Balance Of Electrostatic Forces Research Articles

Electrophoretic Capture and Detection of Nanoparticles at the Opening of a Membrane Pore Using Scanning Electrochemical Microscopy

Stochastic electrophoretic capture of individual nanometer-scale particles at the small opening of a conically shaped nanopore in a synthetic membrane is described. Particle capture is sensed using a scanning electrochemical microscope (SECM) to measure the decrease in the transport rate of a redox-active molecule through the pore. The SECM tip is positioned at the larger backside opening of pore and used to amperometrically monitor the transport rate prior, during, and after particle capture. Following capture, the particle is released by electrophoretically driving it out of the pore opening and back into the solution. The capture and release method is demonstrated by detection of charged polystyrene spheres (43-150-nm diameter) using a polycarbonate membrane with conically shaped pores, the small opening of the pore having a diameter of 60 nm. The inverse of the time to capture polystyrene spheres increases with particle concentration over the range 10(8)-10(10) particles/mL. Selective detection based on nanoparticle charge and size is also demonstrated. A quantitative theoretical description of the rate of particle capture is presented, and the physical mechanism of particle capture, based on the balance of electrostatic and entropic forces, is considered.

Optimization of axisymmetric parametric model for electrostatic force balance

A two-dimensional axisymmetric parametric model was developed to calculate the vector value of the small force between the electrodes of a cylindrical capacitor with a given voltage, which is called an electrostatic force balance and studied to establish a force standard traceable to the SI in the range from µN to nN. The reasons for developing this model are (1) to optimize the design of the electrostatic force balance and (2) to improve the accuracy of the force standard. NIST will check this model by calculating the forces with mechanical forces of their prototype electrostatic force balance loaded with deadweights of several mg.

Progress toward Système International d'Unités traceable force metrology for nanomechanics

Recent experiments with the National Institute of Standards and Technology (NIST) Electrostatic Force Balance (EFB) have achieved agreement between an electrostatic force and a gravitational force of 10−5 N to within a few hundred pN/μN. This result suggests that a force derived from measurements of length, capacitance, and voltage provides a viable small force standard consistent with the Système International d’Unités. In this paper, we have measured the force sensitivity of a piezoresistive microcantilever by directly probing the NIST EFB. These measurements were linear and repeatable at a relative standard uncertainty of 0.8%. We then used the calibrated cantilever as a secondary force standard to transfer the unit of force to an optical lever–based sensor mounted in an atomic force microscope. This experiment was perhaps the first ever force calibration of an atomic force microscope to preserve an unbroken traceability chain to appropriate national standards. We estimate the relative standard uncertainty of the force sensitivity at 5%, but caution that a simple model of the contact mechanics suggests errors may arise due to friction.

Developing an axisymmetric parametric model for an electrostatic force balance

An axisymmetric parametric model, based on a two-dimensional finite element method(FEM), was developed in order to calculate both the electric field around a chargedcoaxial cylindrical capacitor and the force between its electrodes. It enables us tooptimize the design and structure of an electrostatic force balance. This model willbe used as a cross-check by comparing the force derived from its deadweightwith the calculated one, especially in the micro-Newton to nano-Newton range.

Parallelism error analysis and compensation for micro-force measurement

The null balance method for micro- and nano-force measurement is widely used in variousprecision industries. A parallel spring and a lever mechanism are designed andanalysed for an electrostatic force balance by the null balance method. Variousdesign parameters are required to obtain high resolution, a large measurementrange, and fast response. The corner loading error is one of the most dominanterror sources that should be removed. A parallel spring known as the Robervalmechanism, used in the micro-weighing device, can be used for one-axis forcemeasurement since it is very sensitive to the vertical direction force, but insensitiveto other force components. However, precise control of parallelism to below amicrometre is required not only to compensate for but also to remove the cornerloading error. In this paper, the effects of the parallelism error have been analysedusing the Lagrange method and verified by the FEA method. It was found thatparallelism control by the proposed precise compensation mechanism enablesthe simultaneous elimination of and compensation for the corner loading error.

