| 1. |
- Attard, P, et al.
(författare)
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Dynamic surface force measurement. 2. Friction and the atomic force microscope
- 1999
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Ingår i: Langmuir. - 0743-7463 .- 1520-5827. ; 15, s. 553-563
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Tidskriftsartikel (refereegranskat)abstract
- The mechanism and geometry of force measurement with the atomic force microscope are analyzed in detail. The effective spring constant to be used in force measurement is given in terms of the cantilever spring constant. Particular attention is paid to possible dynamic effects. Theoretical calculations show that inertial effects may be neglected in most regimes, the exception being when relatively large colloidal probes are used. Model calculations of the effects of friction show that it can cause hysteresis in the constant compliance region and a shift in the zero of separation. Most surprising, friction can cause a significant diminution of the measured precontact force, and, if it actually pins the surfaces, it can change the sign of the calibration factor for the cantilever deflection, which would cause a precontact attraction to appear as a repulsion. Measurements are made of the van der Waals force between a silicon tip and a glass substrate in air. The evidence for friction and other dynamic effects is discussed. Interferometry is used to characterize the performance of the piezoelectric drive motor and position detector used in the atomic force microscope. It is shown that hysteresis in the former, and backlash in the latter, preclude a quantitative measurement of friction effects. The experimental data appear to underestimate the van der Waals attraction at high driving velocities, in qualitative agreement with the model friction calculations.
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| 2. |
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| 3. |
- Carambassis, A, et al.
(författare)
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Forces measured between hydrophobic surfaces due to a sub-microscopic bridging bubble
- 1998
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Ingår i: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 80, s. 5357-5360
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Tidskriftsartikel (refereegranskat)abstract
- Atomic force microscopy on hydrophobic microspheres in water reveals a strong attraction with a range of 20–200 nm, following an initial steep repulsion at long range. The data are consistent with a single ubmicroscopic bubble between the surfaces, with the attraction due to its attachment and lateral spread, and the repulsion dependent on film drainage and the electric double layer. The results provide direct experimental evidence of the existence of long-lived submicron bubbles, and of their bridging as the cause of the measured long-range attractions between macroscopic hydrophobic surfaces.
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| 4. |
- Carambassis, A, et al.
(författare)
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Interactions of cellulose surfaces: Effect of electrolyte
- 1999
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Ingår i: Langmuir. - 0743-7463 .- 1520-5827. ; 15, s. 5584-5590
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Tidskriftsartikel (refereegranskat)abstract
- The forces between spherical particles of cellulose (20-30 m) have been measured in different solutions using an atomic force microscope, with a view to understanding the interactions in a model papermaking system. At low ionic strength (0.1 mM KBr), the interaction profile is dominated by a long range double layer force and shorter ranged electrosteric force. A qualititatively similar profile is observed at high pH, but in this case both the double layer force and electrosteric force increase as a consequence of cellulose charging. Conversely, the two force contributions both decrease in the presence of calcium ions. At high ionic strength (10 mM KBr) the electrosteric force is absent and the forces appear to be due solely to double layer forces. Overall, the results show that the surface is composed of looser chains that extend out into the solution, the conformation of which is highly sensitive to the solution conditions.
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| 5. |
- Claesson, PM, et al.
(författare)
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Amine functionalized surfactants - pH effects on adsorption and interaction
- 1992
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Ingår i: Progress in Colloid and Polymer Science. - 0340-255X .- 1437-8027. ; 88, s. 64-73
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Tidskriftsartikel (refereegranskat)abstract
- The interactions between layers of two hydrolyzable surfactants, dodecylamine (DA) and tetraoxyethylene dodecylamine (TEDA), adsorbed on negatively charged muscovite mica have been investigated. It was found that the variation of the interaction and adsorption as a function of pH was similar, but not identical, for the two kinds of surfactants. At a concentration of 10-4 M at low pH (below 8) both types of surfactants adsorb electrostatically to form a monolayer. The resulting surfaces have a hydrophobic character, particularly in the case of dodecylamine. A series of events takes place as the pH is increased. For DA the first event is that uncharged molecules adsorb within the monolayer. This does not occur to any significant degree for TEDA due to steric hindrance within the adsorbed layer. In the next step a bilayer builds up on the surface. The binding of the outer layer is stronger for DA than for TEDA. The DA bilayer becomes uncharged at pH 10.3, and liquid dodecylamine droplets, which phase separate from the bulk solution at this pH-value, precipitate on the surface. At pH-values above 11-12 the affinity between both types of surfactants and the surface decreases significantly, resulting in a less ordered first layer. In addition a strong surface charge develops that prevents multilayer build-up.
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| 6. |
- Claesson, PM, et al.
(författare)
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Techniques for measuring surface forces
- 1996
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Ingår i: Advances in Colloid and Interface Science. - 0001-8686 .- 1873-3727. ; 67, s. 119-183
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Tidskriftsartikel (refereegranskat)abstract
- The forces acting between colloidal particles and between surfaces are of utmost importance for determining the behaviour of dispersed systems and adhesion phenomena. Several techniques are now available for direct measurement of these surface forces. In this review we focus on some of these methods. Two techniques for measuring forces between macroscopic solid surfaces; the interferometric surface force apparatus, known as the SFA, and a novel instrument which is based on a bimorph force sensor, the so called MASIF, are described in some detail. Forces between a macroscopic surface and a particle can be measured with the atomic force microscope (AFM) using a colloidal probe, or by employing total internal reflection microscopy (TIRM) to monitor the position of a colloidal particle trapped by a laser beam. We also describe two different techniques that can be used for measuring forces between "soft" interfaces, the thin film balance (TFB) for single foam, emulsion and solid/fluid/fluid films, and osmotic stress methods, commonly used for studying interactions in liquid crystalline surfactant phases or in concentrated dispersions. The advantages and limitations of each of these techniques are discussed and typical results are presented.
