How (and why) liquids flow in the ways they do
The liquids we encounter in the food and confectionery industries are some of the most complex and obscure in the ways they flow. Understanding why they behave in the strange ways they do – and being able to quantify it – is useful and can help understand and avoid some of the problems which can arise from sometimes strange behaviour.
The way a fluid behaves when moving (or when you want it to move) is influenced mainly by its viscosity, which in turn is influenced primarily by temperature, composition and shear rate and in some cases factors particular to that product (for example particle size and moisture content in chocolate). In addition some fluids also have a “yield value”, a force required to initiate flow and below which they do not flow at all.
Note that this review does not consider significant physical or chemical changes in materials due to gelling, coagulation or thermal degradation, as although these are important, they cause large generally step (and frequently irreversible) changes in flow behaviour due to quite separate issues and require a separate and individual treatment.
Whilst straightforward fluids such as oils and low solids solutions behave fairly normally (they are referred to as “Newtonian” fluids and their viscosity is usually only reliant on temperature and solute concentration and to some extent composition) many of the fluids we have to handle are either high solids solutions (eg glucose syrup) or two or even three phase systems (ie liquids containing solid particles such as slurries, fine solids dispersions such as chocolate or foams). The term “rheology” is often used where viscosity is controlled by a number of complex factors.
Materials with complex behaviour are referred to as “non Newtonian” and the viscosity of this type of material is often affected by very small changes in the governing factors.
Natural materials inevitably change in composition with source and season and this can cause changes in flow behaviour which need to be understood so that anticipatory action can be taken where necessary. Even with processed, well controlled materials such as glucose syrup small changes in carbohydrate spectrum (for example from one supplier to another at a constant DE) of solids can cause significant change in viscosity.
Small “in process” changes can have an effect on flow behaviour, for example the level of temper in a chocolate will substantially affect viscosity as the fat crystals effectively increase the solids level in the suspension.
To measure and understand viscosity a range of viscometers are available, the majority being off line instruments. The most common unit of viscosity is the SI (mks system) unit “Pascal second” (Pas) but the Poise (1Pas=10 Poise) is also frequently encountered.
Modern viscometers are capable of producing excellent information on the viscosity of a fluid and the effect of factors such as shear. However they are generally quite delicate (and expensive) instruments and consequently many companies and sectors of the industry have evolved subjective measurement techniques which correspond to their particular requirements. It is generally very difficult to correlate these pieces of equipment with true measured viscosity but that does not diminish their value in the set of circumstances in which they are used. They are frequently rapid, robust and highly consistent in the right hands and are often invaluable in the control and monitoring of processes.
For reasonably low viscosity liquids a “cup” viscometer is quite commonly used, comprising a container with a carefully graduated opening at the base and a second container to collect a known quantity of the fluid. The top container is filled with the exit hole blocked and then the length of time needed to fill the bottom container is measured once the hole is opened. For a low to medium viscosity liquid this method is simple, rapid and the equipment is robust although it is important to control the temperature of the sample and the equipment.
For more viscous fluids and those with more complex rheology a wide variety of (often ingenious) methods have been evolved. Some parts of the chocolate industry have used a “cone” viscometer where a blunt cone is dropped into the sample from a fixed height and the depth of penetration gave a reading. Another method employed (the “Consistometer”) involves releasing a sample along a graduated horizontal track and measuring how far the sample spreads.
Consistometer
Another common technique is to measure the time for a steel ball to fall through a known distance in a tube filled with the fluid. This is obviously restricted to reasonably clear fluids
The viscosity of a fluid can have a significant effect on the way a fluid behaves not just in pump and pipework systems but in depositors, moulds, extruders, spray nozzles, mixers, dryers, etc. This is particularly the case with higher viscosity fluids, but changes in even relatively low viscosity fluids can have an important effect in some cases – particular spray systems.
Whilst controlling viscosity to achieve required performance is in itself important, changes in viscosity affecting system performance can also be indicative of changes in composition or other physical attributes of a fluid and can thus be a useful process control tool
Jul 11