Cause of Viscosity and Surface Tension

name         Kalidindi
status       student
grade        9-12
location     India

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