AFM Colloidal Probe Research Articles

Capillary Adhesion in the Limit of Saturation: Thermodynamics, Self-Consistent Field Modeling and Experiment

We introduce a simple thermodynamic argument for capillary adhesion forces, for various geometries, in the limit of saturation of the bulk phase. For one specific geometry (i.e., the sphere-plate geometry such as that found in the colloidal probe AFM technique), we provide evidence of the validity of our model by comparison with experiment and self-consistent field calculations. With this latter numerical technique, we also discuss deviations from the macroscopic argument both when the system is moved away from saturation and when the capillary bridge becomes so small that macroscopic thermodynamics is no longer accurate.

Fractal analysis of the influence of surface roughness of toner particles on their flow properties and adhesion behavior

In order to quantitatively evaluate the relationship between the minute roughness and powder behaviors such as macroscopic shear behavior and particle–particle interaction of toner particles with different particle surfaces, the FE-SEM image of a toner particle was processed in to a line image, and the particle outline image was analyzed using fractal dimension. In the fractal dimensional analyses, when the scaling was not less than or equal to 1/20 of the particle diameter, the fractal dimension was very large ; it was inferred that the scaling was not suitable for showing a minute structure on the surface of the particle. In this study, the scaling was adjusted to less than or equal to 1/20 of the particle diameter. The relationship between the fractal dimension D and RMS surface roughness of the toner particle, which was determined by atomic force microscopy, had a weak first positive correlation. Subsequently, the influence of the surface coverage of SiO 2 on the fractal dimension was examined. It was observed that the fractal dimension increased with the surface coverage of SiO 2 in the condition of less than or equal to 26%. The relationships between the fractal dimension and the flow properties were examined using the rotary shear tester with conical rotor. When the mean particle diameter was almost constant, the fractal dimension was inversely proportional to the macroscopic shearing torque. Moreover, the relationships between the fractal dimension and the particle–particle interactions were examined using colloid probe AFM. It was observed that the fractal dimension was inversely proportional to the microscopic adhesion force. The relationship between the fractal dimension and the shearing force calculated by using torque was almost identical to that between the fractal dimension and the particle–particle interaction.

Evaluation of the particle–particle interactions in a toner by colloid probe AFM

In order to evaluate the particle–particle interactions in a toner particle with a high accuracy, the changes in the adhesion force between a toner particle glued to an AFM cantilever tip and a particle in a compressed layer of toner powder was measured by colloid probe atomic force microscopy. Toner particle layers compressed by a pressure of 54.9 MPa were used as the substrate in this study. The effects of the scan rate, particle diameter, humidity, and surface coating of silica nanoparticles on the adhesion force were discussed on the base of measurement results. The adhesion force between the toner surfaces decreased with an increase in the AFM scan rate; it then reduced to a constant value at 1.95 Hz. The adhesion force between the toner particles increased proportionally with approximately the second power law of the particle diameter; it exhibited a change that differs from the force characteristic that was directly proportional to the first power law of the particle diameter, as given in van der Waals expression. Next, the additives that consisted of SiO 2 were glued on to the surface of the toner particle, and the influence of the surface coverage of SiO 2 on the particle–particle interactions was examined. The particle–particle interactions consequently decreased in inverse proportion with a surface coverage of SiO 2 26% or less. Moreover, when the SiO 2 surface coverage was 26%, almost no changes were observed in the particle–particle interactions at 80% R.H. It was confirmed that the influence of humidity reduced by coating the toner particle with SiO 2. The relationships between the particle–particle interactions and the flow properties were examined by using a rotary shear tester with a conical rotor. When the mean particle diameters were identical, a first strong positive correlation between the particle–particle interaction and the shearing torque that crossed the zero point was observed. When the shearing force H was calculated from the torque of the conical rotor method by using Rumpf's equation, a first strong positive correlation between the particle–particle interaction Fa and the shearing force H that crossed the zero point was observed.