A Mathematical Model Of Particle Transport In Electrostatic Spray Coating

Although electrostatic spray coating (E-spray) is widely used, its complexity requires optimization based on an empirical understanding of the spray dynamics. The project goal is to develop a mathematical model of the electrostatic field, continuum flow-field, and particle trajectories in an Espray process. By restricting the use of empirically based equations to the atomization phase of the spray process, this model should have the flexibility to tolerate real-world system complexities (i.e. multiple applicators, complicated geometries, etc.) and the ability to be used with any type of Espray gun sharing the same atomization characteristics. This model predicts coupling between three components: the fluid mechanics of the continuum flow field, the electrostatic field, and the particle trajectories. The system is a vertical bell-cup sprayer and a grounded disc centered on the gun axis. An axisymmetric electrostatic model is assumed, while the fluid mechanics and particle trajectories are solved in 3-D. A dilute spray assumption (no particle-particle interactions) allows modeling single-particle trajectories resulting from a balance of electrostatic force, drag and inertia. Varying the particle size generates volume-averaged properties of individual paths to simulate the charge density and fluid drag of a sprayed particle distribution. A turbulence energy-dissipation rate (K-E) model provides the continuum velocity for the particle drag. These individual systems are solved sequentially and that sequence is iterated to convergence. Results include the effect of charged particles on the electrostatic field and identification of the dominant factors affecting coating thickness distribution. Transactions on Engineering Sciences vol 42, © 2003 WIT Press, www.witpress.com, ISSN 1743-3533

Probe Tip Force and Beam States Observer for Position Control of a Scanning Stylus Nano Profiler

The small rocking beam electrostatic force balance sensor is an effective metrology tool for applications such as micro topography of semiconductor devices. During the vertical scanning of the sample surface the capacitor sensor of the beam measures the angular displacement of the rocking beam. To design an effective control system for high speed imaging of a sample surface it is desirable to estimate the force acting between the probe tip and the sample surface, and also the states of the force sensor from the measured signal. These estimated variables are used for feedback position control of the force sensor and the scanning piezo actuator assembly for vertical positioning of the probe tip relative to the sample surface. A force / beam states observer for the rocking beam force sensor is investigated in this paper. Simulated result is presented for performance evaluation of the proposed observer.

Lipid Matrix Properties in Cationic Membranes Interacting with Anionic Polyelectrolytes: A Solid-State NMR Approach

A combined solid-state NMR and small-angle X-ray scattering approach was employed to investigate morphology and lateral packing properties of mixed zwitterionic and cationic lipid membranes in the presence of an anionic polyelectrolyte. Binary mixtures of zwitterionic DMPC and cationic DODAP form stable lipid bilayers over a wide range of mixing ratios. Addition of negatively charged dextran sulfate (DS) leads to the formation of densely packed lipid bilayer stacks with the polyelectrolyte sandwiched between opposing lipid surfaces. Bridging of polyelectrolyte strands between bilayer surfaces provides attractive forces to overcompensate electrostatic and hydration repulsion, resulting in close approach of apposing membranes. The equilibrium surface distance is controlled by a balance of electrostatic, hydration, bridging, and entropic forces. Lipid molecules in the mixed membranes remain homogeneously distributed in both the absence and presence of DS. DS binds exclusively to cationic DODAP. As a result o...

An Electrostatic Servo Capacitive Pressure Sensor

We have fabricated an electrostatic servo capacitive pressure sensor by using bulk silicon micromachining and SODIC(SOot-Deposited Integrated Circuit) wafer. The sensor consists of fixed electrodes and two diaphragms which are connected with pillars to move together for electrostatic servo force balancing. It detects pressure by the servo-voltage which is supplied between the lower diaphragm and the fixed electrodes to keep the diaphragm at the initial position. The diaphragm position is detected by the capacitance change between the upper diaphragm and the fixed electrodes. The capacitances versus pressure and the servo-voltage are measured. The results of these mea-surements show that this sensor can detect the pressure by the servo-voltage and have high sensitivity and wide dynamic range in comparison with the open-loop sensors. The sensor has an advantage that it is not affected by dusts or particles because the capacitor gap is isolated from the external environments.

Precise Micro-Nanomachining of Silicon

Advanced micro sensors for ultrasonic imaging, pressure, acceleration, angular rate and atomic force have been realized based on silicon bulk-micromachining. Novel techniques as deep Reactive Ion Etching (RIE), XeF2 silicon etching have been developed for precise micromachining and applied for the sensors. The deep RIE of silicon has been applied for electrostatic microactuators. Parallel electrodes in a packaged glasssilicon structure enable electrostatic force balancing servo sensors, resonant sensors and electrostatically levitating micromotors. Nano-machining based on the Scanning Tunneling Microscopy (STM) technology have been also developed to fabricate extremely small structures.

