Ricard Alert Zenón

Congrats, Carlos! Super well deserved! Looking forward to seeing what comes out of your lab!

📰 Notícies, agenda científica, descobriments i entrevistes.

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No et perdis res 👉 icreat.cat

Work led by Ashot Matevosyan, who carried out a technical tour-de-force to derive the fluctuating hydrodynamics of an active gel. And we benefited from great discussions with Frank Jülicher. Thanks to both!

Unlike in equilibrium systems, fluctuations in active matter are related not just to dissipation, but also to the microscopic activity. Our work establishes an explicit connection between fluctuations, dissipation, and activity.

In addition to thermal noise, we found an active noise contribution that emerges from the breaking of detailed balance. And we predict how this active noise would impact the motion of a tracer particle in the gel, so that it can be directly compared with microrheology experiments that track them.

In the new work, we generalized this approach to include the fluctuations. So, we derived the fluctuating hydrodynamics of this active gel model. Here's the constitutive equation for the stress in the gel, where the last term is the noise, whose statistics we predict.

In previous work with @davidoriola.bsky.social and Jaume Casademunt, we proposed this model and coarse-grained it to show that it gives rise to the constitutive equations of a viscoelastic active gel.

link.aps.org/doi/10.1103/...

In our work, we wanted to see how active fluctuations emerge from irreversible molecular processes — the source of activity. We modeled an active gel as a network of elastic elements bound by molecular crosslinkers. We introduce activity by breaking detailed balance in the crosslinker binding rates.

Even though they have been measured, predicting active fluctuations theoretically is challenging. So far, most works had taken a phenomenological approach by directly positing a specific dynamics or statistics of active fluctuations in a given system.

In the past two decades, many experiments have measured departures from the fluctuation-dissipation theorem to reveal active, non-thermal fluctuations in living systems, such as the cell cytoplasm, cytoskeletal networks, and chromatin, which behave as active gels.

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

New preprint! We show how mesoscopic nonequilibrium fluctuations in active gels emerge from the breaking of detailed balance at the molecular scale. Warning: Long technical paper ahead! Enjoy! @mpipks.bsky.social @ub.edu @ubics.bsky.social @icreacommunity.bsky.social

arxiv.org/abs/2601.20483

What a night sky!

Also a type of membrane protrusion in cells. Essentially like a blister — a balloon of membrane protruding out of a cell.

Congrats, Mazi!

#NousICREAs2025 | 🧬 Ricard Alert @ricardalert.bsky.social (@ub.edu) investiga en física biològica.

Estudia com les lleis de la física governen la vida, per exemple, investigant com cèl·lules i bacteris es mouen en grups.

short.do/UBTLe-

Our paper on the transition to active turbulence is now out @natcomms.nature.com! With @the-chaotician.bsky.social. Are you curious how activity begets chaos? Check out the paper and the thread below. 👇

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

Congrats! :)

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.

Capillary interactions drive the self-organization of bacterial colonies - Nature Physics Bacteria tend to live in thin layers of water on surfaces. Now the capillary forces in these layers are shown to help organize the bacteria into dense packs.

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!

New preprint! Do you like ocean waves? We found similar waves on bacterial colonies! We found that this collective behavior, known as rippling, is nothing but surface waves on an active nematic. @princeton.edu @mpipks.bsky.social @ub.edu @icreacommunity.bsky.social

www.biorxiv.org/content/10.1...

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.

Super excited to start as an ICREA Research Professor at the University of Barcelona! Really happy to join fantastic colleagues at @icreacommunity.bsky.social and @ub.edu.

www.icrea.cat/community/ic...