Effect of particle size on surface modification of silica nanoparticles by using silane coupling agents and their dispersion stability in methylethylketone

The effect of particle size on the reactivity of hexyltrimethoxysilane (C6S) with the particle surface was studied by using silica nanoparticles (SNPs) with different diameters (30 or 200 nm). In case of 30-nm SNPs, a large amount of isolated silanol was observed. On the other hand, in the case of 200-nm SNPs, the amount of hydrogen bonded silanol and hydrogen bonded water molecules at the surface of the SNPs increased. Since the hydrogen bonded silanol and the hydrogen bonded water enhanced the reaction of C6S with SNPs, the chemisorbed C6S on 200-nm SNPs was larger than that on 30-nm SNPs. Furthermore, the effects of surface modification on the dispersion stability in MEK were studied using viscosity measurements and surface force measurements by the AFM colloid probe method. The viscosity of the dilute SNPs/MEK suspension did not change by the chemisorptions of C6S; however, the viscosity of dense suspension reduced effectively by surface modification. It was estimated that the suspension viscosity reduced effectively when the mean particle surface distance in the suspension was near to the distance where the repulsive force appeared by the surface force measurements using the colloid probe AFM.

Effect of pH and hydration on the normal and lateral interaction forces between alumina surfaces

Interaction forces between alumina surfaces were measured using an AFM–colloid probe method at different pHs. For an α-alumina–sapphire system at acidic pH, the force curve exhibited a well-defined repulsive barrier and an attractive minimum. At basic pH, the interactive force was repulsive at all separations with no primary minimum. Lateral force measurements under the same conditions showed that frictional forces were nearly an order of magnitude smaller at basic pH than those observed at acidic pH. This behavior was attributed to the hydration of the alumina surface. Normal and lateral force measurements with the strongly hydrated ρ-alumina surfaces supported these findings.

Effect of surface interaction of silica nanoparticles modified by silane coupling agents on viscosity of methylethylketone suspension

In order to control the viscosity of a dense silica methylethylketone (MEK) suspension, the surfaces of silica nanoparticles were modified by 3-glycidoxypropyltrimethoxysilane (GPS) or hexyltrimethoxysilane (C6S) in MEK with the addition of a small amount of pH-controlled water. First, the effect of water addition on the amount of chemisorbed coupling agent was investigated. pH-controlled water enhanced the reactivity of the coupling agent in MEK. The amount of chemisorbed coupling agent increased slightly with the addition of pH 3 water and increased remarkably with the addition of pH 12 water. Next, the effect of the organic functional groups of the coupling agent, pH of the additive water, and additive amount of coupling agent on surface interaction were determined by colloid probe AFM. The steric repulsive force between the silica nanoparticles increased due to water addition, particularly when the pH was maintained at 3. The viscosity of the silica MEK suspension reduced effectively when this repulsive force appeared; however, the optimum condition for reducing the suspension viscosity was dependent on the coupling agent species. The viscosity of the dense silica MEK suspension can be controlled by the addition of small amounts of pH-controlled water and the functional groups of the coupling agent.

Colloidal Probe AFM Measurements of the Electrophoretic Force

Colloidal probe atomic force microscopy is a very useful tool in the study of colloidal interactions. Although this technique has been applied to study interactions between a particle and a polarized electrode during electrodeposition, it has never been used to study interactions in high electric fields as encountered in electrophoretic deposition. In this work, a preliminary study was undertaken to verify whether colloidal probe AFM could be used to measure the electrophoretic force on a particle. It was found that the electrophoretic force could be detected by colloidal probe AFM under certain circumstances. In order to prevent that the contribution of the cantilever on the measurement of the electrophoretic force becomes large, the charge on the cantilever should be small compared to the charge of the particle, which is attached to the cantilever. Moreover, the area of cantilever surface which is oriented parallel to the electric field should be small to minimize the contribution of the cantilever.

