A modeling approach to describe the adhesion of rough, asymmetric particles to surfaces

A combined theoretical and experimental study of the adhesion of alumina particles and polystyrene latex spheres to silicon dioxide surfaces was performed. A boundary element technique was used to model electrostatic interactions between micron-scale particles and planar surfaces when the particles and surfaces were in contact. This method allows quantitative evaluation of the effects of particle geometry and surface roughness on the electrostatic interaction. The electrostatic interactions are combined with a previously developed model for van der Waals forces in particle adhesion. The combined model accounts for the effects of particle and substrate geometry, surface roughness and asperity deformation on the adhesion force. Predictions from the combined model are compared with experimental measurements made with an atomic force microscope. Measurements are made in aqueous solutions of varying ionic strength and solution pH. While van der Waals forces are generally dominant when particles are in contact with surfaces, results obtained here indicate that electrostatic interactions contribute to the overall adhesion force in certain cases. Specifically, alumina particles with complex geometries were found to adhere to surfaces due to both electrostatic and van der Waals interactions, while polystyrene latex spheres were not affected by electrostatic forces when in contact with various surfaces.     

Capillary force required to detach micron-sized particles from solid surfaces—Validation with bubbles circulating in water and 2 μm-diameter latex spheres

The adhesion forces holding micron-sized particles to solid surfaces can be studied through the detachment forces developed by the transit of an air–liquid interface in a capillary. Two key variables affect the direction and magnitude of the capillary detachment force: (i) the thickness of the liquid film between the bubble and the capillary walls, and (ii) the effective angle of the triple phase contact between the particles and the interface. Variations in film thickness were calculated using a two-phase flow model. Film thickness was used to determine the time-variation of the capillary force during transit of the bubble. The curve for particle detachment was predicted from the calculated force. This curve proved to be non-linear and gave in situ information on the effective contact angle developing at the particle–bubble interface during detachment. This approach allowed an accurate determination of the detachment force. This theoretical approach was validated using latex particles 2 μm in diameter.     

Influence of adhesion on the sliding and rolling friction

Yet in 1934, one of the authors had developed the molecular friction theory explaining the external friction by dissipation of energy on the molecular unevenness of the bodies in friction. This theory distinctly determines the role of adhesion in the processes of the external sliding friction. The adhesion forces are used in this theory only for explaining deviation from the Amonton's law expressing the proportionality of the friction force to the normal load.

The rolling friction process (in the absence of deformations) represents a process of formation and breakage of adhesion bonds. Using the electron theory as the basis, the mechanism of influence of the electrostatic component of adhesion on the rolling friction is considered, the electrostatic component being attributable to the formation of a double electric layer when solids are in friction, and when its plates are separated as the contact is broken.     

Hydrodynamic crossover in dynamic microparticle adhesion on surfaces of controlled nanoscale heterogeneity

This note documents the crossover from a regime where shear flow hinders microparticle adhesion on collecting surfaces to that where increased flow aids particle capture. Flow generally works against adhesion and successfully hinders particle capture when the net physicochemical attractions between the particles and collector are weak compared with hydrodynamic forces on the particle. Conversely, with strong attractions between particles and collector, flow aids particle capture by increasing the mass transport of particles to the interfacial region. Here, local hydrodynamics still generally oppose adhesion but are insufficient to pull particles off of the surface. Thus, flow actually increases the particle capture rate through the increased transport to the surface. These behaviors are demonstrated using 1 mum silica spheres flowing over electrostatically heterogeneous (length scales near 10 nm) collecting surfaces at shear rates from 22 to 795 s(-1). The net surface charge on the collector is varied systematically from strongly negative (pure silica) to strongly positive (a saturated polycationic overlayer), demonstrating the interplay between physicochemical and hydrodynamic contributions. These results clearly apply to situations where heterogeneous particle-surface interactions are electrostatic in nature; however, qualitatively similar behavior was previously reported for the effect receptor density on bacterial adhesion.     