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| 7. |
- Gölander, C-G, et al.
(författare)
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Structure and surface properties of diaminocyclohexane plasma polymer films
- 1993
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Ingår i: Journal of Applied Polymer Science. - 0021-8995 .- 1097-4628. ; 49, s. 39-51
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Tidskriftsartikel (refereegranskat)abstract
- Plasma polymers of three isomers of diaminocyclohexane (DACH) were deposited on polyethylene, SiO2, and mica at 20 °C. The deposition rate was measured as a function of plasma density and power; a maximum was observed in the latter function. The deposition rate was highest for the monomer with the highest flow rate. The film refractive index was observed to increase with both power density and the degree of fragmentation in the plasma. Film composition was measured by elementary analysis, and was found to be almost identical for each of the three isomers; a mechanism for the polymerization reaction is proposed. The percentage of primary amino groups decreased with increasing power density and with film thickness. Surface force measurements of the thickness and refractive index agreed well with the corresponding ellipsometry values in dry air, and an adhesive force, independent of power density, was measured. When the film was exposed to water vapor, it swelled considerably and the adhesion was determined by capillary forces. Associated with swelling, at high power, was an extremely regular 2-ply rope pattern of protruding material.
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| 8. |
- Hahn Berg, IC, et al.
(författare)
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Lubricating properties of the initial salivary pellicle – an AFM study
- 2003
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Ingår i: Biofouling (Print). - 0892-7014 .- 1029-2454. ; 19, s. 365-369
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Tidskriftsartikel (refereegranskat)abstract
- The role of saliva in the oral cavity is manifold; an important function is to serve as lubricant between hard (enamel) and soft (mucosal) tissues. Intraoral lubrication is of crucial importance in order to maintain functions such as deglutition, mastication and the faculty of speech. A large number of people suffer from impaired salivary functions, displaying symptoms such as 'dry mouth'. This results in a need for methods to assess the lubricating properties of both native saliva as well as potential artificial saliva formulations. Here we measure normal as well as lateral forces, acting between adsorbed salivary films, for the first time by means of colloidal probe atomic force microscopy. We find that the presence of salivary pellicles between hard surfaces reduces the friction coefficient by a factor of 20. This reduction of friction is consistent with the long-range purely repulsive nature of the normal forces acting between the salivary films. The lubricating mechanism is presumably based on a full separation of the sliding surfaces by the salivary films. The friction between salivary films has been investigated at normal loads that well cover the clinical jaw closing forces, and it can be concluded that the lubricating properties are maintained within this load interval. The present study indicates the usefulness of colloidal probe atomic force microscopy, which offers a direct and quantitative measure of the lubrication on a molecular level, in the study of biotribological phenomena. In particular, the results obtained here may have implications for the development of saliva substitutes
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| 9. |
- Imae, T, et al.
(författare)
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Forces between two glass surfaces with adsorbed hexadecyltrimethylammonium salicylate
- 2000
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Ingår i: Langmuir. - 0743-7463 .- 1520-5827. ; 16, s. 1937-1942
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Tidskriftsartikel (refereegranskat)abstract
- Forces have been measured for hexadecyltrimethylammonium salicylate (C16TASal) layers on glass beads. During the inward process, hydrophobic attraction occurred at lower adsorption of C16TASal and electrostatic repulsion interactions happened at higher adsorption. While the jump-in phenomenon was observed for solutions of concentrations below the critical micelle concentration (cmc = 0.15 mM), the step-in phenomenon was characteristic for solutions at the cmc and above the cmc, suggesting the push-out of adsorbed C16TASal layers and/or inserted micelles. The remarkable pull-off phenomenon on the outward process occurred for all solutions, indicating a strong interaction between C16TASal molecules. For aqueous 0.15 mM C16TASal solutions of various NaSal concentrations, on the inward process, the electrostatic repulsive interaction decreased with adding NaSal. This is due to the electrostatic shielding by salt excess. The height of the force wall on the inward process reached a maximum at 0.01 M NaSal, but the interlocking between molecules on two surfaces during the outward process was minimized at 0.1 M NaSal. These tendencies, which are different from that of the electrostatic repulsion interaction, imply the strong cohesion between adsorbed C16TASal layers.
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| 10. |
- Parker, JL, et al.
(författare)
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Time-dependent adhesion between glass surfaces in dilute surfactant solutions
- 1993
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Ingår i: Langmuir. - 0743-7463 .- 1520-5827. ; 9, s. 1965-1967
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Tidskriftsartikel (refereegranskat)abstract
- Measurements of surface forces between glass surfaces in very dilute cationic surfactant solutions at pH 10 are presented. As the surfaces approach the forces are purely repulsive and correspond exactly to the interaction of glass surfaces in aqueous solution at the same pH. However, a small adhesion is observed on separation, the magnitude of which is dependent on the time the surfaces are left in contact. The adhesion arises due to adsorption of surfactant in a narrow gap around the contact area and is induced by the favorable interaction of hydrophobic tails across the gap. The time dependence of the adhesion suggests that the adsorption is diffusion limited.
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