Charging Behavior of the Gibbsite Basal (001) Surface in NaCl Solution Investigated by AFM Colloidal Probe Technique

The charging behavior of the gibbsite gamma-Al(OH)3 basal (001) surface in aqueous solution is important for correctly modeling the overall charging properties of gibbsite particles which controls surface phenomena such as adsorption and crystal growth. However, the question of whether the hydroxyl groups on the basal plane are proton active has been raised recently both from experimental and theoretical points of view. Using gibbsite crystals prepared from industrial Bayer process, the surface potentials of cleaved (001) surfaces were calculated from forces measured by the colloidal probe technique in 1 mM NaCl solution with differing pH. It was surprisingly found that the basal plane is proton active in pH less than 7 and protonation seems to level off at about pH 5. The potential-pH data was accurately fitted with a single pKa surface protonation model with pK(a) = 5.9 +/- 0.2.

Particle-Substrate Interactions in Non-Aqueous Media Studied by Colloidal Probe AFM

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Effect of crosslinking on the elasticity of polyelectrolyte multilayer films measured by colloidal probe AFM

A homemade colloidal probe atomic force microscope was used to perform nanoindentation with a spherical probe of 5 microm in diameter, at different approach velocities in order to extract the Young's modulus, E0, of poly(L-lysine)/hyaluronan (PLL/HA) films. This parameter is of prime importance to control cellular adhesion. The films were either kept in their native form or cross-linked with a mixture of 1-ethyl-3-(3-dimethyl aminopropyl)carbodiimide (EDC) and N-hydrosulfosuccinimide (sulfo-NHS), where the EDC concentration was varied from 1 up to 100 mg mL(-1) (approximately from 5 to 500 mM). A model based on Hertz mechanics was used to account for the interactions between film and probe. It is shown that the Young's modulus varies with the approach velocity for the native (PLL/HA) films, whereas for cross-linked ones, E0 is independent from the velocity over the whole range investigated. It is found that for native films, E0 takes a value of 3 kPa at low approach velocities, a velocity domain that should be relevant in cellular adhesion processes. The Young's modulus increases with the EDC concentration used to cross-link the films and levels off at a value of about 400 kPa for EDC concentrations exceeding 40 mg mL(-1). Thus, it is possible by crosslinking PLL/HA films to control their elastic properties with the aim to alter their behavior as to the cellular adhesion.

Colloid Probe AFM Investigation of Interactions between Fibrinogen and PEG-Like Plasma Polymer Surfaces

Interaction forces between surfaces designed to be protein resistant and fibrinogen (Fg) were investigated in phosphate-buffered saline with colloid probe atomic force microscopy. The surfaces of the silica probes were coated with a layer of fibrinogen molecules by adsorption from the buffer. The technique of low-power, pulsed AC plasma polymerization was used to make poly(ethylene glycol) (PEG)-like coatings on poly(ethylene teraphthalate) by using diethylene glycol vinyl ether as the monomer gas. The degree of PEG-like nature of the films was controlled by use of a different effective plasma power in the chamber for each coating, ranging from 0.6 to 3.6 W. This produced a series of thin films with a different number of ether carbons, as assessed by X-ray photoelectron spectroscopy. The interaction force measurements are discussed in relation to trends observed in the reduction of fibrinogen adsorption, as determined quantitatively by (125)I radio-labeling. The plasma polymer coatings with the greatest protein-repelling properties were the most PEG-like in nature and showed the strongest repulsion in interaction force measurements with the fibrinogen-coated probe. Once forced into contact, all the surfaces showed increased adhesion with the protein layer on the probe, and the strength and extension length of adhesion was dependent on both the applied load and the plasma polymer surface chemistry. When the medium was changed from buffer to water, the adhesion after contact was eliminated and only appeared at much higher loads. This indicates that the structure of the fibrinogen molecules on the probe is changed from an extended conformation in buffer to a flat conformation in water, with the former state allowing for stronger interaction with the polymer chains on the surface. These experiments underline the utility of aqueous surface force measurements toward understanding protein-surface interactions, and developing nonfouling surfaces that confer a steric barrier against protein adsorption.

