In this blog post, I will try to give you some insights into a topic I find a fascinating one related to microfibrillated cellulose (MFC): Rheology of MFC suspensions. Rheology is the study of deformation and flow of material under stress, for example how easily material changes its form when it is pressed, or how easy it is to pump liquid in the pipes.
The rheological properties of MFC are so versatile that you always discover something new when you study it. This time, I will discuss two important rheological properties of MFC: Viscosity, together with shear thinning, and yield stress.
HIGH VISCOSITY AND EXTREMELY SHEAR THINNING BEHAVIOR
Viscosity is the most commonly used parameter related to rheology. It describes a material’s ability to resist flow. In liquids, like water, it comes from the friction between the molecules which in turn is dependent, for instance, on the size of the molecules and interactions between them.
MFC suspension is slightly more complicated since it consists of two different components: MFC fibres and the surrounding water (in an end-application there can be even more components).
As we all know, water has very low viscosity, but a small amount of MFC fibres dispersed in water changes the whole picture: The resulting suspension has much higher viscosity. There are two reasons for that. First, any particles in liquid increase the viscosity since they disturb the flow field of the liquid. Second, MFC fibres are not as single fibres in the suspension, but instead, they form a continuous, strong network that resists flow effectively. As a result, the viscosity of the suspension is very high.
So, this network explains the high viscosity of MFC suspension at rest. However, if we apply force on the suspension, let’s say we start mixing it with a spoon, this network breaks, and the viscosity goes down. The more force we put on the suspension, the more the viscosity decreases. This is called shear thinning, and MFC suspensions are very shear thinning.
A practical consequence is that even very thick MFC suspensions can be pumped or sprayed since it turns easy-flowing under shear. When the shearing is stopped, the fibre network builds up again, and viscosity increases back to (or almost back to) the original value. We can use quite high shear forces, and the suspension will still regain its viscosity almost immediately after stopping the shear.
Figure 1 Graph showing the shear thinning behavior of 0.5% MFC suspension in water: Viscosity drops from 10 Pas to 0.01 Pas when shear rate is increased from 0.025 to 250 s-1.
HIGH YIELD STRESS
MFC network requires quite high force to start flowing. In rheological terms, the required force per certain area to initiate flow is called yield stress. Exactly how much force is required to break the network is dependent on how many contact points there are between the fibres and how strong interactions there are at these contact points.
In practice, at least the following things affect the strength of the network: concentration of MFC in water, how well the fibres are dispersed, concentration of salts in the suspension, possible other components, pH… The highest gel strength is achieved when the fibres are well dispersed, but they still have strong interactions with each other.
High yield stress makes MFC a good stabiliser of suspensions or emulsions since it can trap particles or droplets in the network and prevent them from sinking or floating.
If you want to read more about the scientific background of suspension rheology, I recommend a review article by Mueller and his colleagues (2010).