Everyone knows what liquids are, and is familiar with their basic properties, but there are many behaviours and characteristics of liquids you may have noticed but not understood or acknowledged. Some of these properties can have practical implications when it comes to engineering, for example in liquid pumping and liquid filling machines.
The properties of a liquid are largely defined by its molecular structure, where its molecules are tightly packed together, but are not fixed to each other. This means that liquids are almost incompressible, but can change their shape to suit the container they are held in.
Cohesion and Adhesion
There are two main molecular forces which are responsible for some of the unusual properties liquids can exhibit, cohesion and adhesion. Cohesion is the attractive force between molecules of the same type, and adhesion is the attractive force between different types of molecules.
The molecules within a liquid can either be polar, or non-polar; a polar molecule is one where one end of the molecule is negatively charged and the other end is positively charged, whereas a non-polar molecule does not have a separation of charges across the molecule – it has an even distribution of charge. This polarity is what causes the attraction of the cohesive properties of liquids. Some of the effects of these properties are described below:
Surface Tension
The phenomenon of surface tension is where the cohesive bonds between the liquid molecules at a liquid-gas boundary are stronger than the cohesive bonds in the bulk of the liquid. This is because in the bulk of the liquid, each molecule has a neighbouring molecule to share a bond with, however the molecules at the surface have fewer adjacent molecules to bond with, and therefore the bonds between the molecules at the surface are stronger as they have fewer surrounding molecules to share the bonds with. The result of this effect is that the surface of the liquid has a greater resistance to external forces, which is why objects with a greater density than the liquid can sit on the surface of the liquid.
Capillary Action
Capillary action of a liquid is the spontaneous flow of a liquid through a narrow tube or porous material. The combination of the cohesive and adhesive properties of liquids allows the liquid to flow with no external forces applied to the liquid. In the case of a narrow tube, the liquid molecules have an adhesive attraction to the wall of the tube; if the adhesive attraction if stronger than the cohesive attraction, then the liquid will flow up the tube. For example, if water is mopped up with kitchen paper from one edge, it tends to flow up the kitchen paper.
Viscosity
The viscosity of a liquid is its resistance to flow under an applied force. An example of a high viscosity liquid is honey, and an example of a low viscosity liquid is water.
The viscosity is determined by the friction between the molecules of the liquid caused by intermolecular forces within it. The viscosity of a liquid can vary depending on the temperature of the liquid – heating the liquid increases the kinetic energy of the liquid molecules which increases the separation between the molecules, which reduces the intermolecular force, so friction is lower and the viscosity decreases.
Caviation
Cavitation of a liquid is the process of formation of vapour within a liquid when the static pressure in the liquid falls below the vapour pressure of the liquid. The low pressure leading to cavitation can commonly be caused by the liquid being accelerated to a high velocity, or if the given mass of liquid is expanded, in both cases lowering pressure.
Bubbles
Bubbles in a liquid can be formed when gases which are dissolved within it are released. The gas can be released for several reasons, but typically because of a change in the pressure of the gas in the gas-liquid boundary, or because of a change in temperature of the liquid. Sometimes the dissolved gas in a liquid can rise through it, and get trapped in the top layer of liquid. The surface tensions of the top layer of liquid can act as a net and form bubbles on the top of a liquid.
Evaporation
Evaporation happens when a liquid turn into a gas. Evaporation occurs when the temperature of the top layer of molecules of a liquid is sufficient to give the molecules enough kinetic energy such that they are able to break away from the rest of the bulk liquid and become gas.
The challenges in engineering
These properties of liquids can create some challenges in engineering, for example when trying to pump, dispense and weigh liquids accurately. To be able to dose liquids accurately with a reliable weight, ideally the liquid tube lines should be free from bubbles; the liquid source bottle should be free from dissolved gas; and cavitation should be avoided when the liquid is passing through the pumps. If these conditions are not met, the liquid dispensing can be a mixture of liquid and gas leading to discontinuous dispensing which is inaccurate, and can cause bubbling after settling in the bottle.
The surface tension and adhesion of the liquid may make it difficult to prime the liquid tubes, as the bubbles may be difficult to dislodge.
Cavitation of the liquid can occur within the pump, for example in a centrifugal pump if rotating at high speed, or in the aspiration stroke of a syringe pump if there is a low pressure. In this case, the gas created in the liquids and fluid pressure would need to stabilise before being dosed to get reliable liquid dosing.
Evaporation can have a small impact when trying to weigh a dosed liquid to a high level of accuracy, as the liquid dose can evaporate in the time between being dispensed, weighed and capped.
If the liquid to dispense has a high viscosity, it can be challenging to transport along tubing and get through a pump. This is because the significant extra amount of friction means a high pushing force, or high pulling force (depending on the pump type) is required to move the liquid. In the case of the pumps trying to pull the liquid, if the driving pressure gradient is high, it can lead to cavitation in the liquid, again leading to inaccuracies in filling. This means that the flow rate is limited if to avoid cavitation.
An interesting feature of dosing liquids through a nozzle with certain types of pumps is that a droplet can hang on the end of the nozzle after the dose has been dispensed. This is down to the cohesion between the liquid droplet and the liquid still in the tube, the adhesion between the droplet and the end of the nozzle and the surface tension which keeps the liquid hanging in a droplet shape. This is not necessarily an issue so long as the droplet does not fall off over time, but for some liquids the droplet tends to fall off which can cause issues for subsequent dose accuracies.