Colloid Probe AFM Investigation of the Influence of Cross-Linking on the Interaction Behavior and Nano-Rheology of Colloidal Droplets

The repulsive forces between a glass sphere and immobilized colloidal droplets of poly(dimethylsiloxane) (PDMS) (with various levels of internal cross-linking) have been determined in aqueous solution using colloid probe atomic force microscopy. On initial surface approach, droplet deformation is negligible and interaction forces resemble those expected for electrical double layer interaction of rigid spheres. Upon further approach, droplet flattening results in forces that deviate below rigid body electrical double layer interaction. The extent of droplet deformation has been determined in terms of the deviation from hard-sphere interaction. Droplet deformability is strongly dependent on the droplet cross-linking level and hence controlled by some combination of the bulk rheological and interfacial properties of the droplets. Droplet nano-rheology has been determined from the extent of force curve hysteresis. For liquidlike droplets, with low levels of cross-linking, no force curve hysteresis is observed and the elastic deformation may be described by a single spring constant, which is controlled by the interfacial properties. For highly cross-linked droplets, the extent of deformation is controlled by the droplet's bulk rheology rather than the interfacial properties. Upon retraction of the surfaces, force curve hysteresis is observed and is due to the viscoelastic response of the PDMS. The extent of hysteresis is dependent on the rate of approach/retraction and the loading force and has been theoretically analyzed to determine nano-rheological parameters that describe droplet relaxation processes. Elastic moduli and relaxation times of the PDMS droplets vary over several orders of magnitude as a function of cross-linking.

Relationship between Interfacial Forces Measured by Colloid-Probe Atomic Force Microscopy and Protein Resistance of Poly(ethylene glycol)-Grafted Poly(l-lysine) Adlayers on Niobia Surfaces

Adsorbed layers of "comb-type" copolymers consisting of PEG chains grafted onto a poly(l-lysine) (PLL) backbone on niobium oxide substrates were studied by colloid-probe AFM in order to characterize the interfacial forces associated with coatings of varying architectures (PEG/PLL ratios and PEG chain lengths) and their relevance to protein resistance. The steric and electrostatic forces measured varied substantially with the architecture of the PLL-g-PEG copolymers. Varying the ionic strength of the buffer solutions enabled discrimination between electrostatic and steric-entropic contributions to the net interfacial force. For high PEG grafting densities the steric component was most prominent, but at low ionic strengths and high grafting densities, a repulsive electrostatic surface force was also observed; its origin was assigned to the niobia charges beneath the copolymer, as insufficient protonated amine groups in the PLL backbone were available for compensation of the oxide surface charges. For lower grafting densities and lower ionic strengths there was a substantial attractive electrostatic contribution arising from interaction of the electrical double layer arising from the protonated amine groups, with that of the silica probe surface (as under low ionic strength conditions, the electrical double layer was thicker than the PEG layer). For these PLL-g-PEG coatings the net interfacial force can thus be a markedly varying superposition of electrostatic and steric-entropic contributions, depending on various factors. The force curves correlate with protein adsorption data, demonstrating the utility of AFM colloid-probe force measurements for quantitative analysis of surface forces and how they determine interfacial interactions with proteins. Such characterization of the net interfacial forces is essential to elucidate the multiple types of interfacial forces relevant to the interactions between PLL-g-PEG coatings and proteins and to advance interpretation of protein adsorption or repellence beyond the oversimplified steric barrier model; in particular, our data demonstrate the importance of an ionic-strength-dependent minimum PEG layer thickness to screen the electrostatic interactions of charged interfaces.

Analysis of the action mechanism of polymer dispersant on dense ethanol alumina suspension using colloidal probe AFM

The action mechanism of a polymer dispersant on dense Al 2O 3 ethanol suspension was investigated using a colloidal probe AFM and branched and linear polyethyleneimines (PEIs). To obtain the minimum viscosity and Newtonian flow property of the dense ethanol suspension, the optimum molecular weights of the branched PEIs were determined over the range from 10,000 to 70,000. The linear PEI with Mw 1400 did not reduce the suspension viscosity compared to the branched PEI with the same molecular weight. The amount of adsorbed PEI did not significantly change regardless of the molecular structure and weight of the PEIs. However, the surface interaction between α-Al 2O 3 solids depended on the molecular structure and weight of the PEIs. The branched chain of the PEI adsorbed on the Al 2O 3 surface facilitated the short-range steric repulsion between particles. Based on the results, the increase in steric repulsive force and the disappearance of the adhesion force by the adsorption of the polymer prompted the dispersion of aggregates in suspension and reduced the viscosity of ethanol dense suspension.

