Introduction: Rheology is defined as the science of the deformation and flow of matter (BSR, 1998). It is important for Mechanical Engineers especially dealing with fluid flow and tribology (the science of lubrication). One physical quantity important in the rheology of fluids is viscosity. Viscosity is defined as the ratio of shear stress to shear rate in the fluid. A fluid for which viscosity is constant at all shear rates is called a ‘Newtonian’ fluid. Examples of Newtonian fluids include: water, sugar solutions, glycerin, silicone oils, light-hydrocarbon oils, air and other gases (Schlumberger, 2002). A fluid, whose viscosity is not constant, but varies as a function of shear rate is called a ‘non-Newtonian’ fluid. Examples of non-Newtonian fluids are drilling fluids, some kinds of oils, some kinds of paints, polymer melts, etc. A good overview of the most common types of non-Newtonian fluid behaviors is given in the Brookfield publication “More Solutions to Sticky Problems”.
While many lubricating oils are Newtonian, their viscosities are highly temperature dependent. It is important to know the viscosity and how it varies with temperature in the design of lubrication systems, fluid transport, etc. This lab will introduce the student to one method of making viscosity measurements.
It is particularly important to remember the following points:
(1) Use the correct viscometer size: Measuring ranges overlap, but the best results will be obtained if you are operating near the center of the viscometer’s range. If your sample has a viscosity of about 100 cSt, for example, the size 300 (range 50-250 cSt) viscometer will give more accurate results than either size 200 (range 20-100 cSt) or size 350 (range 100-500cSt). Refer to the table below as a guide to size selection.
(2) Avoid overloading the viscometer: Using either too much or too little sample will produce inaccurate values of viscosity. The volume that fills the tube from E to the inlet of the A tube will approximately half fill the bulb marked H.
(3) Use the correct calibration factor: C. Each viscometer comes with its own serial number and calibration constants, which vary from one instrument to another. Note that the constants listed in the table above are only approximate values. The constant varies with temperature and applies only if the correct amount of fluid has been charged into the viscometer. If the calibration constants have been lost, they can be re determined using viscosity standards.
(4) After loading the sample: wait at least 10 minutes before making measurements. Some time is required to allow the sample to equilibrate at the temperature of the water bath and to allow air bubbles to seggregate.
(5) Use special methods for opaque samples: For high viscosity oil samples, which are sometimes quite opaque, it is often necessary to use auxiliary illumination to improve judgments of the passage of the interface past the starting and ending marks. Another option is to use a “reverse flow” viscometer.
Shear Viscosity: An important mechanical property of fluids is viscosity. Physical systems and applications as diverse as fluid flow in pipes, the flow of blood, lubrication of engine parts, the dynamics of raindrops, volcanic eruptions, planetary and stellar magnetic field generation, to name just a few, all involve fluid flow and are controlled to some degree by fluid viscosity. Viscosity is defined as the internal friction of a fluid. The microscopic nature of internal friction in a fluid is analogous to the macroscopic concept of mechanical friction in the system of an object moving on a stationary planar surface. Energy must be supplied:
(1) To overcome the inertial state of the interlocked object and plane caused by surface roughness.
(2) To initiate and sustain motion of the object over the plane.
In a fluid, energy must be supplied:
(1) To create viscous flow units by breaking bonds between atoms and molecules.
(2) To cause the flow units to move relative to one another.