Viscosity, Rheology & Confusion

In the vast majority of industries, manufacturing products involves subjecting matter to a series of transformations to give it certain functions and forms. The different materials are thus likely to pass through states powders, fluid, pasty, gelled, emulsified, solid, etc.

Among these changes, the ability of the material to flow (its rheology) in pipes, mixers, pumps or devicesapplication is a critical industrial issue. Many instrumental techniques aim to quantify this propensity of matter to flow, which is often a source of perplexity and confusion.

Viscosity, a self-evident concept?

In everyday life, We generally use the concept of viscosity, associated with an intuitive sensory perception of "consistency", to speak of the ability to flow. Observing the movements of honey, water or a fruit juice and classifying them by increasing viscosities by observing their consistency does not pose any difficulty in principle.

In reality, the concept presents certain subtleties that it is absolutely essential to identify in order to understand the variety of behaviors of materials in the industrial context. Unfortunately, and this is one of the first causes of confusion, it is necessary to reach in most scientific and technical courses a Master 2 level for the existence of these refinements to be mentioned.

Intrinsic constant or function?

In a less intuitive and more scientific definition, viscosity is often defined as resistance to flow due to internal friction in the material. Such a definition makes it possible in particular to justify the observation that a fluid to which macromolecules are added tends to thicken and see its viscosity increase. In this definition, viscosity is a specific property of the product concerned. Thus, it is commonly accepted that water, ethanol, raspberry syrup have a certain characteristic value of viscosity.

In reality, viscosity is a function of temperature. It is reported that from antiquity, at the time of the use of the clepsydra, Egyptians and Chinese had noted that in winter it was necessary to heat the water in the clepsydra to maintain the course of time...

Nowadays, most normative measurements impose measurements at a fixed temperature, some recommending measurements at several temperatures.

More viscosity is in many cases not only a function of temperature, it also depends on the conditions under which matter is set in motion.

Newtonian and non-Newtonian fluids

To represent this idea, let's imagine a swimmer in full crawl. If the viscosity of water depended on the conditions in which the swimmer deploys his movement, he would be likely to feel a higher or lower viscosity depending on his effort. Fortunately for the swimmer, and for populations in general, this is not the case. The viscosity of water is independent of the constraints imposed on it: water is a liquid that is described as Newtonian.

On the other hand, if we immerse our swimmer in a sufficiently concentrated cornstarch solution, the faster he tries to go, the thicker the solution will become, almost solid even, thus countering his effort and making his swimming laborious if not impossible. The concentrated cornstarch solution is called non-Newtonian: its viscosity depends on the constraints imposed on it. (In this case, concentrated cornstarch is said to be shear-thickening: the greater the constraints imposed, the more its viscosity increases)

The vast majority of industrial products made up of macromolecules in sufficient quantities - which is the case for formulations - have non-Newtonian behaviors. These effects are moreover exploited as such in numerous applications, whether it concerns the shear-thinning properties of paints, inks or the stringing properties of certain lubricating oils.

The viscosity of non-Newtonian fluids is not an intrinsic property

At the scientific level, the concept of viscosity and the theoretical models have been gradually sketched out since the XNUMXth century to describe these materials whose behavior borrows from the mechanics of solids as much as that of fluids.

Yet, as often observed in the uses of scientific concepts, modern definitions never quite replace traditional or intuitive use.

It is therefore a question of insisting on this fundamental and largely underestimated implication of this property of fluids in general: since the viscosity of a non-Newtonian fluid depends on the conditions under which it flows, then any measurement of such a fluid based on flowing comes to measure a quantity function (implicitly or not) of the concrete conditions of the implementation of the measurement.

In other words, measuring THE viscosity of a non-Newtonian fluid is a pure misnomer, and the industrial consequences of this misunderstanding can be costly.

The instruments and viscosities fair

The study, initially within a research framework, of these non-Newtonian fluids has imposed the development of increasingly sophisticated instrumental techniques for measuring viscosity values ​​under precise operating conditions.

However, there are still a multitude of devices on the market, all claiming the ability to measure viscosity values: viscosity or consistometric cups, capillary viscometers, needle viscometers, Couette viscometers, rheometers, rheometers capillaries and many more. Suffice to say that the business of instrumentation does not really contribute to clarifying the situation.

The catalog is dense, like the plethora of sectoral normative standards recommending this or that device. The concept of millefeuille is not reserved for legislation or administration: the corpus of norms and standards has nothing to envy.

To add further complexity, some devices refer to a measurement of kinematic viscosity, others from dynamic viscosity -notions whose theory nevertheless indicates that they are equivalent to the density factor. We also hear about apparent viscosity - Curious term suggesting that behind appearances hides something else. Absolute viscosity?!…

From imbroglio to confusion

The notions related to viscosity and rheology become obscure as the meanings of the words vary according to the sources.

Such an imbroglio may seem surprising at first sight, but we will show in a future article that it seems to be explained by the history of the development of concepts and instrumental techniques for nearly two centuries.