Preventing and utilising cavitation

Materials World magazine
,
29 Aug 2019

Researchers uncover how to reduce water damage caused by microbubbles to protect infrastructure. Alex Brinded takes a look at how it works.

Objects entering water are exposed to millions of tiny bubbles that burst on the surface and cause damage to infrastructure over time. Engineers at Edinburgh University, UK, have been trying to understand the nature of these formations in order to prevent such damage.

As one of the few groups currently working in this field, the researchers used a supercomputer to model the behaviour of miniscule bubbles that appear on underwater surfaces at an atomic level.

According to the university’s School of Engineering Researcher, Duncan Dockar, during his PhD he met only five other people who specialised in bubbles, indicating that research is lacking in this field which has an effect on the quality of research. ‘It can be frustrating when there seems to be very few analytical equations we can compare results to, but from a research point of view it does mean we have a lot of opportunities for novel work,’ Dockar told Materials World.

Bubbles occur in many situations, such as in turbomachinery and chemical processes, and understanding how they interact informs the design and operation of such systems or processes.

While studying bubble behaviour may seem deceptively simple, in reality, complex physics is needed to understand why bubbles are sensitive to temperature changes such as boiling and evaporation, pressure changes like cavitation, and dissolved gas effects such as the opening of a fizzy drink or buoyancy, where bubbles rise or sink in a pint of Guinness. This sensitivity and changeability makes them difficult to study. Unlike a liquid, which has the same volume regardless of shape, a bubble contains gas or vapour and can change volume rapidly.

‘It is not uncommon in cavitation for bubble diameters to increase up to a thousand times, which is a billion times change in volume. This huge difference in scale is a massive challenge to simulate or observe experimentally,’ Dockar said.

Bursting the bubble

The team modelled cavitation, the spontaneous formation of a bubble in a liquid in response to a local drop in pressure. This process has three stages – initially the low pressure causes bubbles to grow unstably, these rapidly grow large, and then they collapse violently.

A ship propeller causes cavitation because it cycles from high to low pressures as it spins. The collective bubbles created from this can release a strong jet of liquid when they collapse, causing surface pitting and erosion. After thousands of cycles, this can equate to real structural damage.

‘Most bubble equations assume a spherical bubble which is completely surrounded by liquid, which is easier to analyse but not always so close to reality,’ Dockar said. However, surface nanobubbles are hemispherical and attached to a surface, making them behave differently.

The engineers used molecular dynamics software to model each atom in a nanoscale system, and created realistic phenomena of gases, liquids and solids. The liquid pressure was dropped by a small amount at a time until they identified the unstable and rapid growth phases.

‘We have shown that cavitation also occurs on solid surfaces with hemispherical bubbles at the nanoscale,’ Dockar said. ‘We developed a new model for what this new cavitation threshold is, specifically for surface nanobubbles, and explain why it is different than spherical bubbles.’

The model finds the balance between the gas pressure, liquid pressure and surface tension. Theoretically, any machine operating above the new threshold should not risk cavitation damage. Turbomachinery components, such as ship propellers, pumps and hydroelectric turbines, are obvious beneficiaries.

‘I would say the biggest design aspect would be changes in the operating conditions, for example, speed, angle of the blades etc., to prevent these bubbles forming. Prevention is better than cure for cavitation,’ he added.

Exploiting power

The powerful action of surface nanobubble cavitation has the potential to be very useful in other applications. For instance, there is a novel cancer treatment that collapses nanobubbles onto individual tumour cells to rupture and destroy them. Also, ultrasonic cleaning harnesses the power of cavitation as ultrasound forms and collapses bubbles to remove surface debris from items with lots of pores or small features.

According to Dockar, some industries that rely on highly specialised technical equipment are researching the potential to develop a new cleaning technology based on the cavitation process.

For this, surface nanobubbles could be kept stable on rough textures using pressure or removed from electrolysis or catalysts where they can inhibit reactions. The team is currently working on modelling the later stages of cavitation to further understanding.