If I thought I was scaring you with Chemistry Thursday, I must be terrifying you with Physics Friday! Please keep reading...it's really interesting and relevant stuff!
Recently someone asked me if using a giant mixing device on a drill was a good idea for larger batches of products! (See this post...) He brought up the idea of shear, and I thought it would be a good idea to take a look at this concept...
You've probably seen the terms before - high shear or low shear - when looking at data sheets or reading more chemistry minded suppliers' descriptions of products. What the heck does this mean?
Wikipedia) What the heck is shear stress? It means that the surfaces are parallel to each other. (In normal stress, the surfaces are perpendicular to each other. In physics, normal means perpendicular or at a 90˚ angle.)
I like this definition: "Shear is defined as relative motion between adjacent layers of a moving liquid" or "Shear rate is defined as the measure of the extent or rate of relative motion between adjacent layers of a moving liquid." (Pump School)
And a high shear mixer is one that "disperses, or transports, one phase or ingredient (liquid, solid, gas) into a main continuous phase (liquid), with which it would normally be immiscible." (Wikipedia). As you might remember from yesterday's Chemistry Thursday, the water part of our emulsions are considered the external or continuous phase of the product, so a high shear mixer would mix the oil and water to create an emulsion. (When it brings liquid and liquid together, it's an emulsion. Solid into liquid is a suspension.)
Yes, I know I use a hand mixer. There are a few reasons for that, including the fact that Raymond guards his stick blender like it were a first edition Superman comic, and I keep forgetting to buy one for myself. These are not high shear mixers, but we can use them like one for most applications.
For something like gums or polymers, we want low shear mixers, which are designed to move the ingredients around with a low amount of energy. Gums and polymers are called non-Newtonian fluids, meaning they change in viscosity according to shear. Ketchup is a great example of this: It's thick and sticky and won't move until you hit the bottle, then it turns liquid and flows!
What does this all mean to us? Ideally, we will make our oil-in-water emulsions using high shear appliances and we'll make anything to do with non-Newtonian fluids (gums, carbomers, polymers, mayonnaise) with low shear mixers. You can still make your products using a hand mixer, but you will have to put that mixer on higher and longer than a stick blender would take.
Further suggested reading:
Handling shear sensitive liquids (Pumpschool.com)
High shear mixer (Wikipedia)
Shear stress (Wikipedia)
Low and high shear rheology (Sabic Innovative Plastics)
Want to know a lot more? Check out this awesome thesis Droplet Coalescence in the shear flow of model emulsions. Fan-freakin'-tastic!
And check out this video of non-Newtonian fluids, to wit water and corn starch dancing on a speaker!