Talk:Viscosity

Old/undated question and comments

ASTM Cps What is ASTM Cps measured used for viscosity??? Mistatement concerning kinematic viscosity The statement "In many situations, we are concerned with the ratio of inertial to viscous forces, the latter characterised by the fluid density ρ. This ratio is characterised by the kinematic viscosity:" would be correct if modified as follows: In many situations, we are concerned with the ratio of VISCOUS TO INERTIAL forces, the latter characterised by the fluid density ρ. This ratio is characterised by the kinematic viscosity: If no one comes up with a contrary argument in a few days, i'll edit the article elzorro OK. Done. elzorro Bulk viscousity If you add pressure to a viscoelastic sample then it takes time for the volume to relax. In the old days they used to model this with a variant of the Maxwell model (See Maxwell_material). This model contains a term named the bulk viscousity. I don't think that bulk viscosity is relevant to this article. Temperature dependence of viscosity Because viscosity of a fluid is critically dependent on temperature it is important that its value is quoted for a fixed temperature and it is measured in thermostatically controlled environment (equipment and fluid kept constant within, say ±0.1°C). For petroleum oils between a limited range of temperatures the change of viscosity with temperature is near logarithmic. ASTM (American Society for Testing and Materials) has produced a chart somewhat better than an ordinary logarithmic chart, with a near linear temperature function, on which the slope of the viscosity-temperature lines of petroleum lubricants can be suitably compared. The smaller the slope of the line representing the viscosity change of the oil, the better it is, as its viscosity decreases less due to temperature rise caused by internal friction. Eddy Viscosity Would someone be so kind as to expand this section, and maybe add some references? Teaching the concept of viscosity to undergraduates The textbook I'm using as I've seen with other textbooks introduces the concept of viscosity by drawing analogies to solids and shear stress. I'm teaching this concept for the first time and I wanted to ask if there were clever ways of presenting the concept of viscosity to students in their first course in Fluid Mechanics? Any interesting figures or mpegs or anecdotes or examples that students can relate to would be much appreciated. Thanks. Examples In the examples peanut butter has a higher viscosity than chocolate. I thought this was strange as one can spread peanut butter on a sandwich but not milk chocolate! I checked the reference, the test used chocolate which was heated to 50ºC and allowed to cool to 40ºC. Assuming that the human hand is at ~37ºC this would mean melted chocolate was used in the test (as chocolate melts in your hand). It looks ambigious as the moment. --Any test for viscosity should mention the temperature at which it's done. Most viscosity tests are done on liquids. I'd assume that the chocolate was heated to melt it before the test was done. A More Correct Equation for Viscosities of Mixtures The equation given for the viscosities of mixtures (using either the fluidity equation or its equivalent in dynamic viscosity) is at best a very rough approximation. A more correct form uses logarythms. In most regressions I have done on real world data, the plots of mixture viscosities are most often fit extremely well by ln(ln(ʋmix))=SUM{xi·ln(ln(ʋi))} where ʋi = the dynamic viscosity of component i, and xi = mass fraction of component i. Granted, theoretical intuition suggests that using mole fraction might be more accurate (than using weight fraction), however I have not found that to be an enhancement that justifies the extra complexity, assumptions, and resulting increased error bars when dealing with complicated real-world substances for which there is a range of molecular weights. (I am a chemical engineer with 28 years in R&D in the lubricant additives industry.)

conditions of viscosity use

I think the article goes too quickly from fluids viscosities to complex liquids viscosities :

A condition of continuum is missing.

Viscosity of a fluid comes from continuous deformation of a fluid when flowing.

So, concerning substances or complex liquids like blood, honey, sap, slury etc... holding fibres, cells, macromolecules, and, various complex bodies, we cannot speak of viscosity just as for fluids.

Fluid viscosity respects Stokes hyothesis , like it was developped to obtain the conconservative Navier–Stokes equations.

The article about viscosity continuously describes what is permitted for fluids until liquids and shows viscosity diagrams for complex liquids which are not continuous. One can call it "apparent viscosity" or "resulting viscosity" but these models are taken from continuous fluids and are suddenly applied to complex liquids without checking the required assumptions.

In the Newton's model, there is a volume of flowing fluid between dx, dy and dz. In this volume, the fluid particles have to be identical and this volume has to be representative of the full volume of the cell.

This Newton's law is traditionnally applied to any fluid : Newtonian or non Newtonian, complex, loaded etc... but in reality, vscosity applies only to conservative fluids.

I think there is a mistake concerning ethymology :

From Littré and from A Concise Etymological Dictionary Of The English Language by Walter W. Skeat, 1882.

There is, in the article of viscosity a confusion between etymology and meaning. Mistletoo comes from mist, mixture, mix, pitch, moist... but mistletoe is translated "viscum" in latin. Viscum comes from pitch.

So Pitch gave "bisque", "viscous", "moisture", and "mistle". Mixtures are viscous because they hold thicky components, like mistletoo use for glue.

References