In these posts on surfactants I’ve been writing recently, I’ve mentioned viscosity quite a lot, so I thought we should take a look at this concept.
The easy way to think about viscosity is about the thickness of a fluid. Water has low viscosity while something like glycerin has a higher viscosity. I liked this definition that “Viscosity is resistance to flow” (reference), that more viscous things require more force to make them flow. So something like glycerin takes more force to make it flow when compared to water.
I really liked this one minute video on viscosity, which is awesome.
If you want a more science-y definition, I liked this one: “Viscosity is the measure of the internal friction of a fluid. This friction becomes apparent when a layer of fluid is made to move in relation to another layer. The greater the friction, the greater the amount of force required to cause this movement, which is called shear. Shearing occurs whenever the fluid is physically moved or distributed, as in pouring, spreading, spraying, mixing, etc. Highly viscous fluids, therefore, require more force to move than less viscous materials.”(Reference) They also say this internal friction is “caused by molecular attraction”.
There are a few definitions I’d like to look at as they’ll be important when we start talking about other thickeners for surfactants.
Yield stress: This is the “applied stress we must exceed in order to make a structured fluid flow.”
Yield stress point: “Minimum stress value applied to inital flow.” When the fluid is below this point, it displays the properties of being a solid or in that it isn’t flowing the way water might flow. When it’s above it, it’ll flow like a liquid. (Reference)
“Some materials can better be described as soft solids than as fluids. Strong gels and thick pastes possess an inherent structure to such a degree that they will not flow appreciably unless left for a very long time.” (Reference) In other words, there are things that kinda seem like solids when they’re below this yield stress point, but flow when the yield stress point or value required to have the thing flow.
The word “elastic” is important to this discussion, too. This means a solid has the ability to return to its previous shape or size when the force is removed.
Shear: It’s defined as the “relative motion between adjacent layers of moving liquid”. As above, “Shearing occurs whenever the fluid is physically moved or distributed, as in pouring, spreading, spraying, mixing, etc.” A high shear mixer – like an immersion or stick blender – will brings those layers together quicker than something that’s low shear, like a hand mixer. High shear mixers spin quicker in the middle than the outsides.
If we were to talk about something like ketchup, it’s very thick when it’s sitting in the bottle. To get some out for our hash browns or chips, we have to apply force or stress to it by shaking or hitting the bottle. When it reaches its yield stress point, it will start to flow and comes out of the bottle. When we stop applying the stress to the bottle – and, in my case, stop swearing at the damned thing! – it stops flowing.
How is it measured? Viscosity is measured by millipascal seconds or mPa s, and water at 20˚C is 1.0020 mPa.s, which gives us a base line so we can measure the viscosity of other things we use.
If you look at this chart showing the viscosity of Sepimax ZEN, you can see using 3% with 0% salt creates something that’s 100,000 mPa.s, so you know it’s much thicker than water.
Glycerin is 1430 mPa.s, propylene glycol is 40.4 mPa.s, while olive oil is 84 mPa.s, so we know that using 3% Sepimax ZEN creates something much much thicker than olive oil.
You may see some things measured in centistokes or cS. (1 cS = 1 mPa.s) Silicones are measured this way, especially dimethicone. I’ve worked with 5 cS, 350 cS, and 1000 cS, which you might be able to picture as being as almost as thin as water, thicker than olive oil, and less thick than glycerin respectively.
As an aside, temperature can have a huge impact on viscosity, something you’ve no doubt witnessed as your lotion thickened or as a lip balm or lotion bar stiffens as it cools. For a lot of science-y stuff from solubility to viscosity, it’s important to know the temperature in Celsius. If it isn’t listed, you could assume it’s normal temperature and pressure, being 20˚C and 1 atm, or atmosphere, which is sea level.
Why should you care about this concept? Because we adjust the viscosity in just about every product we make. (And because science is #metalaf.) Think how many times you’ve wished your lotion, facial cleanser, bubble bath, and so on was thicker or thinner, and you can see how important it is.
In lotions, we can increase the viscosity by adding thickeners, including fatty alcohols, fatty acids, waxes, butters, and humectants. It’s pretty simple to take a hand & body type lotion and turn it into a thicker, more viscous body butter by using more butter, adding a fatty alcohol like cetyl alcohol or cetearyl alcohol, adding a fatty acid, and more. You can also increase viscosity by adding a carbomer or polymer or gum, too. (Yeah, there are so many ways to thicken a lotion!)
In watery products, like toners, we can increase the viscosity with gums, like xanthan gum or guar gum, with carbomesr like Ultrez 20, or with polymers like Sepimax ZEN or Sepinov EMT 10, to name a few.
When formulating with surfactants, viscosity is such a huge issue! We have to match up the right surfactant with the right thickener – something you know very well if you’ve ever tried to thicken foaming silk protein or disodium laureth sulfosuccinate with Crothix and wondered why you still had what seemed like water – to increase viscosity.
The beads stay suspended as the cleanser is more a solid than a liquid when there’s no stress placed on it, like when it’s sitting on a counter. If we start putting pressure on the system by squishing the bottle, we’ll soon overcome the yield stress point, and it will flow like a liquid. When we return the bottle to the counter, it’ll soon behave as more like a solid until annoyed again, and the beads will remain suspended.
As an aside, that last paragraph is how I ended up down this rabbit hole! Whee! I love rabbit holes!
We’ll be talking a lot about viscosity as we take a look at more thickeners we can use in our products, starting with Rheomer 33T, my new favourite thickener!