field is in the upward direction from the plane. However, in the presence of charge transfer, the force may vary only slightly with the applied field or even show a reverse tendency if the transfer charge density depends significantly on the applied field.

Modelling and analysis of the electrostatic adhesion performance considering a rotary disturbance between the electrode panel and the attachment substrate

ABSTRACTIn real-world applications of electrostatic adhesion technology, the electrostatic adhesion force decreases when the electrode panel slides or rotates relative to the attachment substrate due to an external disturbance. However, little work has been done to study this phenomenon. This paper presents a model and analysis method to evaluate the electrostatic adhesion performance considering a rotary disturbance between the electrode panel and the attachment substrate. First, dynamic variations of the electric fields for any point on the attachment substrate are analysed. Then the decrease in the electrostatic adhesion force is explained based on the analysis results and our previous work. The analysis results and explanations are experimentally validated on three different attachment substrates. The fundamental cause of the decrease in adhesion force is the loss of the lined layer polarization due to the rotary disturbance. The results provide a good explanation to the phenomenon and make the existing electrostatic adhesion theory more self-consistent.