Abraham Maslow the American psychologist said that if all you have is a hammer, everything starts to look like a nail. You can apply this to the world of viscosity testing. If all you have is an instrument for measuring viscosity and liquid behaviour, then you start to think about everything as being a liquid. As we’ll see that’s not always the best option.
So I was with a customer today, who are using a Brookfield RV viscometer fitted with a spindle number seven. Now if you know the Brookfield RV spindle number seven, you’ll know that it’s pretty much just a single straight rod. It’s often what we call the spindle of last resort.
This company were trying to measure a highly structured sample, and they had realised that every other spindle in the box leaves a great big hole in their sample. The only thing that they could stick into the sample was a spindle number seven. The same thing goes for the LV viscometer spindle number four.
So what do we do in this situation? Well, first of all what do we mean by a structured liquid? A structured liquid is a material that doesn’t flow easily when you poke a hole in it. Suspensions, emulsions, pastes, waxes – those kinds of things, are all examples of what we call a structured liquid.
They have an internal structure as a result of association between liquid droplets or suspended particles that create this network that possesses a yield stress. That yield stress has a certain amount of strength, and it’s that strength that prevents the material flowing back around the spindle when you press that into the sample. In order to make a structure liquid flow, you need to exceed that yield stress. You need to apply stress is greater than that yield stress.
That’s exactly what we do when we get our ketchup out of the bottle or we squeeze toothpaste from the tube. We’re applying sufficient stress to get the material to flow like a liquid. What’s usually the case is that that liquid-like flow is only momentary and then the material will undergo a pretty quick recovery back to being that soft solid condition again.
So what we need to decide is, do we measure the structure- and we can use some pretty gentle techniques to understand that. Or do we smash the hell out of that structure and measure the viscosity of the resulting liquid? The spindle viscometry approach.
That decision is best answered by understanding what we call the critical quality attributes of the material. In essence why are we even measuring viscosity in the first place? To put it another way, what happens if the viscosity is wrong? How does that manifest? Is it in the performance, the stability, the process-ability, or the filling characteristics of the material? This is a really critical thought process to go through, but it’s outside of the scope of this video. But if you need some help working through that with your products, then let me know and we can talk through them.
So, how do we measure that structure? Well, in my lab we use a whole bunch of techniques based on oscillatory rheology. This involves very gently nudging and wobbling the structure and measuring its strength and its rigidity as a result.
Here’s an extreme example of the typical sort of thing that we would do to a gel. These oscillatory techniques and some gentle controlled stress approaches, enable us to understand the very delicate structures that may set up in a typical material whether we’re trying to measure.
A cheaper and more rough-and-ready approach is to use a T-bar and helipath accessory, that fits onto the viscometer and then measures the torque required for a rotating pin to cut its way down through the structure in the material.
While this technique lacks several orders of magnitude of range and sensitivity and the results can be scarily erratic and noisy, it can be a cheap and easy way of starting off in structure measurement.
If you find yourself poking holes in your sample with an RV spindle number seven, or an LV spindle number four, then that should be a warning sign that in actual fact it may not be viscosity that’s the most relevant thing for you. It may be that you should be measuring something like yield stress, or modulus, or one of these structural characteristics.
I hope this helps! Get in touch with us at rheologylab.com . Subscribe for more videos and look forward to speaking to you again soon!