Waxing lyrical about rheology

Randy Byrne, Malvern Instruments’ VP for Commercial Development, has formulated this excellent (not a trick-) question: “Which has the higher viscosity at room temperature – mayonnaise or honey? (Randy claims no originality for this question!)  We’ll return to this later……  Now, one factor that seems common to many techniques is the indescribable ‘sample preparation’ which I’ll now attempt to describe….

The specific question is “Does sample preparation affect my sample?’ The obvious and true answer in both cases is “Yes”.  So, clearly we have to reformulate the question – just as the sample preparation reformulates the formulation….  A better question to ask relates to the factor or factors that we are trying to control by applying a specification (‘fit for purpose/’just good enough’) or acceptable tolerances on our material. How will the material be applied (literally or figuratively)?  What is the next step for the material?  Where does it go?  How does it get there?

We need to avoid the situation expounded by Galehouse when talking about particle sizing of sediments:

“It cannot be over-emphasised (Br.) that great care must be taken in preparing any sample for size analysis.  The objective of the investigation is to determine the size distribution of the sample.  What is often determined, however, is the effectiveness or destructiveness of the disaggregating or dispersing technique”

J S Galehouse Sedimentation Analysis in R E Carver (Ed.) Procedures in sedimentary petrology Wiley-Interscience Chapter 4 (1971)  ISBN 471 13855X pp 69 – 92

So why are we ‘preparing the sample’?  Surely, one would want to measure the material ‘as is’ or the state that is delivered to the (side of) the instrument.  That seems logical but often is not the case – a material may become part of a formulation such as a paint or food (such as mayonnaise) where it’s subject to a high shear process in the manufacture and it’s these end properties that are the desired ones – the opacity of the paint or the flow of the mayonnaise for example.  It may be part of a formulation where it’s required to dissolve and in this case the available surface area dictates the dissolution rate.  So this comment of Dr Horace Rose’s becomes more pertinent:

“Very little appears to be known about how the differences of behaviour (Br.) arising from the differences between the conditions of test and use, can be predicted, but there is no doubt that such information is necessary before the control of such materials can be adequately carried out”

Dr. Rose in Discussion (page 143) following paper “Particle Shape, Size and Surface Area” in Powders In Industry SCI Monograph No 14 Society of Chemical Industry, London (1961)

We note that my copy of the above text was never withdrawn from the library as seems to be the fate of Dr Rose’s particle size books.  The exact situation exists for my 1954 copy of “The measurement of particle size in very fine powders” by the same author and also for a book that seems, by title at least, to be potentially very useful “Flow of Liquid Chemicals in Pipes” by Norman Swindin (Benn Brothers, Limited, 1922).  This title pops up in my webinar on “Applications of rheology in the waxes, creams, and emulsions arena” but we’re running ahead of ourselves into post-data manipulation to fit a model and give another Professor another Ph.D student. Swindin comes up with a couple of stunning and entertaining quotes “Logarithms may cover a multitude of sins” and then castigates Osborne Reynolds (of Reynolds Number fame) for using logarithms “This is to be regretted for it has set the fashion in exponential formulae, of which Mr Parry, the Engineer-in-Chief to the English Electric Company, tells me is announced once a month or oftener”.  Notwithstanding whether ‘oftener’ is a real word or not (can it be used in Scrabble?) I’d like to meet the aforesaid Mr Parry for dinner…..

So, back to the theme of sample preparation and taking the mayonnaise example….  Mayonnaise is an oil-in-water emulsion (eggs, oil, vinegar, and spices in the simplest case).   Thickening agents are used to adjust the viscosity and bulking effect of the oil, to enhance mouth feel, to ensure that a stable emulsion is formed, and to obtain the right ‘pour’ properties if this is a pourable product or standing properties if this is a spoonable product.  Now back to the question that was posed at the beginning of this blog.  We have to think about the context and usage of the materials, mayonnaise and honey – in particular read Dr. Horace Rose’s comment again….  His son is a famous eye surgeon in London by the way…  OK, if we consider pouring the sample from the container then the mayonnaise will just sit there and the honey will slowly pour.  On that basis we’ll say that the mayonnaise is of higher viscosity.  But now consider drawing a knife or a spoon through the two materials – you’ll realize that it’s much easier to do with the mayonnaise than with the honey.  So, at higher shear rates, the mayonnaise is of lower viscosity…. This can be summed up in a simple plot:

So the real answer to the question (like many answers in science) is “It depends”.  So both answers in the mayonnaise/honey question could be correct – it depends on what we really want to do or know. It depends on our next or final application of the material; it depends on the conditions we apply (sample preparation); it depends on how we interpret our data over a range of shear rates (a single measurement at a single shear rate simply doesn’t provide this information)   Not just “Give me a number, give me an answer….”  Out of application and sample preparation context, this requirement for a number is meaningless.  Think not thoap….

Waxing lyrical?  Watch the recording of our webinar on “Applications of rheology in the waxes, creams, and emulsions arena” – we may just see the mayonnaise example again…