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Name: Kalidindi
Status: student
Grade: 9-12
Location: N/A
Country: India
Date: N/A


Question:
Molecular attraction is the cause for surface tension of liquids. What is the cause for viscosity of liquids?


Replies:
Kalidindi,

Actually, it is more accurate to say that it is the difference between the molecular attraction between one fluid and that with another at the interface that results in what we measure as surface tension. For example, the surface tension of dichloromethane when measured against air is going to be different than the surface tension of that same dichloromethane if measured with water on top of it. Surface tension results from the preferential cohesion of the liquid particles to itself rather than adhering to the other fluid at the interface.

Viscosity on the other hand results from whatever forces inhibits the flow of the liquid particles. This can be, as one would suspect, coming from the intermolecular attractive forces (ionic dipoles, dipole-dipole interaction, hydrogen bonding, london forces). But viscosity can also come from other factors such as physical entanglement of long chain molecules. Imagine how much different pasta would flow if the pasta is shaped like grains (as in the case of taboule or elbow noodles) as opposed to that same pasta but shaped like long strands (spaghetti or angel hair).

Viscosity is a more complex value since it also depends not only on intrinsic properties but on extrinsic forces such as how much force is applied on the liquid. You may have experienced that honey sometimes will flow faster if gravitational forces alone are acting on the honey, but trying to force the honey out of a squeezable bottle often makes the honey "seize up" and counter-intuitively resist the flow.

Greg
(Roberto Gregorius)

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