Uptake and Intracellular Distribution of 4-Aminofluorescein-Labelled Poly(L-Lysine Citramide Imide) in K562 Cells

4-Aminofluorescein (F1-NH2) was conjugated with various macromolecular carriers of the poly(L-lysine citramide)-type which were hydrophobised by ethyl (C2), heptyl (C7), and dodecyl (CI2) alky] groups attached to the pendent carboxyl of the lysine moieties present in repeating units. The dye was used to label the carriers and monitor their intracellular fate after introduction within the incubation medium of K562 cells. The labelled hydrophobised carriers formed multimolecular compacted aggregates stabilised by the balance of attractive hydrophobic interactions and repulsive electrostatic forces in the aqueous culture medium. The apparent molecular weights and the sizes of these aggregates were determined by Size Exclusion Chromatography (SEC) and by light scattering respectively. Comparison was made of the cell distribution of free and conjugated F1-NH2 in the cell cytoplasm and nucleus by using fluorescence microscopy and laser microspectrofluorometry. It was shown that cell uptakes resulted from adsorptive pinocytosis and depended on hydrophobicity and aggregation of the conjugates. The influence of physical entrapment of free-Fl-NH2 within the hydrophobic microdomains formed by aggregates F1-NH2 conjugates was also discussed.

Vacuum-sealed silicon micromachined pressure sensors

Considerable progress in silicon pressure sensors has been made in recent years. This paper discusses three types of vacuum-sealed silicon micromachined pressure sensors that represent the present state of the art in this important area. The devices are a capacitive vacuum sensor, a surface-micromachined microdiaphragm pressure sensor, and a resonant pressure sensor. Vacuum sealing for these devices is accomplished using anodic bonding, films deposited using low-pressure chemical vapor deposition, and thermal out-diffusion of hydrogen, respectively. These sensors emphasize high sensitivity, small size, and excellent stability, respectively. The silicon-diaphragm vacuum sensor uses electrostatic force balancing to achieve a wide pressure measurement range.

Chain model of electrorheology

We develop a chain model of an electrorheological fluid in steady and oscillatory shear. This model is based on a balance of hydrodynamic and electrostatic forces and focuses on the mechanical stability of chains. With this model we compute the shape and orientation of an unconfined dipolar chain in steady shear as a function of the shear rate and electric field, and show that this leads to the expected shear-thinning viscosity. We then demonstrate that chains confined by electrodes are unstable in some Mason number regimes. The chain model is extended to the case of oscillatory shear by including a kinetic equation for the aggregation and fragmentation of chains. The resultant chain dynamics is found to be strongly nonlinear, as is the rheology. Finally, we consider the effect of local field corrections and long range interactions on the bare dipolar interaction force and derive a self-consistent pair interaction force that demonstrates that the maximum orientation of a chain in shear depends strongly on the dielectric contrast.

Condensation of DNA by basic protiens does not depend on protien composition

AbstractWe have studied by electron microscopy the size and morphology of the complexes obtained with different DNAs (between 500 and 5243 base pairs long) and four different proteins: sea urchin histone H1; sea cucumber histone ϕ0, chicken erythrocyte histone H5, and clupeine. Surprisingly, the type of protein used has only a marginal influence on the complexes formed. The molecular weight and topology of DNA do not show any influence. The size of the complexes depends strongly on the ratio of positive to negative charges and also on the ionic conditions. Our studies have been mainly carried out at a ratio of 0.4. Under these conditions the average thickness of rods and toroids observed varies between 165 Å at 1.5 mM salt to 290 Å at 100 mM salt, with minor variations around these values depending on the type of DNA and protein used. We conclude that the formation of DNA condensates is mainly determined by a balance of electrostatic and intermolecular forces, the influence of specific interactions is only marginal. This conclusion seems to apply not only to the complexes described here, but also to chromatin fibers and to DNA condensed by low molecular weight counterions and other compounds (polyamines, inorganic ions, ethanol, etc.). © 1994 John Wiley & Sons, Inc.

