Infinite slip surface coatings

Materials World magazine
29 May 2019

A new surface coating improves dispensing and transport of yield-stress fluids. Ceri Jones reports.

Liquid-impregnated surfaces (LIS) can reduce up to 100% of wall-friction to allow highly viscous liquids to move with minimal resistance or spreading, as demonstrated by recent research.

When pumping a thick liquid, gel or paste through a tube, the matter often sticks to the sides, slowing down the transfer speed and wasting what remains on the surfaces. This new coating prevents friction-induced drag and the subsequent need for cleaning by lubricating the surfaces.

Created by the team behind LiquiGlide – the non-stick ketchup bottle – LIS is an extension of that research to tackle problems when dealing with non-Newtonian fluids.

‘LIS can induce mobility in yield-stress fluids even below their yield-stress, allowing them to move as a plug without shearing with an infinite slip length,’ read the paper Mobility of yield stress fluids on lubricant-impregnated surfaces, by MIT Professor of Mechanical Engineering, Kripa Varanasi, with former students Leonid Rapoport and Brian Solomon.

‘Our surfaces enable fluids to move by whichever way is more preferable for them. This is very important when you want to maintain the integrity of these materials when they are being processed.’

This coating has a broad range of useful applications, particularly for yield-stress fluids i.e. those that do not flow unaided and require energy to start moving, such as the tap of a bottle or squeeze of a tube. While originally designed to improve the transport of food products within the processing and production, it can be applied in any process that requires friction reduction, from managing dough during industrial-scale breadmaking, to improving efficacy of pharmaceuticals.

Skidding ahead

The liquid-impregnated surface is achieved by etching or grinding a network of fine pillars across the interface at the nano-scale, resulting in a textured surface. When the lubricant is applied, it is kept in place by capillary action and intermolecular forces, in the manner of tree water transport systems. ‘Any material inside a container with this kind of lining essentially only comes in contact with the lubricating liquid, and slides right off instead of sticking to the solid container wall,’ said MIT.

This makes it possible to minimise drag, perhaps up to 100%, achieving what the team calls near infinite slip.

On the surface, it is simple but the benefits are wide-ranging. For mass bread production, a highly slippery surface for a dough mixing bowl means less scraping of ingredients and therefore lower risk of over-kneading, while in pharmaceutical production, it would minimise the shear forces required to mix and move drugs, which tends to damage active compounds, reducing their effectiveness.

Forging ahead, further work is being carried out to assess the potential to support the development of successful flow batteries – those that contain a slurry of conductive particles instead of solid electrodes. Use of superhydrophobic interfaces would overcome design challenges of flow batteries which, due to the thick slurry inside, would not flow.

According to MIT, incorporating the coatings resulted in a fourfold increase in capacity and an 86% saving in mechanical power, compared with the use of traditional surfaces. ‘Engineering surfaces for a flow battery opens up an entirely new branch of applications that can help meet future energy storage demand,’ said Solomon.

Read the paper here: or visit the MIT website to watch a video of the coating in action.