Non-specific adhesion on biomaterial surfaces driven by small amounts of protein adsorption

This work explores how long-range non-specific interactions, resulting from small amounts of adsorbed fibrinogen, potentially influence bioadhesion. Such non-specific interactions between protein adsorbed on a biomaterial and approaching cells or bacteria may complement or even dominate ligand–receptor mating. This work considers situations where the biomaterial surface and the approaching model cells (micron-scale silica particles) exhibit strong electrostatic repulsion, as may be the case in diagnostics and lab-on-chip applications. We report that adsorbed fibrinogen levels near 0.5 mg/m² produce non-specific fouling. For underlying surfaces that are less fundamentally repulsive, smaller amounts of adsorbed fibrinogen would have a similar effect. Additionally, it was observed that particle adhesion engages sharply and only above a threshold loading of fibrinogen on the collector. Also, in the range of ionic strength, I, below about 0.05 M, increases in I reduce the fibrinogen needed for microparticle capture, due to screening of electrostatic repulsions. Surprisingly, however, ionic strengths of 0.15 M reduce fibrinogen adsorption altogether. This observation opposes expectations based on DLVO arguments, pointing to localized electrostatic attractions and hydration effects to drive silica–fibrinogen adhesion. These behaviors are benchmarked against microparticle binding on silica surfaces carrying small amounts of a polycation, to provide insight into the role of electrostatics in fibrinogen-driven non-specific adhesion.     

Electrostatically confined nanoparticle interactions and dynamics

We report integrated evanescent wave and video microscopy measurements of three-dimensional trajectories of 50, 100, and 250 nm gold nanoparticles electrostatically confined between parallel planar glass surfaces separated by 350 and 600 nm silica colloid spacers. Equilibrium analyses of single and ensemble particle height distributions normal to the confining walls produce net electrostatic potentials in excellent agreement with theoretical predictions. Dynamic analyses indicate lateral particle diffusion coefficients approximately 30-50% smaller than expected from predictions including the effects of the equilibrium particle distribution within the gap and multibody hydrodynamic interactions with the confining walls. Consistent analyses of equilibrium and dynamic information in each measurement do not indicate any roles for particle heating or hydrodynamic slip at the particle or wall surfaces, which would both increase diffusivities. Instead, lower than expected diffusivities are speculated to arise from electroviscous effects enhanced by the relative extent (kappaa approximately 1-3) and overlap (kappah approximately 2-4) of electrostatic double layers on the particle and wall surfaces. These results demonstrate direct, quantitative measurements and a consistent interpretation of metal nanoparticle electrostatic interactions and dynamics in a confined geometry, which provides a basis for future similar measurements involving other colloidal forces and specific biomolecular interactions.     

Molecular Basis of the Dynamic Strength of the Sialyl Lewis X—Selectin Interaction

The selectins are Ca²⁺‐dependent cell adhesion molecules that facilitate the initial attachment of leukocytes to the vascular endothelium by binding to a carbohydrate moiety as exemplified by the tetrasaccharide, sialyl Lewis X (sLeX). An important property of the selectin‐sLeX interaction is its ability to withstand the hydrodynamic force of the blood flow. Herein, we used single‐molecule dynamic force spectroscopy (DFS) to identify the molecular determinants within sLeX that give rise to the dynamic properties of the selectin/sLeX interaction. Our atomic force microscopy (AFM) measurements revealed that the unbinding of the selectin/sLeX complexes involves overcoming at least two activation barriers. The inner barrier, which determines the dynamic response of the complex at high forces, is governed by the interaction between the Fuc residue of sLeX and a Ca²⁺ ion chelated to the lectin domain of the selectin molecule, whereas the outer activation barrier can be attributed to interactions involving the sialic acid residue of sLeX. Due to their steep inner activation barriers, the selectin‐sLeX complexes are less sensitive to high pulling forces. Hence, besides its contribution to the bond energy, the Ca²⁺ ion also grants the selectin–sLeX complexes a tensile strength that is crucial for the selectin‐mediated rolling of leukocytes.     

