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Good rheology

6 June 2011 No Comment

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A quick scan of the modern technology that I use every day – my smart phone being an obvious example – reminds me that actually I use only a few of its dazzling capabilities. I get the impression from customers that some feel the same about their high spec rotational rheometers. How to get relevant rheological data is something we’re often asked, and this blog provides me with an opportunity to explore some of these issues further, starting today and over the coming months.

If there are particular areas you’d like me to cover, then please leave a comment or drop me a line at stephen.carrington@malvern.com

So, why are we running this test?

One of the very first questions to ask before starting any test is, ‘Why are we running this test?’, since understanding why you are doing a measurement helps you to determine how to do it.

Take hand cream for example.

Hand cream in a pot is under low shear conditions – acted on only by gravity. Pumping from the dispenser subjects it to high shear, but once resting on the hand, low shear conditions apply again. Vigorous hand rubbing during application is also a high shear activity.

It’s quite easy to define some desirable characteristics for a hand cream. Thick and creamy with great ‘scoopability’ will give a luxurious feel whereas something that drips off the hands is clearly to be avoided. Too thick, however, and it won’t cover well.

Choosing the right test conditions

Measuring a hand cream’s viscosity at a single, high value of shear would be useful in assessing how easily it might pump from a dispenser or be worked over the hands. But what about its behaviour in the pot? Will it remain stable while it sits on the bathroom shelf?

The majority of fluids, gels and pastes in industrial use, especially those in personal care products are Non-Newtonian, where viscosity varies according to the applied shear rate. Many materials shear thin, flowing more easily at higher applied shear. Others shear thicken. Both phenomena are exploited commercially.

This means that measuring viscosity at just one shear rate won’t necessarily reflect the behaviour of interest. A flow curve – a plot of viscosity against shear rate – is far more informative and enables the development of a bespoke viscosity profile for a specific product. A smart use of rheology.

Engineering the products

Returning to the hand cream, it becomes clear that shear thinning behaviour offers advantages and that engineering the product this way delivers what the consumer desires – thick and creamy in the pot (low shear) and easy to apply (high shear). Constructing a flow curve provides some necessary rheological insight to support the necessary formulation work.

But this isn’t always the case. Other applications may call for different rheological strategies. The key is to tailor the testing strategy to address the questions that need to be answered.

Help when and how you want it

At Malvern we recognise that our rheometer users range from experts to complete beginners – and we aim to support you every step of the way. Follow the rheology posts here over the coming months, visit the Education Centre, comment via the blog, or just pick up the phone. We’re here to help.