Dynamic surface forces

Luckham P F and Manimaaran S 1997 Investigating adsorbed polymer layer behaviour using dynamic surface forces apparatuses—a review Adv. Coiioid interface Sc/. 73 1 -46... 

The interferometric SFA has served as an invaluable tool in studying the hydrophobic attraction among other things due to the fact that it is the only technique available today that enables direct observation of occurrence of cavitation. For instance, recently Lin et al. [89] employed a dynamic surface forces measurement method to study interactions between DODAB LB coated surfaces. High-speed camera images of FECO revealed that there are no bubbles on the surfaces prior to contact. However, short-lived cavities, typically lasting 3 ps before disappearing, have been observed to form upon separation (Fig. 6). 

Ducker, W. A., and Cook, R. F. 1990. Rapid measurement of static and dynamic surface forces, Appl Phys Lett 56, 2408-2410. 

Dynamic surface forces include hydrodynamic interactions and viscoelastic properties of adsorbed layers, as well as frictional forces. All of the techniques described above can be used for measuring hydrodynamic interactions. However, in order to study viscoelastic properties and frictional forces, some modifications to the techniques need to be introduced. This may include oscillating the surfaces in the normal (66-69) or tangential... 

Luckham, P. F. and Manimaaran, S., Investigating adsorbed polymer layer behaviour using dynamic surface force apparatuses - a review, Adv. Colloid Interface ScL, 73, 1 46 (1997). 

The modification of the surface force apparatus (see Fig. VI-4) to measure viscosities between crossed mica cylinders has alleviated concerns about surface roughness. In dynamic mode, a slow, small-amplitude periodic oscillation was imposed on one of the cylinders such that the separation x varied by approximately 10% or less. In the limit of low shear rates, a simple equation defines the viscosity as a function of separation... 

The flotation of mica has been correlated to the adhesion force measured from surface force (SFA—see Section VI-4) experiments although, to these authors, it is clear that dynamic effects prevent an absolute comparison [69, 70],... 

Israelachvili, J.N., Intermolecular and Surface Forces. Academic Press, San Diego, 1992. Landman, U., Luedtke, W.D., Burnham, N.A. and Colton, R.J., Atomistic mechanisms and dynamics of adhesion, nanoindentation, and fracture. Science, 248(4954), 454-461 (1990). 

Molecularly motivated empiricisms, such as the solubility parameter concept, have been valuable in dealing with mixtures of weakly interacting small molecules where surface forces are small. However, they are completely inadequate for mixtures that involve macromolecules, associating entities like surfactants, and rod-like or plate-like species that can form ordered phases. New theories and models are needed to describe and understand these systems. This is an active research area where advances could lead to better understanding of the dynamics of polymers and colloids in solution, the rheological and mechanical properties of these solutions, and, more generally, the fluid mechaiucs of non-Newtonian liquids. 

The hrst apparatus for nanotribology research is the Surface Force Apparatus (SFA) invented by Tabor and Win-terton [1] in 1969, which is used to study the static and dynamic performance of lubricant him between two molecule-smooth interactions. 

The emphasis of this chapter is on the dynamic nature of adhesion and its interrelations to friction, but before discussing the dynamics a brief introduction to the surface forces responsible for creation of adhesion is given in the following. 

Bubble size at departure. At departure from a heated surface, the bubble size may theoretically be obtained from a dynamic force balance on the bubble. This should include allowance for surface forces, buoyancy, liquid inertia due to bubble growth, viscous forces, and forces due to the liquid convection around the bubble. For a horizontally heated surface, the maximum static bubble size can be determined analytically as a function of contact angle, surface tension, and... 

Ishii, M., Thermo-Fluid Dynamic Theory of Two-Phase Flow . Eyrolles, Paris, France (1975). Israelachvili, J., Intermolecular Surface Forces . Academic Press, London, UK (1991). 

Each dimensionless group represents a rule for scale-up. Frequently these individual scale-up rules conflict. For example, scale-up on dynamic similarity should depend chiefly upon a single dimensionless group that represents the ratio of the applied to the opposing forces. The Reynolds, Froude and Weber numbers are the ratios of the applied to the resisting viscous, gravitational and surfaces forces, respectively. 

Most treatments of bubble growth dynamics assume that, as the bubble expands, Eq. (7) is still valid although the pressure may drop due to the expansion from both temperature variations in the bubble and the increase in radius—to lessen the surface forces, i.e., at any time in the growth. 

The combination of careful chemical synthesis with NSE and SANS experiments sheds some light on the fast relaxation processes observed in the collective dynamics of block copolymers melts. The results reveal the existence of an important driving force acting on the junction points at and even well above the ODT. Modelling the surface forces by an expression for the surface tension, it was possible to describe the NSE spectra consistently. The experimental surface tension agrees reasonably well with the Helfand predictions, which are strictly valid only in the strong-segregation hmit. Beyond that, these data are a first example for NSE experiments on the interface dynamics in a bulk polymer system. 

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