Modeling the Effect of the Relative Humidity on the Manipulation of Nanoparticles with an Atomic Force Microscope

In the manipulation of nanoparticles, different behaviors are typically observed including sliding, rolling and rotation. Most of investigations in this field have so far focused on describing the interaction forces under vacuum (dry air) environmental condition, while the effect of the relative humidity has been poorly considered. In this work we developed a model for simulating the dynamic nanoparticle motion (rolling and sliding) in an AFM-based manipulation of nanoparticles in a humid environment. In our method, the interaction forces include the adhesion force, mainly consisting of the capillary force and van der Waals force, the normal force and friction forces. We calculated the adhesion force by considering the contributions from the wet and dry portions of the particle. Our stimulations show that nanoparticles smaller than the AFM tip tend to slide before rolling, while in large nanoparticles the rolling occurs first. The particle motion is achieved if the applied force exceeds a critical value and the direction of the rolling movement depends on the applied force angle. Furthermore, small nanoparticles are more easily manipulated by the tip in low-humidity conditions while the manipulations with large nanoparticles need high-humidity conditions. Preliminary results can be used to adjust proper handling force for the accurate and successful assembly of particles.     

Interactions between apolar,basic and acidic model oils and a calcite surface

Abstract

In this study, the atomic force microscopy colloidal probe technique was employed to investigate the interaction between apolar, basic and acidic model oil probes and a calcite surface in solutions containing different concentrations of NaCl, CaCl₂ and Na₂SO₄. In the presence of SO₄^2?, hydration and structural forces were observed between apolar model oil probes and a calcite surface on approach. Relatively low adhesion forces were observed between the basic model oil probes and the calcite surface, while higher adhesion forces were observed between the acidic model oil probes and the calcite surface. Furthermore, the adhesion forces between the basic model oil probes and the calcite surface significantly increased in the presence of SO₄^2?, while the adhesion force between the acidic model oil probes and the calcite surface decreased in the presence of Ca²⁺ or SO₄^2?. The differences in the adhesion forces are related to electrostatic attraction and ion bridging forces between the model oil probes and the calcite surface.     

Determination of interaction strength between corrole and phenol derivatives in aqueous media using atomic force microscopy

Atomic force microscopy (AFM) was used to measure single interaction forces between corrole (host) and phenol derivatives (guests) in aqueous media. A gold tip was modified with thiol derivatives of corrole via the Au–S covalent bond. Such a tip was used to measure adhesion forces with a planar gold substrate modified with thiol derivatives of phenol and ortho-nitrophenol in aqueous solutions. The mean force between the corrole and ortho-nitrophenol was higher than that between corrole and phenol, probably reflecting stronger hydrogen bond interaction in the former complex. In the presence of a supporting electrolyte (0.1 M K₂SO₄), the mean force increased, suggesting that electrostatic and π–π interactions play an essential role in the adhesion force. In addition, the adhesion force measured at pH 6.0 was larger than that at pH 10, reflecting the electrostatic repulsion at the higher pH. These behaviours are consistent with the potentiometric responses of a liquid membrane based on corrole to phenolic compounds. Also, the values of forces for the interaction between corrole and phenol derivatives showed the same tendency as energy calculated for these complexes. The Poisson method was used for the calculation of the single force of the chemical bond between the corrole host and the phenolic guests.     

Adsorption Modification of Dispersed Aluminum: The Effect of Surfactant and Polymer Adsorption on the Colloidal and Mechanical Properties of Aluminum Organosuspensions

Adsorption of surfactants (stearic acid, octadecylamine) and polymers (butadiene and nitrile synthetic rubbers with carboxyl end groups) on bare and modified surfaces of aluminum dispersed in polar and nonpolar solvents (isopropyl nitrate, octane) was studied. Adsorption data are compared with the results of the study of aggregative and sedimentation stabilities of aluminum dispersions, as well as with the mechanical characteristics of sediments. Notions of fractal nature of particle aggregates are used to describe data on organosuspension sedimentation. Experimental results are analyzed in terms of donor–acceptor properties of organosuspension components and interactions at the particle–solvent and particle–modifier interfaces.     

