Computer-Controlled Experiments in the Surface Forces Apparatus with a CCD-Spectrograph

Grünewald, T.; Helm, Christiane A.

doi:10.1021/la960002q

Cited by 26 publications

(12 citation statements)

References 19 publications

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“…The surface force apparatus (SFA) (Mark 4, ANUTECH, Australia) is a well-established tool for the investigation of surface interactions. , Briefly, the force is measured as a function of distance between two molecularly smooth surfaces in a crossed-cylinder configuration. The distance D is measured interferometrically with a resolution of ≈0.2 nm. One of the surfaces is mounted on a cantilever spring of known spring constant; the force is determined from the spring deflection with a resolution of 10 -7 N. The data are presented as the force acting between the surfaces, F ( D ), normalized by the mean radius R of curvature of the surfaces (1−2 cm).…”

Section: Methodsmentioning

confidence: 99%

Molecular Mechanisms Controlling the Self-Assembly Process of Polyelectrolyte Multilayers

1998

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The distance dependent interaction between polyelectrolyte-covered mica surfaces in aqueous solution was investigated with the surface forces apparatus. We find the following: (i) The surface charge changes sign, when an oppositely charged polyelectrolyte from a concentrated polyelectrolyte solution is adsorbed. (ii) Tails and loops of the adsorbed polyions dangle into the bulk phase, inducing a small steric force. If polycations and poyanions are adsorbed on top of each other, a strong short range attractive force is seen due to ion-pair formation after crossing a large repulsive electrostatic/steric barrier. (iii) Obviously, after polyelectrolyte adsorption, there are still nonoccupied binding places (point charges) on the substrate. We show that these adsorption properties regulate the build-up of polyelectrolyte multilayers: Ion pairs between oppositely charged polyion segments and the substrate are formed, until the surface charge is inversed. The electrostatic barrier limits the adsorbed amount, guarantees the equal thickness of consecutively adsorbed layers, and furthermore causes a strong adsorption hysteresis, which leads to conveniently stable polyelectrolyte multilayers in various environmental conditions.

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Section: Methodsmentioning

confidence: 99%

Molecular Mechanisms Controlling the Self-Assembly Process of Polyelectrolyte Multilayers

1998

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“…An effective method of locating the fringe position automatically, which forms the basis for a number of image processing algorithms described in the literature, [33][34][35][36] is to fit an appropriate curve to the extrema in the measured intensity profile. In our system, we adopted the algorithm of Quon et al 33 mainly because it is designed to also extract the shape of the fringe ͑which is particularly advantageous when studying deformable fluid surfaces͒.…”

Section: Fringe Analysis Using Image Processingmentioning

confidence: 99%

Extending the surface force apparatus capabilities by using white light interferometry in reflection

Connor

Horn

2003

Review of Scientific Instruments

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An important factor in the success of the surface force apparatus ͑SFA͒ in measuring interactions between surfaces over nanometer separations has been the optical interference technique used to measure the surface separation. Until recently, this technique has only been used when both of the materials are transparent. As a result, thin sheets of mica have been the material of choice. We describe a simple method to extend the capabilities of the SFA so that a wide variety of material surfaces can be studied while retaining an optical measurement technique. The key to this technique is to modify the optics so that reflected, rather than transmitted, light is used to produce the interference pattern. Now, only one material is required to be thin and transparent while the other can be any material providing it is at least partially reflective. To succeed with this technique, it is necessary to maximize the visibility of the interference fringes. This is achieved by optimizing the thickness of a partially reflective coating ͑often silver͒ deposited on the back side of the transparent material.

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“…From the imaging point of view, long-range interactions are probably less important, because these forces are likely to contribute only to a slowly varying background and to distribute over a large area on the specimen, resulting in small local deformations, provided that they are not very large in magnitude (Shao et al, 1996;Butt, 1991a,b;. However, at short ranges, the hydration force (or solvation force in fluids other than water) is known to be important (Israelachvili and Pashley, 1983;Israelachvili, 1991;Cleveland et al, 1995;Grimson et al, 1980a,b;O'Shea and Welland, 1992;Grünewald and Helm, 1996;Meagher, 1992;O'Shea et al, 1994). It has been shown with two parallel plates in water that the pressure can easily exceed 100 atmospheric pressures at a distance below 2 nm (Israelachvili and Pashley, 1983;Israelachvili, 1991).…”

Section: Introductionmentioning

confidence: 99%

Hydration Force in the Atomic Force Microscope: A Computational Study

Yuan²,

Shao

1998

Biophysical Journal

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Using a hard sphere model and numerical calculations, the effect of the hydration force between a conical tip and a flat surface in the atomic force microscope (AFM) is examined. The numerical results show that the hydration force remains oscillatory, even down to a tip apex of a single water molecule, but its lateral extent is limited to a size of a few water molecules. In general, the contribution of the hydration force is relatively small, but, given the small imaging force ( approximately 0.1 nN) typically used for biological specimens, a layer of water molecules is likely to remain "bound" to the specimen surface. This water layer, between the tip and specimen, could act as a "lubricant" to reduce lateral force, and thus could be one of the reasons for the remarkably high resolution achieved with contact-mode AFM. To disrupt this layer, and to have a true tip-sample contact, a probe force of several nanonewtons would be required. The numerical results also show that the ultimate apex of the tip will determine the magnitude of the hydration force, but that the averaged hydration pressure is independent of the radius of curvature. This latter conclusion suggests that there should be no penalty for the use of sharper tips if hydration force is the dominant interaction between the tip and the specimen, which might be realizable under certain conditions. Furthermore, the calculated hydration energy near the specimen surface compares well with experimentally determined values with an atomic force microscope, providing further support to the validity of these calculations.

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Computer-Controlled Experiments in the Surface Forces Apparatus with a CCD-Spectrograph

Cited by 26 publications

References 19 publications

Molecular Mechanisms Controlling the Self-Assembly Process of Polyelectrolyte Multilayers

Molecular Mechanisms Controlling the Self-Assembly Process of Polyelectrolyte Multilayers

Extending the surface force apparatus capabilities by using white light interferometry in reflection

Hydration Force in the Atomic Force Microscope: A Computational Study

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