Offshore oil producers have long wished to have a magical remote control for the viscosity of crude oil. An ideal device like this will allow them to reduce the viscosity of crude oil for a few hours, while it is pumped through pipelines to onshore tanks, where it would be restored to its original thick and viscous conditions. Scientist R. Tao and X. Xu have finally cracked it!
Viscosity is a fundamental material property when studying fluid flow for any application. The two most common types of viscosity are dynamic and kinematic. The relationship between these two properties is quite straightforward.
In a recent post, we discussed how some liquids containing bacteria swimmers can display negative viscosity. This can be a misleading since the apparent negative viscosity occurs due to swimmers pushing the surrounding fluid so that the overall system shows negative resistance to flow (i.e. it flows by itself!). One of the best examples of super fluidity is super cooled helium.
It is widely accepted that viscosity plays a crucial role in current printing and coating industries. In a very similar way, viscosity has a strong influence on the texture of artistic paintings. Textures may not be as obvious as colors in a painting, but it can be just as important in conveying the artist’s idea or concept behind it.
Scientists from Trinity College Dublin captured the falling drop of very high viscosity bitumen or asphalt in a pitch-drop experiment. As Rheologists say, “Everything flows”. Well…in the case of pitch, you need to be extremely patient. This material appears to be an elastic solid to the naked eye. As the drops from these experiments show, however, it is actually an extremely viscous fluid! The Dublin pitch-drop experiment was set up in 1944. This drop took so long to form and fall (1 drop/10 years) that the scientists actually forgot about the experiment and only recently, started recording it with a webcam. Viscosity estimates from the last drop put it at 2 x 107 Pa-s or twenty billion times greater than water. Imagine sitting in front of the funnel with your notebook and your stop watch. Now imagine that you fall asleep and then the drop finally falls!
Units of Viscosity: Part I
The most commonly used unit for dynamic viscosity is centipoise (cP), which is equivalent to 0.01 Poises (P). This unit is used in honor of French physicist Jean Léonard Marie Poiseuille (1797-1869) who worked with Gotthilf Hagen on the widely known Hagen-Poiseuille’s law which applies to laminar flow through pipes. It is not a coincidence that the viscosity of distilled water at 20°C was used to define 1 cP. In order to give you an idea of the viscosity of some conventional fluids we have collected their viscosities in Table 1. The SI unit for viscosity is the Pascal-second (Pa-s), which corresponds to the force (N) per unit area (m2) divided by the rate of shear (s-1). However, since the viscosity of most fluids is below 1 Pa-s (See table 1), the cP equivalent or milipascal-second (mPa-s) is often used.