On the retention mechanisms and secondary effects in microthermal field-flow fractionation of particles

The behavior of nanometer or micrometer-sized particles, dispersed in liquid phase and exposed to temperature gradient, is a complex and not yet well understood phenomenon. Thermal field-flow fractionation (TFFF), using conventional-size channels, played an important role in the studies of this phenomenon. In addition to thermal diffusion (thermophoresis) and molecular diffusion or Brownian movement, several secondary effects such as particle–particle and/or particle–wall interactions, chemical equilibria with the components of the carrier liquid, buoyant and lift forces, etc., may contribute to the retention and complicate the understanding of the relations between the thermal diffusion and the characteristics of the retained particles. Microthermal FFF is a new high-performance technique allowing much easier manipulation and control of the operational parameters within an extended range of experimental conditions in comparison with conventional TFFF. Consequently, in combination with various other methods, it is well suited for a detailed investigation of the mentioned effects. In this work, some contradictory published results concerning the thermal diffusion of the colloidal particles, studied by TFFF but also by other methods, are analyzed and compared with our experimental findings.     

Numerical study on the adhesion and reentrainment of nondeformable particles on surfaces: the role of surface roughness and electrostatic forces

In this paper, the reentrainment of nanosized and microsized particles from rough walls under various electrostatic conditions and various hydrodynamic conditions (either in air or aqueous media) is numerically investigated. This issue arises in the general context of particulate fouling in industrial applications, which involves (among other phenomena) particle deposition and particle reentrainment. The deposition phenomenon has been studied previously and, in the present work, we focus our attention on resuspension. Once particles are deposited on a surface, the balance between hydrodynamic forces (which tend to move particles away from the surface) and adhesion forces (which maintain particles on the surface) can lead to particle removal. Adhesion forces are generally described using van der Waals attractive forces, but the limit of these models is that any dependence of adhesion forces on electrostatic forces (due to variations in pH or ionic strength) cannot be reproduced numerically. For this purpose, we develop a model of adhesion forces that is based on the DLVO (Derjaguin and Landau, Verwey and Overbeek) theory and which includes also the effect of surface roughness through the use of hemispherical asperities on the surface. We first highlight the effect of the curvature radius on adhesion forces. Then some numerical predictions of adhesion forces or adhesion energies are compared to experimental data. Finally, the overall effects of surface roughness and electrostatic forces are demonstrated with some applications of the complete reentrainment model in some simple test cases.     

Controlling oriented aggregation using increasing reagent concentrations and trihalo acetic acid surfactants

Zinc oxide particle growth from homogenous solutions prepared with isopropyl alcohol was monitored using in situ UV–vis spectroscopy, and results show that the rate of ZnO particle growth and the final ZnO nanoparticle size depend strongly upon the concentrations of precursors and the identity of surfactants used. In addition, particle size versus time data was fit using the coarsening model and the simultaneous oriented aggregation and coarsening model in order to evaluate the effect of changing synthetic variables on the mechanism of nanoparticle growth. In general, an increase in growth by oriented aggregation with increasing precursor concentrations was observed, a result that was consistent with results from high-resolution transmission electron microscopy (HRTEM) characterization. The increase in precursor concentrations resulted in an increase in the number concentration of ZnO nanoparticles, which resulted in a higher probability of particle–particle interactions and hence increased growth by oriented aggregation. Additionally, particle growth in solutions of trifluoro-, trichloro-, and tribromoacetate surfactants was studied, and growth by oriented aggregation followed the trend expected based on the number concentration of zinc oxide particles. Growth with trifluoroacetate was an exception, with growth by oriented aggregation substantially suppressed.     

Glass and Structural Transitions Measured at Polymer Surfaces on the Nanoscale

This paper reviews our recent progress in determining the surface glass transition temperature, T_g, of free and substrate confined amorphous polymer films. We will introduce novel instrumental approaches and discuss surface and bulk concepts of T_g. The T_g of surfaces will be compared to the bulk, and we will discuss the effect of interfacial interactions (confinements), surface energy, disentanglement, adhesion forces, viscosity and structural changes on the glass transition. Measurements have been conducted with scanning force microscopy in two different shear modes: dynamic friction force mode and locally static shear modulation mode. The applicability of these two nano-contact modes to T_g will be discussed.This revised version was published online in November 2005 with corrections to the Cover Date.     

