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In physics, viscosity is the force applied in a unit area times time or basically Pascal times second. We could say that it is a measure of its resistance to gradual deformation by shear stress or tensile stress.

But what does it mean in the chemistry world? I believe it is related to the distance between molecules in a solution. When molecules are farther apart then viscosity is lower than when molecules are closer.

Could you tell me if my mature concept about viscosity is on the right track or far too off? Could you also define viscosity in terms of chemistry?

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    $\begingroup$ There are intermolecular forces but also molecules are not point masses so they don't just slide past each other smoothly. This would give rise to the resistance to shear. $\endgroup$ – Zhe Oct 17 '17 at 15:27
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    $\begingroup$ Viscosity is defined in chemistry and physics pretty much equally. For exact definitions of various viscosities open IUPAC Gold Book and search for "viscosity" (left toolbar) – you will see it's the same physical terms. $\endgroup$ – andselisk Oct 17 '17 at 15:28
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    $\begingroup$ I agree with @andselisk a̶s̶ ̶a̶l̶w̶a̶y̶s̶. Perhaps you meant your title to read: "Viscosity: The molecular perspective" or something like that ;) $\endgroup$ – paracetamol Oct 17 '17 at 15:31
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More or less similar…I will try to put brief concepts of viscosity in terms of pharmaceutics which is somewhat another branch of chemistry, where it is very much applicable in drug design.


Viscosity is an expression of the resistance to flow of a system under an applied stress. The more viscous a liquid is, the greater is the applied force required to make it flow at a particular rate.

Viscosity is defined in terms of the force required to move one plane surface past another under specified conditions when the space between is filled by the liquid in question. More simply, it can be considered as a relative property, with water as the reference material.

Viscosity implies that the liquid flows even under the smallest stress and does not return to its original shape or form once the stress is removed.

Viscous deformation, i.e. viscous flow, occurs in accordance with Newton’s law,

$\ce{σ=ηỳ}$

where the applied stress σ results in flow with a velocity gradient ỳ or rate of shear. The proportionality constant η is termed viscosity, while its reciprocal is called fluidity.

Viscosity has also been described as the internal friction in the fluid as it corresponds to the resistance of the fluid to the relative motion of adjacent layers of liquid.

The viscosity of simple liquids (i.e, pure liquids consisting of small molecules and solutions where solute and solvent are small molecules) depends only on composition, temperature, and pressure. It increases moderately with increasing pressure and markedly with decreasing temperature.

There is quite a lot to discuss on viscosity in depth but these are just shallow concepts to understanding the term.


References

  1. Martin’s Physical Pharmacy and Pharmaceutical Sciences : Rheology
  2. Remington: Essentials of Pharmaceutics: Rheology
  3. Ansel’s Pharmaceutical Dosage Forms and Drug Delivery Systems : Special Solutions and Suspensions
  4. Aulton ’s Pharmaceutics The Design and Manufacture of Medicines 4th edViscosity Rheology, and flow of fluids
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