The article is written by Riya Veluri, an editorial team member of Industrial Lubricants. After her graduation, Riya works as a website developer & SEO specialist in Lubrication & Tribology Industry & writes technical articles on Lubricants, Lubrication, Reliability & sustainability.
Rheology – The Science Behind Material Flow
Rheology is the science of flow and deformation of a particular matter that describes the interrelation between force, deformation and time. It is the study of how materials respond to applied stress or strain.
Rheology performs a vital role in formulating everything from cosmetics to food to inks and coatings. Rheology impacts all stages of material use, including pumping, storage, production, stability, transportation, spreading, and even product performance.
The term ‘rheology’ was derived from the Greek words “RHEO” (flow) and “LOGOS” (science) and is used to describe liquids’ flow and solids’ deformation. Viscosity is an expression of a fluid’s resistance to flow: the higher the viscosity, the greater the resistance.
Essential elements that affect materials flow and deform
We have to consider four essential elements when a material flows or is deformed.
- The first is the materials inner structure. What is its molecular makeup? How is the material built?
- The second essential element revolves around morphology.
- The third element concerns the outside forces that stress the material and causes it to deform or flow. For example, materials can be pulled apart, compressed or sheared. In each case, their flow will behave differently.
- The fourth element is the ambient conditions. What temperature is the stress for the material? These are the essential elements of rheology.
Let’s take a brief look at the most important differences between materials and their rheological behaviour. First, we can divide all materials into liquids and solids and say that liquids flow and solids don’t, but that’s not a good scientific approach.
In the real world, materials are more complex, from oil, glue and shampoo to facial creams, jelly and car tires. These materials should not be defined simply by two words. Then how can we define these materials better?
Well, we can define them with Rheology.
Rheologically speaking, most materials are viscoelastic. Nearly every material is made with a dense portion and an elastic portion. If the material is denser (Viscous), then it’s a liquid. If a material is more elastic, it’s solid. The materials with the highest viscous portion are called viscous liquids or Newtonian liquids. Newtonian liquids will always show the same viscosity at constant ambient conditions, no matter how stressed.
Viscoelastic liquids are liquids with an elastic portion. Whenever these materials are stressed, they flow, but they also exhibit a certain level of stiffness. When viscoelastic solids are deformed by an outside force that is not too large, the inner structure of these materials sticks together and tries to retain the materials original form.
Thus, elastic solids always show the same level of stiffness as long as their structure is not destroyed. Generally, when we take a flow, particularly with liquid materials, we typically think of viscosity. But viscosity is only one aspect in rheology.
Viscosity is a measure of a fluid’s resistance to gradual deformation or flow by shear stresses. It describes the internal friction of moving fluid and, therefore, its thickness. Therefore, fluid with a large viscosity like honey resists motion because its molecular makeup and morphology result in a lot of internal friction between neighbouring particles moving at different velocities. But fluid with low viscosity like water flows easily because its molecular makeup and morphology result in very little internal friction when it’s in motion. So a liquid viscosity depends on its chemical structure, its morphology and the attractive forces between them.
The rheometer is a device that is used in accessing the rheological properties of lubricants. Here is an example of a sample loading in a standard rheometer:
Credit for video: Dr Febin Cyriac
Now we examine deformation forces to understand how they affect the materials flow behaviour and give a more in-depth understanding of the term viscosity.
Different forces can act on any state of matter
- Tension – these are aligned forces pulling apart the material.
- Compression – these are aligned forces pressing the substance.
- Bendy- unlined forces are applied to opposite sides of the material.
- Torsion – forces twisting a substance.
- Shear – unaligned forces push one part of the material in one direction and another in the opposite direction.
Let’s ensure we fully understand the meaning of some defamation terms by reviewing shear rate and shear stress.
Materials that flow are comprised of velocity gradients which are layers that flow with different velocities. For example, imagine water flowing through a pipe; the liquid layers do not flow with even velocity throughout the bulk. This is because there are frictional forces between the pipe wall and the interface in the liquid layer and frictional forces between all the liquid layers themselves. The difference in velocity between the liquid layer closest to the pipe wall and the liquid layer in the middle divided by their relative distance is known as shear rate.
