For the last 69 years, there has been an experiment running at Trinity College Dublin where I work, consisting of a glass funnel with pitch tar inside that is very slowly dripping out. Pitch is an extremely slow-moving fluid, so each drop takes about ten years to form and fall, and this is the first time it’s been captured on video:
Even knowing that pitch is extremely slow-moving and viscous, I still felt an expectation watching the video that once the drop fell, it would merge with the bottom pitch. But of course it doesn’t: it falls over, and that merging will take a long time. Pitch is twenty billion times more viscous than water; for comparison, honey is only about a thousand times more viscous. So pitch flows more slowly and is harder to stir, but why?
Picture a drop of liquid up close. The liquid is comprised of molecules, which are moving somewhat freely but also interacting with each other (if they were moving completely independently, the substance would be a gas). But the molecules at the edge of the drop are experiencing additional forces, interacting with either the air or the container around the drop in addition to the other molecules in the drop. So if we tilt the drop, or try to push it through a tube, those edge molecules are going to flow more slowly, if they are next to a containing wall, or more quickly, if they are next to air. Different layers in the liquid are more or less easy to move, and this means that under the same impetus to flow, the layers end up moving at different speeds. So there is actually friction within the drop, between different layers of molecules! The friction due to intermolecular interactions is stronger in some liquids than others, which is where we get high-viscosity liquids like pitch and low-viscosity liquids like water. Both are subject to the same physical laws, but the strength of intermolecular forces slows the pitch down by an incredible amount. And that’s the microscopic explanation for the macroscopic phenomenon we call viscosity.