ColloidTek blog: Why measuring just viscosity is not enough?

Why measuring just viscosity is not enough?

We regularly attend in meetings where we are asked if Collo is a viscosity meter. Since the time is often limited I usually give a short answer that goes like this:

Collo measures something beyond viscosity, in fact many of the primary causes of it.

That was the short answer. Long answer is a little more complex, be warned, I am going to try science in the following.

Those primary causes affect viscosity – often without knowing. Much like syrup, honey or cane sugar can make the cake sweet but only the master chef can distinguish between them. Similarly to master chef Collo can help to understand what causes the changes in viscosity.

 


What is viscosity (in Liquids)


Viscosity is a quantified measure that indicates how much liquid resists deformation. Viscosity is calculated by dividing shear stress1 by shear rate2.

For instance, water has a low viscosity, you can mix it easily with a spoon whereas syrup has a higher viscosity and needs more force to gain the same shear rate.



To exactly know how much force is needed per mixing speed, viscosity is measured with a viscometer according to a measuring standard. There are many different methods and devices to measure viscosity, but they should yield a quantified result in Pascal-seconds [Pa*s] or poises [P] to be called a viscometer.

To get the benefits of online analysis3 apparent viscosity can be measured with tools which connect directly to production process, but they usually give a more qualitative result that still correlates to the one that is measured with a lab viscometer.

In the following I lead you in on the primary causes of viscosity that Collo® measures. This needs a short intro on what really causes the viscosity.


What causes viscosity


Liquids are formed of molecules that interact with each other. In plain solutions, viscosity is caused by molecular friction: the closer these molecules get together and the more attraction they feel to each other the higher the viscosity. Water molecules do not attract each other very strongly, thus low viscosity. With more complex liquids, also the viscosity behavior is more complex. For example, the syrup that has a high viscosity contains a lot of dissolved sugars which, being complex molecules, act on each other and increase the viscosity.

Things get even more complicated when the liquid contains more than just dissolved items as then the particle-particle forces start to contribute to viscosity as is explained in the figure below.



In more complex liquids the electrical forces on the molecules of the surrounding solution affect the viscosity together with the electrical forces of the particles.

Particles in solution have inherent electrical forces and forces that are caused by ions that accumulate on the particles4. Electrical forces may attract or repulse the particles, this effect on the viscosity.

So, both the molecules and particles can resist motion as they may repulse each other with the electrical forces inherent or gathered on them. This is close to what Collo® measures.

Before the closing words, I will still shortly visit on some things that change viscosity, since they are inherently linked with the changes in these primary causes and thus also measurable with Collo®.


Variables that change viscosity


Temperature and pressure


Now that we know what causes viscosity it is a little easier to explain what happens when liquid heats and its viscosity decreases. This decrease is caused by the thermal expansion of liquid that moves the molecules further away from each other and decreases the molecular and particle-particle interaction forces. This is evident when measuring with Collo® and due to our clever engineers, we can also make the effect of temperature disappear from the measurement. Pressure then again has little effect on the viscosity since liquids do not compress5.

Agitation dependency


You know how ketchup sometimes does not come out of the bottle? That is because it is too viscous for the small nozzle. Still, let’s not curse on the engineer who designed it because he had to optimize. He knows that ketchup is shear thinning (this can be seen with Collo® online) due to a small amount of added xanthene-gum. This shear thinning causes the ketchup to flow through the nozzle when you shake and squeeze the bottle. Viscosity of ketchup quickly becomes lower and had the engineer drilled a nozzle with a larger diameter you would then really have a reason to curse him. Had the chemist not added the xanthane-gum would the ketchup not stay on your french fries.

Thixotropy


Thinning can also have a time dimension in some very special liquids such as clay slurries, this phenomenon is called thixotropy. Viscosity in thixotropic liquid decrease for a certain time that can be minutes or even hours. This is caused by breakage of bonds between such particles that take time to rebind with each other. With Collo® we can have a derived feature to measure this online.

As for the science behind why these kinds of phenomena take place there is a lot of professional research but as far I know, no one can completely explain them yet – hope we can help in this with Collo.


What Collo measures


Now that I have explained some of the basics on liquids and their viscosity, you probably realize what I mean by saying that Collo measures something beyond viscosity. My longer and more scientific answer is:

Collo sensor is a resonator that creates an electric field on the liquid and vibrates the particles and/or the molecules on them. This in turn enables us to get information online and in many cases, we can see changes that will affect the viscosity and other liquid variables before they are noticed by viscometers.

Getting down on the primary causes of viscosity can help you to understand and tailor your liquids and to react more quickly to sudden changes. So, use Collo® and don’t make too shear thinning ketchup – your customers hate that.


Dr. Matti Järveläinen


1) How much strength you use to mix with a defined sized tool
2) Mixing rate
3) See what they are from Chapter 1 in my doctoral thesis
4) The Finns can read more about this from our article in Materia.
5) As a scientist I must add a footer note saying that this is false: liquids do compress, but just a little.


Afterword: What does this matter?

Viscosity and their primary causes can be quite important in many liquids such as paint, ink, ketchup, drugs, mineral slurries, pottery, oils, and much more. But don’t take my word on it: I’ll ask about the importance of these factors from someone who works with them daily and report back in this blog later.

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