Liquids respond to the force of mixing. These responses are critical considerations when selecting an appropriate stirrer. Below, we cover five types of liquids that each have a unique response to shear force: Newtonian liquids, dilatant liquids, pseudoplastic liquids, thixotropic liquids, and rheopectic liquids.
Newtonian liquids
Newtonian liquids, like water or alcohol, retain the same viscosity regardless of the shear force applied. As the mixer speed increases, the flow proportionally increases, and viscosity remains constant. When selecting a stirrer for use with Newtonian liquid samples, only the initial viscosity needs to be considered. For low-viscosity Newtonian liquids, magnetic stirrers are usually sufficient.
IKA C-MAG MS Series Magnetic Stirrers come in 5-, 10-, and 15-liter capacities.
If using an overhead stirrer for a Newtonian fluid with low viscosity, a radial flow pattern is ideal. Impellers that offer a radial flow pattern and are suitable for low-viscosity liquids include straight, coil, and radial impellers. For an in-depth comparison of impellers and their applications, see our previous post.
A straight impeller, like the Sciologex straight impeller, creates radial flow suitable for low-viscosity Newtonian liquids.
For high-viscosity Newtonian liquids, a magnetic stirrer is typically insufficient, and an overhead stirrer and an appropriate impeller are required. For highly viscous fluids, an anchor-type impeller can be used at low speeds, a radial flow impeller at average speeds, and a propeller-type impeller at high speeds. For detailed considerations for highly viscous fluids, see our previous blog post on the topic.
The IKA EUROSTAR 200 Control P4 can accommodate a capacity of up to 100 L and a maximum viscosity of 150,000 mPa*s.
Dilatant Liquids
Dilatant liquids exhibit increased viscosity with the rate of applied shear stress. Dilatant liquids generally contain a high concentration of solids, such as polymers, metals, and oxides. These liquids behave like a fluid under low shear conditions, but act more like a solid under high shear forces. An example we are all familiar with is oobleck, the mixture of cornstarch and water. Other examples include clay suspensions, polyvinyl alcohol, and electrorheological fluids.
The greater the shear force applied to the dilatant liquid, the greater the resistance encountered. This can inhibit the stirring process and damage the stirring equipment. For dilatant liquids, using a high torque stirrer at low speeds is preferable. The selection of the impeller is also critical to reduce shear. An impeller providing a tangential flow pattern will offer the lowest shear. For more information on impeller selection, see our previous blog post.
The IKA EUROSTAR 200 Control P4 accommodates a maximum viscosity of 150,000 mPa*s and has a torque of 660 N·cm. The Control P4 may be particularly well suited for mixing dilatant liquids owing to its high torque and low speed range (4 to 530 RPMs).
Anchor impellers, like the anchor impeller from Jeio Tech, provide a tangential flow pattern with lower shear force.
Pseudoplastic Liquids
Pseudoplastic liquids behave in the opposite manner to dilatant fluids, where the viscosity decreases as the shear force increases. Common pseudoplastic examples include gels, latex paints, ketchup, lotions, and nail polish.
The initial viscosity must be considered when selecting a stirrer to ensure that torque is sufficient to initiate mixing. Additionally, an impeller that provides high shear, such as a dispersion or saw tooth impeller, that creates a radial flow pattern is optimal.
The turbine impeller from Jeio Tech offers a high shearing force.
Thixotropic Liquids
Thixotropic liquids are similar to pseudoplastic liquids in that viscosity decreases as shear increases. However, where pseudoplastic liquid viscosity change is immediately reversible (i.e., when the force ceases, the fluid returns to the initial viscosity immediately), thixotropic liquid viscosity is both time- AND shear force-dependent. When shear force ceases, a thixotropic liquid will return to its initial viscosity over a certain time period. Some examples of thixotropic liquids include glue, peanut butter, tar, and soap. Considerations for thixotropic liquids are the same as those for pseudoplastic liquids: the torque and viscosity ratings must be sufficient to initiate mixing since viscosity will be highest at low shear.
Rheopectic liquids
Rheopectic liquids behave in the opposite manner to thixotropic liquids. The viscosity of a rheopectic fluid increases with increased shear force. However, like a thixotropic liquid, the change in velocity is time-dependent. When shear force is removed from rheopectic liquids, like gypsum paste, the viscosity will decrease over time. Considerations for rheopectic liquids are the same as those for dilatant liquids: a higher torque stirrer at lower speeds is ideal.
The Velp PW Overhead Stirrer is well-suited for highly-viscous rheopectic liquids, with a relatively high torque (N·cm), accommodating viscosities up to 100,000 mPa*s, and with speeds ranging from 20 to 1,200 RPMs.