Eliminating foam on industrial surfaces with bubble-capturing technology

Materials World magazine
,
4 Mar 2020

A bubble-capturing system could eliminate foam build-up on surfaces improving efficiency for industrial processes. Shardell Joseph reports.

Bubble-attracting sheets have been used to create a passive system for reducing or eliminating foam build-up, with textured mesh that collapses bubbles as quickly as they form. Researchers from MIT University, USA, claimed the technology can be used to prevent foam from forming in bioreactors and fermentation tanks, eliminating the use of antifoaming agents. As stated by the researchers, the industrial applications could be used in industries such as biofuel generation, oil and gas production and processing, and chemical processing.

Using a surface the researchers called aerophilic, the system can attract and shed bubbles of air and gas, similarly to how typically hydrophilic materials allow water droplets to cling, spread out and then fall away from surfaces.

‘This system is made from simple and cheap materials - porous SST plates are a commodity so is SST wire meshes - and allows for removal of foam without any additives,’ said MIT Varanasi Research Group Researcher, Leonoid Rapoport. ‘These additives are an active way to remove foam as you need to add them to the solution, then filter them out, and then make sure that they are filtered to an acceptable level.’

Frothy bubbles have been known to hinder industrial processes by taking up space, limiting the volume available for making the product, and by blocking up pipes and valves, damaging living cells. As the device is stated by the team to be simple, inexpensive and completely passive, it could reduce or even eliminate the need for chemical additives called defoamers, which companies spend an estimated US$3bln a year on.

Bubble behaviour

The flat device is a range of designed surface textures varying in size scales, was developed after conducting an in-depth study of how bubbles behave when in contact with a surface.

This was achieved by creating a high-speed video, and in doing so they discovered that the bubbles tend to bounce similarly to a rubber ball, bouncing several times before eventually sticking in place.

This resembles the way in which droplets of liquid behave once hitting a surface, only in reverse – because the bubbles rise they bounce downwards.

‘In order to effectively capture the impacting bubble, we had to understand how the liquid film separating it from the surface drains,’ said Rapoport. ‘And we had to start at square one because there was not even an established metric to measure how good a surface is at capturing impacting bubbles. Ultimately, we were able to understand the physics behind what causes a bubble to bounce away, and that understanding drove the design process.’

Air-filled surfaces

During the study, the team learned that when a rising bubble approaches a horizontal surface, which is separated from the bubbles by a the thin liquid film, it needs to be drained for the bubble to be captured by the surface.

To approach this challenge, the researchers designed aerophilic surfaces that can efficiently drain liquid films upon bubble contact, demonstrating their capacity to catch bubbles relates to their texture parameters. According to the paper, Capturing bubbles and preventing foam using aerophilic surfaces, published in Advanced Materials in February 2020, an aerophilic surface is able to capture the bubbles because they are a class of surfaces that are sufficiently non-wetting, meaning they can sustain a very thin layer of air in-between features when submerged.

This layer, known as a plastron, is also used by insects and spiders for underwater respiration, and has a similar texture to some feathers on diving birds that help keep the animals dry under water. For this device, the plastron contributes to making the bubbles stick to the surface and dissipate.

The team tested the concept using the principles to create a device in the lab, which passively captured a rising flow of bubbles in a surfactant-rich solution. The device contained a bubble-capturing surface and inserted it into a beaker with bubbles rising through it.

Placing the beaker next to an identical one containing foaming suds and same sized sheets but without the textured material, they analysed the speed at which the foam dissipated in both beakers. In the one containing the bubble-capturing surface, the foam quickly dissipated to very little, while in the other the full layer remained.

Rapoport stated that the system can be used in any industry that produces foam as a byproduct, such as paper and pulp, waste removal, pharmaceuticals and foods. ‘It can also be applied to industries where capturing bubbles can provide value such as methane capture from wetlands or in industries that need to boil water to create steam, this is ubiquitous in the energy industry for example,’ he said.

As the system can be applied to many different processes in different industries, the team is now seeking a suitable application to start a trial.