Nematic phase changes unravel soft material properties

Materials World magazine
10 May 2016

Research returns surprising results regarding the function of cell membrane shape changes.

The oily membranes of living cells stand up to significant stretching and bending. Now, researchers at the University of Wisconsin-Madison, USA, are trying to recreate their design principles synthetically to understand how.

In a study published in Proceedings of the National Academy of Sciences in May 2016, they found that previously unappreciated parameters shape soft materials similar to biological membranes.

The researchers made vesicles – tiny synthetic orbs – of materials similar to the membranes surrounding living cells, then suspended them inside a liquid crystal, which can exist in different states.

Like most liquids, liquid crystals move around freely in all directions, but at specific temperatures or electromagnetic conditions, the molecules adopt similar orientations – leading to the nematic phase, where they all point in the same direction.

Objects floating in a liquid crystal can influence the alignments of this phase, but rather than accommodate the change, the complex fluid pushes back on rigid objects. The researchers did not know what would happen when a soft material was suspended in the liquid crystal.

While larger spheres remained round overall, smaller spheres became elongated like rugby balls. Counterintuitively, using numerical techniques, the researchers found that a competition between surface tension and elasticity of the liquid crystal was a driver in the distortion of the vesicles, independent of the stiffness or flexibility of the membranes.

‘Going into the problem, there was no obvious reason to think that surface tension would be a relevant piece of the puzzle’, said Wisconsin-Madison Mathematics Professor Saverio Spagnolie. ‘Usually when people think about membranes, the primary forces they consider are associated with elasticity, but it turns out that the bending stiffness has absolutely nothing to do with the shapes that we see in this work.’

The team plans to further clarify the source of surface tension and investigate whether similar forces could mould the local compositions of membranes made from mixed components akin to the surfaces of living cells.