Dispersion behavior control of fine Al2O3 aqueous suspension by combination of molecular structure of polymer dispersant and pH -Analysis by colloidal probe AFM method-

Dispersion behavior control of fine Al2O3 aqueous suspension by combination of molecular structure of polymer dispersant and pH -Analysis by colloidal probe AFM method-

Influence of molecular weight of PEI on rheological properties of alumina nano-particles ethanol suspension. -Analysis by colloidal probe AFM method-

Influence of molecular weight of PEI on rheological properties of alumina nano-particles ethanol suspension. -Analysis by colloidal probe AFM method-

Adhesion between Hydrocarbon Particles and Silica Surfaces with Different Degrees of Hydration As Determined by the AFM Colloidal Probe Technique

Adhesion between oily contaminants and soil particles is a key factor controlling oil release during soil decontamination using wet separation processes. Previous research reports showed that hydration of the soil components, mainly silica particles, governs the oil release efficiency. In this work, the adhesion force between polyethylene particles and silica surfaces with different degrees of hydration obtained by thermal treatment and silanation was measured in an aqueous environment using the atomic force microscopy colloidal probe technique. The adhesion force increases significantly with dehydration of the silica surface and the subsequent increase in hydrophobicity. It was found that experimental adhesion forces measured in the polyethylene−water−silica system are in good agreement with the values calculated theoretically using the Lifshitz/van der Waals−Lewis acid/base interaction theory for highly hydrophobic silica substrates. This agreement is less satisfactory for hydrophilic and slightly hydrophobized silica. It is hypothesized that a thin film of water is strongly adsorbed to the silanol groups on the silica surface and protects the silica surface against its “dry contact” with the polyethylene particle during the atomic force microscopy surface force measurements.

Immobilization and Characterization of Poly(acrylic acid) Graft Layers

This study reports the covalent immobilization of poly(acrylic acid) (PAAC) onto thin films deposited using radio-frequency glow discharge from a vapor of n-heptylamine (HApp). The successful immobilization of PAAC onto HApp was demonstrated by XPS analyses and AFM colloid probe force measurements. The force profiles obtained between silica particles and the PAAC coating were repulsive in nature, roughly exponentially decaying and of long range. The interaction measurements for the PAAC surfaces were not purely electrostatic in nature but also a result of compression of the covalently attached PAAC layer by the silica surface (i.e., electrosteric). The concentration of EDC/NHS added during the coupling influenced both the range and magnitude of the interaction force profiles between the silica colloid probe and the grafted PAAC layer; i.e., the concentration of EDC/NHS added during coupling of PAAC influenced the structure of the layer formed. This result was in good qualitative agreement with XPS analyse...

The Electrical Double Layer on Gold Probed by Electrokinetic and Surface Force Measurements

Gold surfaces, obtained by vacuum deposition of 15-nm gold films on glass and silica wafers, were studied in aqueous solutions by streaming potential measurements and colloidal-probe AFM force measurements. In the force measurements both a bare and a gold-coated silica particle (6 μm in diameter) have been used as colloidal probes. From the streaming potential measurements we determined the zeta-potential of the gold surface, while from the force measurements the diffuse double-layer potential ψd was obtained by fitting the data to the DLVO theory or to the nonlinear Poisson–Boltzmann equation. Measured interactions were found to be entirely due to overlap of electric double layers with no indication of attractive Van der Waals forces. Results of both types of measurements are in good agreement. The double layer potential strongly depends on the pH, probably as a result of the presence of oxide species on the gold surface. Insight in the double layer potential of polarizable interfaces such as the gold/electrolyte solution interface is the first step for understanding the effect of externally applied potentials on the adsorption behavior of charged species.

Analysis of Action Mechanism of Various Additives in Dense Alumina Suspension by using Colloid Probe AFM

Analysis of Action Mechanism of Various Additives in Dense Alumina Suspension by using Colloid Probe AFM