What is Viscosity?

Sep 30 2013

It is important to understand that there is no simple relationship between viscosity and particle size that covers all types of liquids. Liquid viscosity is caused by forces between its particles, which are also known as molecules.

There are different kinds of forces in different liquids. For instance, water has strong hydrogen bonds whereas mercury has a metallic interaction between its atoms and molecules. Also oils and fats have van der Waals forces that are considerably weaker.

Some examples of different types of forces and viscosity in millipascals (mPa) at 25°C are:

Hydrogen bonds water (H₂O) 0.890
Metallic interaction mercury (Hg) 1.526
Van der Waals forces tetrachloromethane (CCl₄) 0.908
Polar forces tetrachlorosilane (SiCl₄) 99.4

The forces grow with particle size in the case of van der Waals forces. There is almost a linear increase of viscosity from C1 (methanol) to C10 (decanol) in hydrocarbons.

This is because of the increasing length of the linear hydrocarbon chain. There are long molecules in vegetable oil so the van der Waals forces are large and hence the viscosity is high. In the case of hydrogen bonds, the number of bonds that can be formed by a molecule has a major effect on its viscosity.

Consider three simple liquids, all having very similar molecular size with one, two and three hydrogen bond forming groups respectively. The viscosity in millipascals (mPa) at 25°C is:

Propanol 2
Propandiol 40
Propantriol (glycerol) 934

The reason for this significant increase is that more hydrogen bonds per molecule help strong 3 dimensional networks between the molecules in the liquids, while single hydrogen bonds can only form into linear chains, or at best, rings.

What is the Unit of Viscosity?

Dynamic (absolute) Viscosity

When the measured values are based on the basic physical units of force [N], length [m] and time [s],

Dynamic viscosity = [N/m²] • [s] = force/length² • time = [Pa] • [s]

The basic unit of viscosity measurement is poise. A material that needs a shear stress of 1 dyne/cm² to produce a shear rate of one reciprocal second has a viscosity of one poise, or 100 centipoise.

One can encounter viscosity measurements expressed in Pascal-seconds (Pa•s) or millipascal-seconds (mPa•s), which are units of the International System and are sometimes used in preference to the metric designations.

One Pascal-second is equal to ten poise, one millipascal-second is equal to one centipoise.

In the SI system the dynamic viscosity units are N s/m², Pa.s or kg/ms where:

1 Pa.s = 1N s/m² = 1 kg/ms

The dynamic viscosity is normally expressed in the metric CGS (centimeter-gram-second) system as g/cm.s, dyne.s/cm² or poise (p) where

1 poise = dyne.s/cm² = g/cm.s = 1/10Pa.s = 1/10N.s/m²

For practical purposes, the poise is too large and is normally divided by 100 into the smaller unit known as the called the centiPoise (cP) where:

1p = 100cP

1cP = 0.01poise = 0.01g/cm.s=0.001Pa.s = 0.001N.s/m²

Water at 68.4°F (20.2°C) has an absolute viscosity of 1cP.

Kinematic Viscosity

Using the SI-system the theoretical unit is m²/s or commonly used Stoke (St) where

1St = 10^-4m²/s

As the stoke is an unpractical large unit, it is usual divided by 100 to give the unit called Centistokes (cSt) where

1St = 100cSt

1cSt = 10^-6m²/s

Since the specific gravity of water at 68.4°F (20.2°C) is almost one (1), the kinematic viscosity of water at 68.4°F is for all practical purposes is 1.0 cSt.

Saybolt Universal Seconds (or SUS, SSU)

Another unit for viscosity measurement is saybolt universal seconds. The efflux time is Saybolt Universal Seconds (SUS) required for 60 ml of a petroleum product to flow through the calibrated orifice of a Saybolt Universal viscometer, under carefully controlled temperature and as prescribed by test method ASTM D 88.

The kinematic viscosity method has largely replaced this method.

Saybolt universal seconds is also called the SSU number (Seconds Saybolt Universal) or SSF number (Saybolt Seconds Furol).

Kinematic Viscosity

Kinematic viscosity versus dynamic or absolute viscosity can be expressed as

ν = 4.63 µ/SG

where

ν = kinematic vicosity (SSU)
µ = dynamic or absolute viscosity (cP)
SG = Specific Gravity

Material with Highest Viscosity in the World

Viscosity is a measure of a liquid's resistance to flowing. Thin, low viscosity liquids flow easily. Thick high viscosity liquids flow more slowly or require the shear stress application to induce flow. Eventually all solids will flow in response to shear stress, so solids can, in principle, be considered as liquids with a very high viscosity.

Determining what is the highest viscosity in the world is based on one’s opinion of the dividing line, if any, between liquids and solids. The dividing line is around 10¹² Pa's.

In case we accept all solids are liquids then diamond has the highest viscosity.

About Fungilab

Fungilab manufactures advanced and high quality rotational viscometers. Fungilab is a strong and solid brand, recognized around the world for its best practices in products and services in the viscosity market.

The values that Fungilab brand transmits to market for improving the client’s confidence are high quality equipment with an accurate, easy and evolved working system, featuring the latest advances in technology; specialized customer and technical service to differentiate fungilab from the rest of the brands in the viscosity market; pre-sales and post-sales support to end customers and dealers; and equipment repairs.

Fungilab Presentation Final