Hi I’m Neil Cunningham, and this is Rheology School.
Today, we’re talking structured liquids. I’d like to show you a really great way of rheologically describing materials. It’s great not only because it comprehensively sums up the key attributes and rheological behaviours of a lot of products, but it’s also great because it’s dead easy to understand.
It works on the principle that suspensions, emulsions, gels and polymer solutions all exhibit liquid-like characteristics under some circumstances, but solid-like characteristics under others.
Jelly is a solid. Solids have “bounce-back-ability”. They deform elastically, storing energy whenever a stress is applied. When the stress is removed they bounce back-they return to their original shape.
Syrup on the other hand is a liquid. It exhibits viscous or inelastic behaviour. Liquids flow. Just dissipating energy as they do so.
Mayonnaise lives a double life of a “structured liquid”. Under low stresses its structure remains intact and that’s why we can see the wobbling, that we see in the jelly through the elastic deformation, but when we apply sufficient stress to it, we disrupt that structure and we elicit liquid like flow behaviour
albeit briefly, and it pretty quickly returns back to being the soft solid that we started with.
It’s the measurement of this structure that makes rheology such a powerful technique when compared to simple viscosity measurements.
One mustard… one mayonnaise… one decent rheometer. Let’s do some rheology!
So we’ve tested our samples, let’s bring up the results and see what we’ve got.
Okay, so let’s call up our results. Let’s look at the mustard first of all. I’m using a very useful rheological parameter called the phase angle, “delta”, here on the y-axis and plotting that
against the stress that was applied to the sample throughout the test on the x-axis. The phase angle ranges from zero degrees to ninety degrees and it essentially tells us whether the material deforms elastically like a solid, which would result in a low phase angle or flows viscously like a liquid, which results in a high phase angle.
So, you can think about phase angle as being a structure detector, and what we can see with the mustard here is that we have a plateau here at no stresses, at a low phase angle and we’re getting values of around about 14° along here. So, this tells us that the mustard has an intact elastic structure at those low stresses, but as the stress rises, we can see the ongoing yield, the structure breaks up and the phase angle heads up towards 90°.
Let’s now compare this to the mayonnaise.
So now what we can see with the mayonnaise is, firstly, we can see the mayonnaise has a lower phase angle in the plateau region here. So that tells us that the mayonnaise structure is a little bit better developed than that of the mustard. More strikingly, we can see that the mayonnaise yields at a significantly higher stress than the mustard. Remember this is on a logarithmic scaling.
Now we’ve talked about yield stress as if it’s a single number, but obviously the yield occurs over a range of stresses. So it’s a process rather than a single event. However, undeniably a single value for yield stress would be very useful for us, from the point of view of comparing products.
So one method that we can use for getting a simple, single number yield stress value is to interpolate the stress at which the line crosses the 45° phase angle threshold. That is effectively the point at which the sample makes the transition from being what we call ‘elastic dominant’ i.e. more solid than liquid, to being ‘viscous dominant’, more liquid than solid.
It is really easy to do that in this software. You click on the curve and from this box of available analysis here, we select what we call G crossover point we hit the traffic light and the software will report our interpolated yield stress value and it’s coming in here at 69.49 Pascals for the mustard. Let’s do the same thing for the mayonnaise and the mayonnaise is coming in at 217.1 Pascals, significantly higher than that of the mustard.
What we’ve done here, is we’ve performed a test that has applied some very gentle stresses to our samples, identified that there’s some structure present, that’s the low phase angle value we’re getting here for both of these samples, and then identify the stresses that are required to disrupt that structure, signified by the raise of the phase angle, the ongoing yield.
The mayonnaise has a yield stress of around about 217 Pascals the mustard has a yield stress of only 70 Pascals. That means that the mayonnaise is 3 times stronger than the mustard. In other words, you have to give the mayonnaise 3 times as hard a push to get it moving than the mustard and that’s going to show itself in how these materials look and feel to the consumer. Yield stress is one of the most underappreciated material properties that we can measure.
More than viscosity, it contributes to texture handling appearance and a whole host of processing and performance behaviours.
Oscillation stress sweeps, such as we’ve run here, are just one of many techniques that we have available to us for capturing and quantifying this most useful of numbers.
Thanks for watching and see you again soon!