Cause of Viscosity and Surface Tension
Molecular attraction is the cause for surface
tension of liquids. What is the cause for viscosity of liquids?
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.
Update: June 2012