Ricard Alert Zenón
@ricardalert.bsky.social
Physics 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.
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!
We now found that, even when the non-flowing state is linearly stable, it can experience a nonlinear instability: a sufficiently strong perturbation will produce a discontinuous transition to spontaneous flows. In this regime, the quiescent and flowing states are bistable.
June 10, 2025 at 9:17 AM
We now found that, even when the non-flowing state is linearly stable, it can experience a nonlinear instability: a sufficiently strong perturbation will produce a discontinuous transition to spontaneous flows. In this regime, the quiescent and flowing states are bistable.
Since the early 2000s, active nematics are known to experience the so-called spontaneous flow instability: the ordered non-flowing state is linearly unstable, and this instability produces a continuous transition to spontaneous flows.
iopscience.iop.org/article/10.1...
iopscience.iop.org/article/10.1...
June 10, 2025 at 9:17 AM
Since the early 2000s, active nematics are known to experience the so-called spontaneous flow instability: the ordered non-flowing state is linearly unstable, and this instability produces a continuous transition to spontaneous flows.
iopscience.iop.org/article/10.1...
iopscience.iop.org/article/10.1...
We also use the Hamiltonian to study the role of external driving. When suitably engineered, the driving allows one to tune the interactions. For example, spins on a square lattice can be made to interact only along chains, hence destroying the transition to a ferromagnetic phase.
May 9, 2025 at 3:32 PM
We also use the Hamiltonian to study the role of external driving. When suitably engineered, the driving allows one to tune the interactions. For example, spins on a square lattice can be made to interact only along chains, hence destroying the transition to a ferromagnetic phase.
We developed Monte Carlo simulations based on the extended Hamiltonian. Unlike a previous “selfish energy” approach, our simulations accurately match the phase transitions of the non-reciprocal system, as in its original Langevin equations! We show this equivalence as well in a formal derivation.
May 9, 2025 at 3:32 PM
We developed Monte Carlo simulations based on the extended Hamiltonian. Unlike a previous “selfish energy” approach, our simulations accurately match the phase transitions of the non-reciprocal system, as in its original Langevin equations! We show this equivalence as well in a formal derivation.
In the SI, we show how to do it for 4 other systems with non-reciprocal interactions.
May 9, 2025 at 3:32 PM
In the SI, we show how to do it for 4 other systems with non-reciprocal interactions.
To build a Hamiltonian, the trick is to add degrees of freedom, such that the extended system is reciprocal. We derive the equations of motion from the Hamiltonian and eliminate the auxiliary degrees of freedom. This procedure is general; we illustrate it with XY spins with vision-cone interactions.
May 9, 2025 at 3:32 PM
To build a Hamiltonian, the trick is to add degrees of freedom, such that the extended system is reciprocal. We derive the equations of motion from the Hamiltonian and eliminate the auxiliary degrees of freedom. This procedure is general; we illustrate it with XY spins with vision-cone interactions.
Many systems — from active colloids to cells to animals — have non-reciprocal interactions, which don’t follow Newton’s action-reaction law. Given that these systems don't have an energy function, what is their statistical mechanics? Our work makes progress in this direction.
May 9, 2025 at 3:32 PM
Many systems — from active colloids to cells to animals — have non-reciprocal interactions, which don’t follow Newton’s action-reaction law. Given that these systems don't have an energy function, what is their statistical mechanics? Our work makes progress in this direction.
New preprint! Non-reciprocal interactions don’t arise from a potential. Yet, we found a way to encode them in a Hamiltonian, which captures the phase transitions of non-reciprocal systems! With Yubo Shi, Roderich Moessner, and @marinbukov.bsky.social @mpipks.bsky.social.
arxiv.org/pdf/2505.05246
arxiv.org/pdf/2505.05246
May 9, 2025 at 3:32 PM
New preprint! Non-reciprocal interactions don’t arise from a potential. Yet, we found a way to encode them in a Hamiltonian, which captures the phase transitions of non-reciprocal systems! With Yubo Shi, Roderich Moessner, and @marinbukov.bsky.social @mpipks.bsky.social.
arxiv.org/pdf/2505.05246
arxiv.org/pdf/2505.05246
Second, Adam created channels with uniform stiffness but with a friction gradient. And cells did move up the friction gradient. So cells do perform frictiotaxis!
April 24, 2025 at 11:03 AM
Second, Adam created channels with uniform stiffness but with a friction gradient. And cells did move up the friction gradient. So cells do perform frictiotaxis!