Shear stress is a force applied over a given unit area. There is a direct relationship between shear stress and shear rate. With higher shear stress, we can expect a higher shear rate given the same system, and the more viscous liquid is, the more force is required to flow at the same rate as the less viscous liquid. So water requires less force to flow through a pipe than, for example, honey.
Now that we understand shear stress and shear rate.
Let’s revisit viscosity once again so that we can define it more precisely.
The viscosity of the material is the shear stress divided by the shear rate.
Viscosity is measured as dynes seconds per square centimetre or more commonly known as poise or centipoise.
Let’s use a golf stroke as another analogy to deformation forces. When we put a golf ball, we exert a slight force or stress onto the ball, which travels slowly. On the other hand, the dragon golf ball exerts a tremendous force, which propels the ball at a much faster rate. Likewise, if we run a sand trap in a denser medium, we would have to exert a relatively higher force to achieve a reasonable speed and distance to reach the flagpole.
So let’s look at this intuitively. First, if we have to apply more shear stress to achieve the same shear rate, the numerator is larger, so the viscosity must be higher.
Conversely, if we have to apply less shear stress to achieve the same shear rate, the numerator is smaller, and therefore the viscosity must be lower.
Alternatively, if we achieve a higher shear rate from given shear stress, the denominator is larger, so the viscosity must be lower. Conversely, if we achieve a lower shear rate from given shear stress, the denominator is smaller, and therefore the viscosity must be higher.
Rheologically Different Flow Profiles
First, let’s examine various types of fluids by grouping them into two basic flow profiles.
Newtonian and Non-newtonian.
Newtonian fluids are liquids whose viscosity does not change with a change in shear rate. Only a small group of fluids exhibit such constant viscosity, and a good example would be water. When we plot viscosity against the shear rate, the rheology profile of water exhibits no change in viscosity with higher shear rates and no change when the shear rate is reduced or removed.
However, with most liquids, the viscosity does change with shear rate and the flow of those liquids is called non-Newtonian fluids.
Non-Newtonian fluids are liquids whose viscosity does change with a change in shear rate. There are three types.
A good example of pseudoplastic flow is mayonnaise. Its viscosity will be reduced by applying shear stress such as vigorous mixing. Mayonnaise is a shear-thinning material whose viscosity continuously drops with increasing shear rate and recovers along the same path when the shear rate is reduced. The question is why pseudoplastic materials would become thinner with an increase in the shear rate? Pseudo plastic systems contain polymers that at rest are coiled up due to stabilising molecular forces. When you apply shear through mixing or shaking, the polymer chains begin to untangle. With increased agitation, the polymers will align themselves in the direction of the flow, and the internal resistance will decrease.
Here a viscosity rise with increasing shear rate, which also recovers along the same path when the shear rate is eliminated. This rheological phenomenon is known as dilatancy. At the same time, this profile may be somewhat counterintuitive. A good example would be quicksand. You struggle to get out of the quicksand, exerting high shear, the quicksand will instantly thicken up and restrain you, and if you stand still with just gravitational shear, you will slowly sink. The flow profile is called dye latent because the system increases in volume with an increase in shear rate.
Thixotropic fluids are similar to other shear-dependent materials like pseudo plastics but with one added variable. They are time-dependent as well. Therefore the viscosity is affected by both shear rate and the time it takes to recover. Ketchup is a good example of this flow behaviour. You shake the bottle vigorously to pour the ketchup, but you only have a short time to deliver this condiment onto your food before it reaches its rest high viscosity. Many formulators of coatings and inks take advantage of this shear and time-dependent behaviour as it contributes to multiple benefits.
In Thixotropic fluids, an increase in the shear rate will also decrease the viscosity; however, reducing the shear rate will result in a slower path or a long time to reform the structure and achieve full viscosity recovery. The difference between the downward and upward curves in thixotropic systems is known as the degree of hysteresis. Since products with higher molecular weight particles take longer to recover, their degree of hysteresis will be larger than systems with lower molecular weight particles.