We’re excited to have explained a striking collective behavior in biology (rippling) as an active-matter phenomenon (surface waves on an active liquid crystal)!

Second, we varied the substrate polymer concentration and composition, which affects its affinity for water. The more polymer concentration (which also means a stiffer substrate), the higher the cost to extract water. And the wavelength again varies as expected.

We tested these predictions in experiments in two ways: First, adding a surfactant to vary surface tension. As predicted, the wavelength increased with surface tension.

Here, water provides restoring forces that compete with active stresses to produce to waves. We predict that the wavelength is controlled by the capillary length of the bacterial film’s interface, which depends on the surface tension of water and the energy cost of extracting it from the substrate.

In recent work, we found that bacteria are covered by a meniscus of water, which is extracted from the underlying hydrogel substrate (agar gel).

www.nature.com/articles/s41...

So, we propose a new view of rippling as surface waves on an active nematic, similar to previous findings in microtubule-kinesin mixtures.

www.science.org/doi/10.1126/...

Previous work proposed that rippling arises from synchronized cell reversals occurring when two wave fronts collide. But we found no evidence for reversals happening preferentially at wave crests.

We found that ripples are standing waves with a period of ~20 min, a wavelength of ~100 µm, and an amplitude of 6 to 20 cell widths on top of a thick film of cells (with many cell layers).

Aaron and Josh measured the height of the bacterial colony with a laser-scanning microscope called a profilometer, which reveals the waves very clearly.

Experiments by Aaron Bourque in Josh Shaevitz’s lab @princeton.edu and theory by Peter Hampshire at @mpipks.bsky.social and @csbdresden.bsky.social. Check the full thread below!

A thousand thank yous to all my mentors and collaborators, especially to Jaume Casademunt, @xaviertrepat.bsky.social, Ned Wingreen, Jean-François Joanny, and Frank Jülicher for their continued support over the years.

Amazing story! Congrats!

Awesome work! :)

Theory and data analysis work done by postdoc MJ Franco-Oñate in my group.

This collaboration led by @katecavanaugh.bsky.social from @oweinerlab.bsky.social was particularly inspiring for me: The ideas of active wetting that we developed some years ago actually helped us understand a key biological process. We could not have hoped for more!

www.nature.com/articles/s41...