Role of Interfacial Interactions in the Deposition of Colloidal Clay Particles in Porous Media

Colloidal clay particle transport under saturated conditions is believed to be controlled by its interactions with the surrounding environment. The dominating forces among these interactions are electrostatic forces that are determined by colloidal clay particle and porous medium surface charge density and Lifshitz–van der Waals forces that are determined by colloidal clay particle and porous medium surface thermodynamic properties. Electrostatic forces are greatly affected by solution chemistry in terms of solution ionic strength and pH. In this research, electrostatic and Lifshitz–van der Waals forces of natural colloidal clay particles with a model porous medium of silica sand were quantified at different ionic strength and pH conditions. At the same time, colloidal clay particle transport in the model medium of silica sand was conducted in a laboratory column. The maximum electrostatic forces, F ^EL (max), which occurred when the separation distance between colloidal clay particles and the porous medium was in the range of the sum of the double layer thicknesses of the colloidal clay particles and the porous medium, was found to be the determinant factor for colloidal clay particle deposition in the porous medium. Colloidal clay particle desorption in the porous media was related to the net effect of attractive Lifshitz–van der Waals forces and repulsive electrostatic forces, evaluated at the equilibrium distance where physical contact between the colloidal clay particle and silica sand actually occurred (i.e., affix force). Higher colloidal clay particle desorption was found to coincide with smaller affix force values.     

Some Remarks on the Removal of Adhered Particles by Oscillating Air Flows

On the background of technical concepts for the removal of particles from moving textiles by linear as well as oscillating air flows, some theoretical remarks on particle adhesion forces and the hydrodynamical and acoustic forces acting on particles are given. As the main removal forces in linear air flows, drag and lift are taken into consideration and analytical expression derived from the Bernoulli equation. For a hypothetical example of particles in the size range from below 0.5 to 7 μm on a cotton fiber, hydrodynamic forces are compared to van der Waals interaction, electrostatic forces and capillary forces. In this work, a special focus is placed on the application of high-frequency oscillating air flows which results in a direct coupling of ultrasound to the particles. The theoretical calculation of resulting forces showed that the detachment threshold is determined not only by sound intensity level, but also by the frequency of the ultrasonic wave. If capillary forces act, the coupling of ultrasound alone does not suffice to detach particles within the considered size range.     

Contribution of Colloidal Forces in Non-contact Area to the Adhesion Between a Micro-probe and a Substrate Surface: Theoretical Analysis

Colloidal forces outside the microscopic probe (particle)–substrate adhesion contact area were analyzed theoretically. Equations describing the van der Waals, electrical double layer, and hydrophobic forces were derived for the non-contact area of a probe–substrate system assuming a simple sphere–flat geometry. Two cases were considered: particles freely resting on the substrate surface and particles pulling off the substrate. The results of modeling presented in this communication suggest that the adhesion of fine particles (microscopic and sub-microscopic particles) to flat surfaces can be affected by the forces acting outside the contact area. However, due to increased distance between the particle and substrate during separation, both the van der Waals and electrostatic forces acting outside the contact area are negligibly small compared to the short-range adhesion forces and they do not contribute to the measured pull-off force to any great extent for most systems. On the contrary, our calculations suggest that the long-range hydrophobic forces can contribute to the strength of adhesion between hydrophobic fine particles and hydrophobic substrates.     

Tribocharging of Three-Component Powder Mixtures

Adhesion of charged toner particles used in electrophotography is dominated by electrostatic forces. In this paper we discuss a model which describes the process by which toner particles acquire their electrostatic charge, tribocharging.

In previous papers, we have presented a model of tribocharging of two-component mixtures of powders based on the assumptions that:

1. The surface of each powder is populated with electron accepting and donating sites;
2. The density of states of the donating and accepting sites can be represented by narrow bands, all of which have the same energy; and
3. Charge is exchanged between donor and acceptor sites until thermodynamic equilibrium is established.

In this paper, we show how to extend this model to multi-component mixtures. The extended model can be used to calculate the charging behavior of three-component mixtures of electrophotographic toners and carriers based on measurements with two component mixtures. Experimentally-measured charging behavior agrees with the model predictions.

These results confirm it is possible to assign charging site densities to individual materials empirically. The site densities can then be used to predict charging behavior of the materials in mixtures which have not been studied experimentally. The success of the model also implies that toner particles migrate freely from carrier particle to carrier particle and that triboelectric interactions take place between toner particles of different compositions in mixtures.     

