Understanding Viscosity : Adhesion vs Cohesion

Adhesion and cohesion

Adhesion is quite simply the tendency of fluid molecules to stick to another different material (hence adhesive) while cohesion is how the molecules themselves to stick to each other. Generally, both the cohesion and adhesion increase with viscosity in a linear manner, but certain materials have a tendency towards one or the other. This can cause different behaviour when pumped.

One material with high cohesion compared to adhesion is tomato paste. While also shear thinning, it may be noted that over time it will slip down inside the container leaving the sides relatively clean. This is partly why it’s easy to pump using a positive displacement pump with a follower plate, ie. ViscoFlux mobile S - the follower plate easily separates the paste from the sides of the container and the material is easily moved up the pump tube, due to its limited adhesive properties.

In comparison, honey has completely Newtonian properties, ie. it does not shear thin, and is notoriously hard to pump. Honey has very high adhesive properties in comparison to its cohesion. Purely based on the viscosity number, honey should be relatively easy material to pump. However due to its high adhesion, it is difficult to force through a hose or pipe. Pressure losses can be very high. As most high viscosity pumps have small cavities through which the the media is moved (thus providing yet more surfaces for the honey to stick to), this means that pumps have to be run at low speeds, with as wide a cavity as possible to avoid overworking the motor.

Experience has shown that honey requires a Viscopower F580 pump with high-torque direct drive motor, when on paper (considering the commonly stated viscosity figure of ~10,000 mPas) a gearbox driven pump eg. Viscopower F570 with single-phase motor would appear to suffice.

In conclusion, tread carefully when specifying pumps for high viscosity fluids. Some fluids, such as petroleum jelly, have very high viscosity at room temperature but are easy to pump due to their shear-thinning and low adhesive nature. Other fluids with considerably lower viscosity are much harder to pump eg. honey, epoxy resin, due to their Newtonian, adhesive properties.

For this reason, Flux offers pumps for testing. This eliminates the risk associated with selecting the right pump for high viscosity fluids. Rather than guess the viscosity, behaviour and pumpability of an unknown fluid, we can test and demonstrate how the fluid will move when pumped, plus measure actual flow rate, thus ensuring a particular pump will meet your process requirements.