Ricard Alert Zenón
@ricardalert.bsky.social
ICREA Research Professor at Universitat de Barcelona. Research Group Leader at MPI-PKS and CSBD in Dresden. Theory of living matter. Collective phenomena in biology through the lens of active matter physics.
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)!
December 2, 2025 at 8:02 PM
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.
December 2, 2025 at 8:02 PM
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.
December 2, 2025 at 8:02 PM
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.
December 2, 2025 at 8:02 PM
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.
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/...
www.science.org/doi/10.1126/...
December 2, 2025 at 8:02 PM
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/...
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.
December 2, 2025 at 8:02 PM
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).
December 2, 2025 at 8:02 PM
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.
December 2, 2025 at 8:02 PM
Aaron and Josh measured the height of the bacterial colony with a laser-scanning microscope called a profilometer, which reveals the waves very clearly.
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...
www.biorxiv.org/content/10.1...
December 2, 2025 at 8:02 PM
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...
www.biorxiv.org/content/10.1...
Warm goodbye to Dresden's @mpipks.bsky.social, my scientific home for the past 4 years. It's been amazing to start my group here! Thanks to the Max Planck Society, group members and everyone at the institute! Looking forward to the new chapter in Barcelona @ub.edu
September 21, 2025 at 1:57 PM
Warm goodbye to Dresden's @mpipks.bsky.social, my scientific home for the past 4 years. It's been amazing to start my group here! Thanks to the Max Planck Society, group members and everyone at the institute! Looking forward to the new chapter in Barcelona @ub.edu
Check our new preprint on active surface simulations of amoeboid migration! We show how this mode of migration can be mechanically guided by gradients in friction, viscosity, pressure, or confinement, and even by nearby cells. Work led by Hanna Gertack and Sebastian Aland.
arxiv.org/abs/2509.11801
arxiv.org/abs/2509.11801
September 19, 2025 at 9:37 AM
Check our new preprint on active surface simulations of amoeboid migration! We show how this mode of migration can be mechanically guided by gradients in friction, viscosity, pressure, or confinement, and even by nearby cells. Work led by Hanna Gertack and Sebastian Aland.
arxiv.org/abs/2509.11801
arxiv.org/abs/2509.11801
n these streams, bacteria are densely packed but can still freely move past one another. Capillary forces allow bacteria to pack densely without cell-cell adhesion, avoiding jamming.
August 2, 2025 at 2:59 PM
n these streams, bacteria are densely packed but can still freely move past one another. Capillary forces allow bacteria to pack densely without cell-cell adhesion, avoiding jamming.
Capillary forces organize gliding bacteria into different phases. In the movie, menisci are initially wide but weak, and bacteria form a gas. When water is made less available, capillary forces become strong and organize the colony into nematic streams. Watch until the end!
August 2, 2025 at 2:59 PM
Capillary forces organize gliding bacteria into different phases. In the movie, menisci are initially wide but weak, and bacteria form a gas. When water is made less available, capillary forces become strong and organize the colony into nematic streams. Watch until the end!
Capillary attraction also makes bacteria arrange side by side after dividing.
August 2, 2025 at 2:59 PM
Capillary attraction also makes bacteria arrange side by side after dividing.
When their menisci touch, bacterial cells experience capillary attraction. Check out the video! This attractive interaction promotes bacterial aggregation.
August 2, 2025 at 2:59 PM
When their menisci touch, bacterial cells experience capillary attraction. Check out the video! This attractive interaction promotes bacterial aggregation.
Matt and Josh developed an apparatus to control water availability, which directly impacts the pressure, and hence the width of the menisci.
August 2, 2025 at 2:59 PM
Matt and Josh developed an apparatus to control water availability, which directly impacts the pressure, and hence the width of the menisci.
On hydrated environments, like soil, textiles, and hydrogels, bacteria are surrounded by a meniscus of water. We fit the meniscus profile with a model based on surface tension and the osmotic pressure of extracting water from the substrate.
August 2, 2025 at 2:59 PM
On hydrated environments, like soil, textiles, and hydrogels, bacteria are surrounded by a meniscus of water. We fit the meniscus profile with a model based on surface tension and the osmotic pressure of extracting water from the substrate.
On a two-dimensional square lattice, the particles can align in stripes, with either regular or stochastic widths, or in polar domains.
June 25, 2025 at 5:11 PM
On a two-dimensional square lattice, the particles can align in stripes, with either regular or stochastic widths, or in polar domains.
On one-dimensional chains, these interactions lead to a variety of ordered states.
June 25, 2025 at 5:11 PM
On one-dimensional chains, these interactions lead to a variety of ordered states.
We studied the emerging orientational order of these active crystals, which depends on how the polarity-bond interactions vary with distance. The figure shows a few possible cases of the distance dependence, for different experimental systems.
June 25, 2025 at 5:11 PM
We studied the emerging orientational order of these active crystals, which depends on how the polarity-bond interactions vary with distance. The figure shows a few possible cases of the distance dependence, for different experimental systems.
We now asked: What happens if the particles are on a lattice to start with? Due to self-propulsion, the particles displace away from the lattice sites.
June 25, 2025 at 5:11 PM
We now asked: What happens if the particles are on a lattice to start with? Due to self-propulsion, the particles displace away from the lattice sites.
We study particles with what we call polarity-bond interactions, whereby they turn either towards or away from each other. In previous works, we found that these interactions emerge in metal-dielectric Janus particles. They can also be relevant in cells or in walker robots.
June 25, 2025 at 5:11 PM
We study particles with what we call polarity-bond interactions, whereby they turn either towards or away from each other. In previous works, we found that these interactions emerge in metal-dielectric Janus particles. They can also be relevant in cells or in walker robots.
We also understood the nonlinear instability by mapping the resulting active nematic patterns to a particle in a potential. We can then interpret the different patterns as particle trajectories in this potential.
June 10, 2025 at 9:17 AM
We also understood the nonlinear instability by mapping the resulting active nematic patterns to a particle in a potential. We can then interpret the different patterns as particle trajectories in this potential.
In the language of dynamical systems, what happens is that the spontaneous flow instability switches from being a supercritical pitchfork bifurcation to a subcritical one. Check the bifurcation diagram below!
June 10, 2025 at 9:17 AM
In the language of dynamical systems, what happens is that the spontaneous flow instability switches from being a supercritical pitchfork bifurcation to a subcritical one. Check the bifurcation diagram below!