Adhesion between silica particles in an alcohol medium

The destruction times of the sediment column structures of hydrophobic and hydrophilic silica particles were measured using a simple device. Experiments were carried out for different fractions of both silica particles in alcohols ranging from ethanol to decanol. On the basis of linear relationships between the reciprocal of the destruction times of the silica sediment column structures and average diameters of the silica fractions, the density of alcohols and the work of cohesion of alcohols, and the critical values of these parameters were determined. For the silica particle/alcohol/silica particle system having a critical value of the average diameter of the particle (or the critical density of alcohol, or the critical work of cohesion of alcohol), the detachment force of one silica particle from another was found to be equal to the attachment force between them. The values of the detachment force were found to decrease with increasing length of the hydrocarbon chain of the alcohols. Using these force values and the critical work of cohesion of alcohol, the radii of the contact planes between two silica particles and then the attachment forces were calculated. The attachment force values increased with increasing length of the hydrocarbon chain of the alcohols. It has also been found that the critical parameters (diameter of silica particles, density and work of cohesion of alcohols) depend on the surface properties of the solid. Hydrophilization of the silica surface caused an increase of the average critical diameter and the critical work of cohesion and a decrease of the critical density and, as a consequence, an increase of the attachment force. The increase of the destruction time with increasing length of the hydrocarbon chain of alcohols was caused, on one hand, by the decrease of the detachment forces and the perimeter of the contact planes, and, on the other, by the increase of the attachment force.     

Adhesion of charged particles

The adhesion properties of charged particles are of considerable importance in the electrophotographic process. Measurements on irregularly-shaped, pigmented particles, called toner in the electrophotographic industry, show that adhesion increases with toner charge but that the magnitude is much larger than expected from a simplified electrostatic image force model. An enhanced electrostatic adhesion is also seen in electric field detachment measurements on spherical charged particles. In both cases, this unexpected large adhesion can be attributed to a nonuniform distribution of charge on the surfaces of the particles.     

Measurement of the Aerodynamic Drag Force on Single Aerosol Particles

The “picobalance” (quadrupole) was used to measure the aerodynamic drag force on individual solid particles and droplets by suspending the object in a laminar jet of gas introduced through the bottom electrode. Particles ranging in diameters from about 1 to 150 μm can be studied in this manner. The DC voltage required to maintain the particle position against the opposing forces of aerodynamic drag and gravity was measured to determine the drag force. The flow velocity at which the aerodynamic drag force balances the gravitational force yields information on the aerodynamic size, and the DC voltage required to suspend the particle against gravity with no flow provides a measure of the particle mass. Particle mobilities for spherical and irregularly shaped solids are presented. Light-scattering measurements for spherical particles provide an independent determination of size; the results are generally in good agreement with the aerodynamic size. It is shown that the electrodynamic balance can be used to measure drag forces much larger than the particle weight.     

Effect of particle characteristics on particle pickup velocity

Particle entrainment is investigated by measuring the velocity required to pick up particles from rest, also known as pickup velocity. Pickup velocity is a function of individual particle characteristics and interparticle forces. Although 5-200 μm particles are investigated, the work presented here focuses on the pickup of particles in a pile in the size range of 5-35 μm. These smaller particle sizes are more typical for pharmaceutical and biomedical applications, such as dry powder inhalers (DPIs). Pickup velocities varied from 3.9 to 16.9 m/s for the range of particle sizes investigated.There is a strong correlation between particle size and the dominating forces that determine the magnitude of the pickup velocity. Preliminary data investigating pickup velocity as a function of particle size indicate the existence of a minimum pickup velocity. For larger particle sizes, the mass of the particle demands a greater fluid velocity for entrainment, and for smaller particle sizes, greater fluid velocities are required to overcome particle-particle interactions. Pickup velocity remains relatively constant at very small particle diameters, specifically, less than 10 μm for glass spheres and 20 μm for nonspherical alumina powder. This can be attributed to the negligible changes in London-van der Waals forces due to a hypothesized decrease in interparticle spacing. At intermediate particle diameters, electrostatic forces are dominant.     

