dynamic viscosity
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What is Dynamic Viscosity?
Dynamic viscosity is a measure of molecular frictional resistance to flow of a fluid
under some applied force. In the region of the fluid it can also be considered as the
ratio of the stress to the mean rate of change of strain. The SI unit is pascal-seconds,
so for a fluid of viscosity one pascal-second this means that in one second the fluid
will move a distance equal to the thickness
of the layer if it is placed between two plates and pushed by one plate sideways with
a shear stress of one pascal.
The viscosity of a fluid depends on the molecular forces or interactions
between the different components of the mixture, and is a measure of the
molecular transport or diffusion of momentum. Viscosity is high for fluids
whose components have strong molecular interactions and is low
if the molecular forces are weak. There
are many factors which affect dynamic viscosity, including temperature.
Usually an increase in temperature will result in a decrease in the viscosity
and vice versa. Another factor which can affect dynamic viscosity is high pressure;
with increasing pressure the dynamic viscosity also increases.
Measurement of dynamic viscosity
A device called a viscometer is used for the measurement of dynamic viscosity,
and a number of different types are available.
In the measurement of dynamic viscosity the effect of temperature is very
important and must be maintained constant for precise measurement.
Some fluids such as lubricants change their dynamic viscosity with
even a small change in temperature. Dynamic viscosity can be measured
using vibrating viscometers, capillary viscometers, falling sphere viscometers or
rotational viscometers. These devices all measure the resistance of the fluid
to some shearing force which is generated in a variety of ways.
The specific field of rheology is concerned with the flow of liquids under the action of
an externally applied force. An important application is in the study of lubricants
where a thin layer of fluid separates moving parts; for example axles in bearings.
A device called a rheometer is a specialised type of viscometer which measures
this property, which in many cases involves non-Newtonian liquids that can not
be defined in terms of a single value of viscosity.