3D Monte Carlo simulations of a magnetic disk-like particle dispersion

We have investigated the aggregate structures of a colloidal dispersion composed of ferromagnetic disk-like particles with a magnetic moment normal to the particle axis at the particle center, by means of 3D Monte Carlo simulations. Such disk-like particles have been modeled as a circular disk-like particle with the side section shape of spherocylinder. We have attempted to clarify the influences of the magnetic field strength, magnetic interactions between particles and volumetric fraction of particles. In order to discuss quantitatively the aggregate structures of clusters, we have focused on the radial distribution and orientational pair correlation functions, etc. For no applied magnetic field cases, long column-like clusters come to be formed with increasing magnetic particle–particle interactions. The internal structures of these clusters clearly show that the particles incline in a certain direction and their magnetic moments alternate in direction between the neighboring particles in the clusters. For applied magnetic field cases, the magnetic moment of each particle inclines in the magnetic field direction and therefore the column-like clusters are not formed straightforwardly. If the magnetic field is much stronger than magnetic particle–particle interactions, the particles do not have a tendency to form the clusters. As the influence of magnetic particle–particle interactions is significantly strong, thick chain-like clusters or column-like clusters or brick-wall-like clusters come to be formed along the magnetic field direction.     

An application of mean-field perturbation theory for the adsorption of polar molecules in nanoslit-pores

In the present research study, we present the development of a model for characterizing and predicting the adsorption of polar molecules between two parallel plates based on mean-field perturbation theory. The electrostatic forces between fluid–fluid molecules in the slit shaped pore are modeled by considering permanent dipole–dipole interactions and permanent dipole-induced dipole moment interactions. The intermolecular potential for the electrostatic interactions was obtained by considering statistical averages over all possible orientations of the molecules. The proposed model is then used to study the sorption of water molecules in the slit shaped pore and an explicit equation for the Helmholtz free energy of the pore phase fluid is derived. Adsorption isotherms for different pore sizes are simulated and the relative contributions of fluid–wall and fluid–fluid interactions to the Helmholtz free energy are calculated as an illustration and compared with the results of existing models in the literature.AMS subject classification: 82B03, 82B05, 82B26, 82B30, 82D15, 31B10, 41A25     

Direct measurements of interactions between polypeptides and carbon nanotubes

The interactions of various polypeptides with individual carbon nanotubes (CNTs), both multiwall (MW) and single wall (SW), were investigated by atomic force microscopy (AFM). While adhesion forces arising from electrostatic attraction interactions between the protonated amine groups of polylysine and carboxylic groups on the acid-oxidized multi-wall carbon nanotubes (Ox-MWCNTs) dominate the interaction at a low pH, weaker adhesion forces via the hydrogen bonding between the neutral -NH2 groups of polylysine and -COO- groups of the Ox-MWCNTs were detected at a high pH. The adhesion force was further found to increase with the oxidation time for Ox-MWCNTs and to be negligible for oxidized single-wall carbon nanotubes (Ox-SWCNTs) because carboxylate groups were only attached onto the nanotube tips in the latter whereas onto both the nanotube tips and sidewall in the former. Furthermore, it was demonstrated that proteins containing aromatic moieties, such as polytryptophan, showed a stronger adhesion force with Ox-MWCNTs than that of polylysine because of the additional pi-pi stacking interaction between the polytryptophan chains and CNTs.     

Transitions from static wetting to steady dewetting and deposition of liquid-film on partially wettable polymer surface