Particle transfer to a plate in uniform flow

A theoretical analysis of particle deposition from a dilute suspension onto a plate immersed in a uniform laminar flow with zero angle of incidence is presented. The complete transport equation was solved numerically, taking into account specific surface interactions such as London-van der Waals dispersion and electrical double-layer forces, as well as external forces such as gravity and electric fields. From these solutions, the dimensionless mass transfer Sherwood number was determined for a variety of conditions characteristic for suspensions and aerosols. It is shown that the theoretical analysis applies also to particle deposition within the entrance region of a parallel-plate channel, provided that the hydrodynamic boundary layer is much smaller than the distance between the plates. It is found that the effects of specific surface interactions are important for distances close to the suspension inlet point (leading edge) and for large particles. The effects are generally less pronounced for aerosols in which gravity and electrostatic forces (if present) play a dominant role.     

Some Features of Physical Forces Between Biological Cell Membranes

We have applied several advances in the theory of electrostatic and electrodynamic (van der Waals) forces to the problem of biological cell adhesion. Long-range interactions (i.e., those acting across separations much greater than interactomic distances) are strong enough to hold cells together or to artificial substrates. There is a wide range of attractive energies depending on the interacting substances, in particular a ten-fold range with the artificial materials and an energetic specificity between cells of like type to allow a population of mixed cell types to aggregate with likes sticking to likes (as is commonly observed experimentally).

The present physical approach can provide a useful logic for designing techniques to probe the cell surface and points out several hitherto neglected aspects of the cell surface germane to the study of cellular adhesion.     

Particle-particle interactions during normal flow filtration: Model simulations

Although particle trajectory calculations have been used previously to analyze the behavior of membrane systems, these studies have ignored the effects of particle-particle interactions. Particle motion was evaluated by numerical integration of the Langevin equation accounting for the combined effects of electrostatic repulsion, enhanced hydrodynamic drag, Brownian diffusion, and interparticle forces. In the absence of Brownian forces, particles are unable to enter the pore unless the drag force associated with the filtration velocity can overcome the electrostatic repulsion. The presence of a second particle alters the particle trajectories, forcing the particles to attain equilibrium positions located symmetrically about the pore centerline. Interparticle forces can effectively push the particle over the energy barrier, significantly reducing the magnitude of the critical filtration velocity required for particle transmission. Brownian forces also allow particles to enter the pore, with the particle transmission increasing with increasing filtration velocity.     

Some Features of Physical Forces Between Biological Cell Membranes

We have applied several advances in the theory of electrostatic and electrodynamic (van der Waals) forces to the problem of biological cell adhesion. Long-range interactions (i.e., those acting across separations much greater than interactomic distances) are strong enough to hold cells together or to artificial substrates. There is a wide range of attractive energies depending on the interacting substances, in particular a ten-fold range with the artificial materials and an energetic specificity between cells of like type to allow a population of mixed cell types to aggregate with likes sticking to likes (as is commonly observed experimentally).

The present physical approach can provide a useful logic for designing techniques to probe the cell surface and points out several hitherto neglected aspects of the cell surface germane to the study of cellular adhesion.     

Numerical simulation of microscopic motion and deposition of nanoparticles via electrodynamic focusing

A computational model was developed to simulate microscopic motion and deposition of charged aerosols during the nanoparticle patterning process utilizing electrodynamic focusing concept (Kim et al., 2006). Our computational model includes Brownian random force, Coulomb and image forces, fluid drag and van der Waals force for determining Lagrangian particle trajectories after solving electrostatic fields in the deposition chamber. Our results are in agreement with the previous experimental findings. The effects of operation parameters such as surface charge density, applied voltage and particle charges were investigated. It was found that the electric field-induced motion of particles dominated over Brownian random motion of 10 nm nanoparticles near the surface and the inertial motion of charged nanoparticles under high electric field would be important to determine the precise deposition pattern within submicrometer scale structures.     

Effect of Press-on Forces on Drug Adhesion in Dry Powder Inhaler Formulations

Press-on forces play a major role in dry powder inhaler (DPI) formulations. In this work, we have quantified the press-on forces on different substrates and drugs under controlled conditions. Externally applied forces significantly affect drug adhesion to surfaces and were found to be independent of the type of substrate and drug. A conservative estimate of press-on forces was made and was found to depend inherently on the size and shape of the drug particle and also on the contact area of the drug and substrate. These press-on forces were positively correlated to the mixing energy. It was found to increase quadratically with an increase in mixing speed. This work is of relevance to the pharmaceutical process of mixing DPI formulations, as it relates dispersion performance to the mixing time, batch size and carrier and drug properties.     

