Stirring low-viscosity materials is typically very straightforward. For smaller amounts, a magnetic stirrer can provide a low-cost and effective option. Overhead stirrers can easily get the job done for virtually any batch size, with the right impeller attached.
Stirring viscous materials, on the other hand, can be the cause of headaches. Depending on the nature of the material and other factors, if you don't use the right equipment and process, you could end up with problems such as unwanted aeration or a partially mixed batch.
In this post, we examine some of the factors you need to take into account when stirring viscous materials.
Type of Stirrer
When dealing with viscous materials, a magnetic stirrer is unlikely to get the job done. The resistance to movement of the stir bar caused by viscous materials prevents the bar from spinning rapidly. This means it won’t be able to provide the turbulence required for thorough mixing. It can also cause the stir bar to become decoupled from the internal magnet, preventing stirring altogether.
From left to right: MS-P Series Magnetic Stirrers, MS-H-S Circular-top Analog Hotplate Stirrer, and a Eurostar 20 Overhead Stirrer
A hotplate stirrer might be of use in situations where the application of heat lowers the viscosity of the material to such an extent that desired mixing can be achieved. Of course, applying heat would have to be acceptable and not damage the material or cause unwanted reactions.
For most other applications involving viscous materials, an overhead mixer fitted with an appropriate impeller will be required. When choosing a stirrer, you’ll want to be sure that it’s capable of providing the power needed to stir the viscous material. You’ll also want to know whether it will be compatible with your chosen impeller (more on those next).
Type of Impeller
Impellers come in various shapes, sizes, and materials. The one you choose will depend on several factors, including:
- The viscosity of the material you are working with.
- The size of the vessel you’re using.
- The desired outcome of the application, for example, high shear or aeration.
- The flow pattern you want to create to achieve your desired outcome, for example, axial flow or radial flow.
- The nature of the material you’re using in terms of whether it will react with the impeller (there are PTFE coated options available).
For example, a radial flow impeller (on the left in image below) is suited to average speed applications involving low or average viscosity materials.
From left to right: A radial flow impeller, an anchor-type impeller, and a propeller-type impeller.
On the other hand, an anchor-type propeller can be suitable for low speed applications involving materials with medium viscosity. The propeller-type impeller can be used at high speeds for medium to high viscosity materials. It creates an axial flow and is ideal for suspensions.
Mixing Non-Newtonian Fluids
Some very viscous materials (often those with viscosities above 1,000 cP) exhibit non-Newtonian behavior. They do not follow Newton’s Law of Viscosity and can resist fluid motion. This can make it very difficult to mix the entire contents of a vessel as the motion created by the mixer can quickly diminish.
For example, say you use a relatively small impeller to mix a fluid with the same viscosity as water. The fluid motion created by the motion of the impeller will eventually result in the contents of the whole vessel being mixed. The same setup for a non-Newtonian material could simply result in a hole being made in the material in the center of the vessel (or wherever the impeller is placed).
In this case, an anchor paddle or square blade impeller will often be the best option. Even then, for very viscous materials, you may find that the whole batch moves around uniformly without actually mixing.
Stirring Speed
Another key factor to consider when mixing viscous materials is the stirring speed. When dealing with materials that don’t resist fluid motion but are more viscous than something like water, using too high a speed can cause unwanted aeration. Depending on the nature of the material, this aeration can be difficult to remove after the fact. Therefore, it’s often better to start off at a slow speed and gradually work your way up the optimal mixing velocity.
Although stirring viscous materials can represent a challenge, when you break the problem down into these individual components, it’s easier to make decisions and arrive at the right setup. Consider which type of stirrer, and if applicable, impeller is ideal for your application and start at a reasonable speed. This way, you avoid common issues such as aeration or a partially mixed batch.