The transitions from static to steadily moving wetting perimeter and further to deposition of a liquid-film on partially wettable surface were studied with the same system under the same conditions. A polyethylene terephthalate (PET) tape was vertically withdrawn at constant velocity from glycerol–water mixture. Elevation L of the three-phase contact line above the liquid level was measured under static, steady, and dynamic wetting. The static receding Θ_R and the apparent dynamic angles Θ_app at different withdrawal velocities U were calculated from the static relationship Θ(L). It was found that the limiting static angle Θ_R,min, at which the wetting perimeter starts moving, depends on withdrawal velocity. Extrapolation of the Θ_R,min/U dependence to U = 0 yields the quasi-static value of this parameter , that coincides with the relaxation static angle Θ_R,rlx achieved after meniscus motion ceases. This conclusion holds also for the wetting mode, where the limiting static advancing angle = Θ_A,rlx. Both the limiting and relaxation angles could be used for calculation of the effective Young's contact angle on non-ideal surface following Adam's suggestion [N.K. Adam, Adv. Chem. Ser. 43 (1964) 53.].The critical velocity U_cr anfd apparent dynamic angle Θ_app,cr, at which transition between steady dewetting and dynamic wetting occurs, were determined. The value of Θ_app,cr = 0° ± 5° agrees with our previous results [R.V. Sedev, J.G. Petrov, Colloids and Surfaces, 53 (1991) 147] implying a quasi-static shape of the moving meniscus up to U_cr. At U > U_cr, the speed V of the contact line relative to the solid wall is independent of withdrawal velocity and thickness of the deposited film. The present data confirm the earlier findings [J.G. Petrov, R.V. Sedev, Colloids and Surfaces, 13 (1985) 317, T.D. Blake, K.J. Ruschak, Nature, 282 (1979) 489] that at U = U_cr, V reaches its maximum value V_max, which is most important parameter of dewetting kinetics.Weak linear decrease of Θ_app with U was found above Ca = 2.4 × 10⁻⁵ up to the critical capillary number Ca_cr = 4.1 × 10⁻⁴. Below Ca = 10⁻⁵ the apparent receding angle depends much stronger on withdrawal velocity. The hydrodynamic (HD), and the simple and more general versions of the molecular-kinetic (MK) and molecular-hydrodynamic (MHD) theories of the wetting dynamics were used for quantitative characterization of the system in the steady dewetting regime. The effective Young's angle was used in the MK and MHD treatment of the experimental data following our previous publication [J.G. Petrov, J. Ralston, M. Schneemilch, R. Hayes, J. Phys. Chem B, 107(7) (2003) 1637]. The HD theory only qualitatively satisfies our experimental data giving physically unreasonable value of the hydrodynamic cut-off (slip) length and too small static receding angle at U = 0. The MK theory gives acceptable values of the oscillation frequency K₀ of the molecules at the contact line. Its more general version, including the viscous dissipation in the contact line vicinity, yields higher oscillation frequency. Very large distance λ between adsorption centers on the solid substrate (about five times the diameter of a glycerol molecule) was obtained with both MK and MHD theories. The too small frequency K₀ obtained with the simple MHD theory is removed by the more general version, accounting for contact line and viscous friction in the inner and intermediate zone of the moving meniscus. All theories show discrepancies between theoretically expected and experimentally estimated values of some of the parameters of wetting dynamics.     

Deposition of particles under external forces in laminar flow through parallel-plate and cylindrical channels

A theoretical analysis of particle deposition kinetics onto walls of parallel-plate and cylindrical channels is presented. Rigorous transport equations are formulated by taking into account specific surface forces as well as external forces, e.g., gravity. By solving the transport equations numerically, the dimensionless mass transfer Sherwood number is determined as a function of various dimensionless parameters introduced such as Pe, Gr, Ad, and Dl, accounting for convection and diffusion, and for gravity, dispersion, and electrical double-layer interactions, respectively. The influence of attractive surface forces and gravity on the deposition kinetics is graphically presented and discussed. For large particles, i.e., about 1-μm diameter (Pe > 1), and for short distances from the point where deposition starts, a considerable increase in particle flux (up to an order of magnitude) is predicted over previous analytical values when strong attractive double-layer forces are present. For particles smaller than 0.1-μm diameter (Pe < 10^-4) our numerical results show that particle deposition rates may be successfully predicted by an analytical formula derived for particles of negligible size even in the presence of double-layer attractions (provided external forces are absent). Experimental results reported in the literature obtained under conditions of negligible gravity force are reinterpreted in terms of the present theory. A somewhat closer agreement with experimental data as compared to the analytical formula mentioned above is found in cases of strong double-layer attractions.