Hi all,
I'm testing the viscoelasticity of some gels with oscillatory tests (oscillation with strain sweep and frequency sweep). I am wondering what the normal force (the force that's compressing the gel, in the direction perpendicular to the oscillation) should be? Thank you!
Hi,
In a standard protocole like described by you, after placing a stiff sample in the geometry, normal force can be a few Newtons and will asympototically decrease for a few minutes. The normal force should be reset to zero before starting the experiment. This will be a way for you to ensure that there is no drifting during experiment. Hope this is clear. Regards
Hi Feichen Yang,
assuming you use a parallel plates measuring geometry, the aim of setting a normal force is to have ideal contact between sample and measuring geometry.
Therefore, it highly depends on sample properties. Both higher stiffness and slippery surface, e.g. oily or greasy, require higher normal force to maintain good contact.
However, a too high normal force will (1) squeeze the sample out of the measuring gap (will require trimming, if possible) and/or (2) will lead to an unrelaxed state of the sample, and this might also modify your measuring results.
As David Sturm mentioned, the values are typically something between 1 and 5 N (depends of course also on the plate diameter).
By the way, if your sample contains polymeric components you will observe an increase of the normal force during e.g. the frequency sweep, which is perfectly fine as it reflects the relaxation behavior of the unrelaxed sample state when measured (even if you measure within the LVE range).
Best wishes
Christopher
1. In case of Cone and plate system the normal force acting on the stationary plate is
FN = (π R2/2)(first normal stress difference = τϕϕ - τθθ) (using θ ϕ r) coordinate system)
2. Using a concentric cylindrical viscometer
Force acting on the surface of rotating cylinder 1. Shear force 2. the normal force (= centrifugal force)
3. Force acting on the base of the stationary cylinder = ρ g h
br gupta,
retd prof.
I I T KGP
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