The Science of Viscosity Measurement3rd Installment

The Science of Viscosity Measurement3rd Installment

The importance of rheology

In our last and previous-to-last installments in this column, we explored how control of viscosity is an intimate and significant part of society and our daily lives. Rheology, where viscosity plays such a vital role, is viewed as the science which deals with the flow and deformation of substances. Rheology is now a major technology involving related research in industry on substances such as polymers, paints, and inks and development in the bio field on living organisms, foodstuffs, natural fibers, etc. Interestingly, the rheo in rheology comes from the Greek work meaning flow.
Rheology is sometimes referred to as a science which links basic research and applied development. Great strides are now being made in technologies which can unravel the chemical properties of simple substances. These efforts have led naturally to attempts to combine, for example, the properties of substance A with substance B and substance C to create a substance imbued with new properties. It is often the case that the key to the development of a revolutionary new industrial product lies in the development of substances endowed with new properties. In applications involving such substances, their physical characteristics need to be understood in order to identify their functions and to consider them from the production line standpoint. Certainly these needs underscore the expectations regarding rheology.
Realistically however, rheological indicators such as viscosity and visco-elasticity are difficult to determine with accuracy. Specialized knowledge and technology, briefly discussed below, become necessary.

Is viscosity measurement difficult?

Determining the suitableness of a substance say, a paint or texture of a foodstuff from a practical standpoint may be obvious from a glance. However the conditions under which such substances are used do not remain constant. The manner in which paint drips or the texture of a foodstuff changes in response to factors such as the amount of force being applied by the painter or the person eating, the place and temperature at the time, and the type of equipment used. We may thus appreciate the difficulties inherent in viscometer measurement under such conditions. Without suitable knowledge of the properties involved or without implementing a suitable means of measurement, it is possible that results obtained may be completely off the mark.
As prelude to further discussion on this subject, the following is a brief outline regarding the fundamentals of fluid rheology as it applies to viscosity measurement.
Viscosity is a proportionality constant of matter which indicates a fluids resistance to flow or degree of tenacity. This is referred to as the coefficient of viscosity or simply viscosity. When force is applied to a fluid to effect flow, internal friction is generated which acts to retard this flow. In other words, viscosity (viscosity = shear stress/shear rate) is the property of a fluid that resists flowing when subjected to an applied force. When measuring such physical properties, the terms Newton fluid or Newtonian fluids are used to refer to fluids which do not change relative to the speed of flow or applied force. Such fluids exhibit the same viscosity regardless of how they are measured (that is, fluids of viscosity which are not shear rate and time dependent). Low-molecular-weight fluids such as water and benzene are usually considered Newtonian fluids.

Expressed as an equation,

------Shear rate
Shear rate expresses change in shear deformation over time and is a type of strain rate. It is the rate of change per unit distance in the direction of the shear, in other words, the velocity gradient.
------Shear stress
Shear stress is the applied force per unit area on the shear flow surface.

This equation however is not applicable to all fluids. In fact it would not be an exaggeration to say that the majority of polymeric liquids which are most prevalent in industry do not fall in this category. For example, the viscosity of some substances may change according to the rate of shear and time. Such substances are referred to as non-Newtonian fluids.
Take the example of paints. A lowered viscosity caused by the speed at which the paint is applied with a brush (shear rate) would make it easier to spread. Once the paint is applied however, the paints higher viscosity would return because the shear rate drops to zero. Paint with such qualities can offer dual advantages of smoother application without runs. As in this example of paint, the property where apparent viscosity of a fluid, when subjected to a constant shear rate, decreases over time and which, in the absence of shear stress, gradually recovers its viscosity is referred to as thixotropy. (See Installment 1 of this column for further details.) The opposite of thixotropic behavior is called rheopexy which refers to the fluid property where the viscosity increases over time as it is sheared at a constant rate.
In addition to the above two types of fluid properties which are time dependent, there are other properties of non-Newtonian fluids such as pseudo plastic, dilatent, and plastic (Bingham, non-Bingham).

A wealth of measurement knowhow at your service

Whether you are dealing with Newtonian fluids, non-Newtonian fluids, or a specific property of non-Newtonian substances, your rheology measurement should be reliable and true. However, unless the measurement method, equipment used, measurement conditions, and other aspects are properly set up, the results you obtain may not be accurate and your efforts may involve an inordinate amount of time.
Based on the preceding discussion, you may be able to appreciate how proper control of the fluidity (viscosity) of substances is indispensable for major industries, and by extension, in our daily lives as well. For basic, high utility materials such as colloids, including gels and polymers, it is important to be able to control visco-elasticity as well as deal with the retarded flows and viscous ness of liquids. There may also be certain cases where high level rheology measurement employing rheometers and, especially for food textures, creep meters are necessary for effective verification (results acquisition).
Toki Sangyo is an integrated manufacturer offering a comprehensive range of activities and services, encompassing engineering & development, manufacturing, sales, and service with a solid lineup of viscometer products. We welcome you to take advantage of our wealth of knowhow. We provide ideal solutions with optimum system configurations incorporating our analytical software.

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