Adsorption of organic molecules from aqueous solutions on carbon materials

Adsorption of organic molecules from dilute aqueous solutions on carbon materials is a complex interplay between non-electrostatic and electrostatic interactions. Non-electrostatic interactions are essentially due to dispersion and hydrophobic interactions, whereas the electrostatic or coulombic interactions appear with electrolytes when they are ionized at the experimental conditions used. Both interactions depend on the characteristics of the adsorbent and the adsorptive and the solution chemistry. Among them the carbon surface chemistry has a great influence on both electrostatic and non-electrostatic interactions, and can be considered one of the main factors in the adsorption mechanism from dilute aqueous solutions. In this paper the current knowledge about the fundamental factors that control the adsorption process from aqueous phase will be presented.     

粘性糊料颗粒间的作用力及其机理分析

在阴极炭块组装过程中，粘性糊料颗粒间的相互作用导致颗粒的成团和破碎，由此影响糊料的微观和宏观行为。本文对两颗粒间的毛细力、范德华力、粘性力、接触力、静电力和重力六种相互作用力的计算进行了阐述。通过对这些相互作用力的量级分析，得到了导致糊料中颗粒成团和破碎行为的主要作用力，并分析了颗粒尺寸对相互作用力的影响。最后对碰撞变形过程中主要作用力随碰撞时间的变化规律进行了研究，通过对比这些力在碰撞过程中的相对大小对颗粒成团的机理进行力学解释。     

Interactions between cellulose surfaces: effect of solution pH

The forces acting between cellulose surfaces have been studied using the interferometric surface force apparatus. The cellulose surfaces were prepared by Langmuir-Blodgett deposition of trimethylsilyl cellulose (TMSC) onto hydrophobized mica. Prior to measurements, the surfaces were desilylated to obtain pure cellulose. The degree of silylation and the molecular weight of the TMSC both affect the structure of the deposited layer. This was observed from the surface pressure-area isotherm, force versus distance curves, and atomic force microscopy images. The forces between the cellulose surfaces were found to depend on the pH of the solution. In dilute electrolyte solutions, the cellulose film was uncharged and rather compact when the pH of the solution was 6.0. However, when the pH was increased to 7.3, the cellulose film swelled considerably and a long-range steric force was measured. The swelling of the film is interpreted as being due to the dissociation of a few carboxylic acid groups present along the cellulose chain. The forces measured were, however, dominated by steric interactions. The repulsion does not increase substantially when the pH is increased from 7.3 to 9.5. Our results suggest that the pK_a of the acid groups present within the cellulose film is larger than it would be in the bulk aqueous solution.     

New Concept for Simulating Particle Packing in Colloidal Forming Processes

A new simulation concept based on the discrete element method (DEM) has been developed for studying particle-packing dynamics during colloidal forming processes. Long-range potential interactions, medium influences, solid-body contact, and adhesion are considered. DEM regards each particle as an individual element and enables the observation of the spatially and temporally resolved microscopic details. The importance of different competitive interaction forces is compared. The electrostatic repulsion constrains the Brownian motion for all particle sizes. The simultaneous, competitive influences of the colloidal chemical and process parameters on the particle-packing structure during pressure filtration and centrifugal casting have been simulated and analyzed. In the pressure filtration test, the well-stabilized monomodal particles form a close-packed hexagonal structure, and this regularity is strongly dependent on the filtration rate. The higher the filtration rate, the more packing defects are formed. Although well-stabilized suspensions with high zeta-potential tend to form particle chains, suspensions with low zeta-potential form agglomerate structures. The limitations of the long-range interaction functions derived from the Hamaker theory and from the electrostatic Poisson-Boltzmann equation are discussed.     

Electrostatic dispersion of fine particles in the air

The paper studied the method of keeping fine particles from aggregating in the air by electrostatic dispersion. The effects of electrode voltage, diameter, humidity and rest time, as well as van der Waals forces, electrostatic forces and liquid bridge forces between particles on electrostatic dispersion of powder were discussed. It was shown that optimal electrostatic dispersion effect of calcium carbonate and talcum particles can be achieved with corona voltage of 29 kV, particle size of 2–25 μm, and proper rest time of 48 h. Criteria for electrostatic dispersion were put forward on the basis of experimental results. Theoretical calculation indicated that the criteria for electrostatic dispersion were in good agreement with experimental results.