Using Rheology and Tribology to Predict Sensory Properties

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Video Transcript for Using Rheology and Tribology to Predict Sensory Properties:

So I’d like to show you how we use rheology and tribology data to help formulators predict the sensory properties of cosmetics, skin care products, and pharmaceutical topicals. Now predicting sensory properties using these in vitro methods is unarguably a tough gig and we’re certainly not going to put any sensory panellists out of a job in the near future.

However, the benefits of being able to rapidly screen multiple formulations and to compare those formulations to benchmarks of known sensoriality, is undeniably a very desirable capability. We have to make use then, of the tools that we’ve currently got available to us, to get us started down that road. You can think about the sensory experience as being a bit like listening to a piece of music. You have multiple components contributing to the experience and it’s a constantly changing mix, with a second-by-second shifting landscape.

If you think about the sequence of events for using the skincare products, well you’ve got the initial appearance of the product as the user opens up the tub or the jar and then you’ve got the first touch, the pick up and carry over behaviour, the initial application onto the skin and the beginning of spreading, then the later stages of spreading and finally we get into the rub out and the after feel. There are a whole range of physical processes going on throughout that chain of events. A compression, shear, abrasion, adhesion, temperature changes and composition changes.

Let’s look at a typical application and how we combine three metrics; yield stress, viscosity and coefficient of friction, to generate this three-dimensional scatter-plot that we can use it to give us an initial what we call an establishing shot, of a range of products in the marketplace. Yield stress is a measure of the strength of the soft solid structure that we see in a cream, an ointment or even in a lotion.

Yield stress is highly relevant in the early stages of the experience, the appearance, the first touch and the initial application onto the skin. Once we start and rubbing the product onto the skin then we break up that structure and at that point viscosity starts to become highly relevant, and specifically we’re interested in the high shear viscosity, because rubbing on to the skin is a pretty high shear process.

In the later stages of the application, that layer of product gets thinner, and thinner. We then get into a direct skin-on-skin interaction and at that point, where we’ve got these two surfaces interacting with each other, we’re into a tribological situation more than a rheological situation. The tribology, the friction and the lubricity of the product becomes more relevant.

When we then combine these three metrics together to generate this three-dimensional scatter-plot, it gives us a nice visual indication of where these individual product types are located within that space. What we could then do is if we’re interested in, let’s say for example, day creams.

We could zoom in on the day cream area, get a bunch of different day creams and do the same thing, characterise them, add them to this plot and then we can look in the much more granularity, and in more detail as to where the day creams sit in this certain place.

This is a nice simple tool that enables formulators to compare their products to other products of known sensoriality in the marketplace and also to identify the impact of changes in formulation, process, ageing and a whole bunch of other factors. Looking at these three metrics; yield stress, viscosity, and coefficient of friction, is a great start.

You have to start somewhere, but it’s important not to underestimate how complex it is to predict sensory properties and typically what we would do from here is we start to gather some further metrics and we’ll combine them using some other statistical tools and multi-dimensional analysis techniques, to visualise and build up models of this sensory landscape.

Hopefully this has been useful for you, if you have any questions, don’t hesitate to get in touch through the website and subscribe for more videos, I look forward to talking to you next time!

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Neil Cunningham, Founder and CEO