INDEX hydrodynamic force

Much of the fundaments we know about surface forces are based on experiments with the SFA. With the SFA, surface forces are measured between two atomically smooth mica surfaces. Distance is measured interferometrically, which allows absolute determination of separation distance with a resolution of typically 0.1 nm (with down to 25 pm achievable). Its absolute force sensitivity is not as high as in several other methods, but in terms of the usually more relevant force per unit area, its sensitivity is excellent. Lateral (friction) forces can be measured in addition to normal standard force versus distance measurements and have contributed much to our understanding of lubrication by thin films. Additional information such as refractive index and contact area can be obtained. The main reason for the limited number of groups using this instrument is the difficult operation of such a system that needs a very experienced and skillful expert. The large interaction areas demand a contamination-free surface preparation and can lead to significant hydrodynamic forces in highly viscous media, which could make equilibrium measurements hard to achieve. 

The particle contribution to the stress, Opa ide, is given as the sum of the stress Opot due to a conservative force resulting from the interparticle potential and the stress Ohyd due to a dissipative, hydrodynamic interaction. Opot is expressed in terms of the positions of the particle centers rl l (a is the particle index) and the conservative forces acting on the particles as (cf. Appendix... 

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