Electron and ion emission from liquid metal surfaces

The application of positive or negative electric fields to small-area liquid metal surfaces leads to very high brightness DC ion or pulsed electron emission. The stabilization of a cone-shaped structure by the balance of electrostatic and surface tensions forces is described. Electron and ion emission occurs by field emission and field evaporation mechanisms.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

The effect of adsorbed lysozyme on interaction forces and coalescence of triglyceride droplets in aqueous salt solutions

We have used optical reflectance to measure the thickness of the draining film between droplets of a synthetic triglyceride (MCT810) brought together in aqueous solutions containing lysozyme (10 −3% w/w) and sodium chloride at 10 −1 to 10 −5 M. Drainage was essentially complete after ca. 30 s, the draining film then being close to plane parallel with a thickness in agreement with that calculated from a balance of electrostatic repulsive forces and capillary and van der Waals attractive forces, except at the highest salt concentration (10 −1 M) examined. Thicknesses ranged from 30 to 145 nm. The aqueous film collapsed after a time ranging from a few seconds to several hours, producing droplet coalescence. The median time to coalescence and the interquartile range increased with increasing film thickness. These data are not consistent with the concept of a “critical thickness” for the collapse of protein-stabilized films. The implications of the results for the use of proteins as emulsion stabilizers are discussed.

X-ray diffraction studies on cation-collapsed DNA

The polyamines spermidine, spermine and putrescine are now known to induce tertiary collapse of DNA. In this collapsed state DNA assumes a compact toroidal conformation. However, the structural details of DNA in these compact particles and the forces that stabilize the collapsed state are not clear. We show here that the structural arrangement of DNA in this tertiary conformation is determined by the chemical structure of the agent used to collapse. We have used aliphatic triamines (NH 3 +-(CH 2) 3-NH 2 +-(CH 2) n -NH 3 + with n = 3, 4, 5 and 8) and diamines (NH 3 +-(CH 2) x -NH 3 + with x = 2, 3, 4 and 6) to collapse DNA. We find that the Bragg spacing and the calculated interhelical spacing for a hexagonal packing model vary systematically with the length of the methylene bridge. We also find that the ionic strength of the solution has no effect on the Bragg spacing. This observation suggests that the arrangement of DNA strands in the complexes is determined by the structure of the polycation, and argues against suggestions that the structure of the collapsed state is maintained by the balance of long-range electrostatic repulsive and attractive forces. Instead we propose that DNA helices form a hexagonal array with counterions in the interstices between the helices resulting in a stable three-dimensional phase with high structural order. Arguments are presented favoring such a model in terms of stabilizing and destabilizing thermodynamic forces.

A study of the effect of sodium dodecyl sulfate on bovine β-casein self-association

The effect of low concentrations of sodium dodecyl sulfate on the self-association of β-casein in solution has been reinvestigated at neutral pH by using instrinsic fluorescence measurements, analytical ultracentrifugation, gel filtration chromatography, and the fluorescent properties of the probe, anilinonaphthalene sulfonate. Sodium dodecyl sulfate was found to interact with the protein so that the normal equilibrium between monomers and micellelike polymers was displaced toward polymer formation. At higher concentrations of sodium dodecyl sulfate, the β-casein polymers became smaller while the monomer-polymer equilibrium remained displaced toward polymer formation. It seems likely that there is a limited number of sites on the β-casein molecule that bind sodium dodecyl sulfate strongly. As a consequence of this binding, the balance of electrostatic and hydrophobic forces is altered to increase the degree of self-association at low concentrations of sodium dodecyl sulfate, despite the increase in net negative charge per protein monomer.

X-ray diffraction studies of the filament lattice of striated muscle in various bathing media

The low-angle equatorial X-ray reflections from the filament lattice of vertebrate striated muscle have been investigated as a function of sarcomere length and the nature of the bathing medium. In glycerinated (rabbit psoas) muscle, the lattice dimension was found to be a function of sarcomere length, and the pH, ionic strength and valency of cations in the inter-filament medium. The total variation of the myosin—myosin centre-to-centre separation according to these parameters was 370 to 570 Å. Similar effects were also demonstrated in living (toad sartorius) muscle: the myosin—myosin centre-to-centre separation varied between 310 and 510 Å according to the sarcomere length, and the pH and ionic strength of the bathing medium. The dependence of the filament separation on the electrical parameters of the inter-filament medium indicates that there are long-range electrostatic forces between the filaments. It is suggested that the contractile apparatus of muscle behaves as a colloid in which the separation of the filaments is determined by the balance of electrostatic double-layer repulsive forces and van der Waals' attractive forces.