Alexandre Morin


I work on experimental active-matter physics at the Laboratoire de Physique at the ENS de Lyon, France, under the supervision of Denis Bartolo. I am interested in the behavior of active matter and collective phases of active matter in disordered environments. I address these questions by performing and analyzing experiments based on self-propelled colloids that spontaneously organize into a flock at high density.
+33 6 75 59 29 65

Active colloids self-organize to form a polar-ordered flock which propagates through a microfluidic channel.
Video is real time. Channel width is 1 mm.


  1. Journées de la Matière Condensée, oral presentation, (Grenoble, France 2018).
  2. Gordon Research Conference - Soft Condensed Matter Physics, poster presentation, (New London, NH, USA 2017).
  3. Liquids 2017 - 10th Liquid Matter Conference, oral presentation, (Ljubljana, Slovenia 2017).
  4. Laboratoire International Associé - Matière : Structure et Dynamique, invited oral presentation, (Lyon, France 2017).
  5. APS March Meeting, oral presentation, (New Orleans, LA, USA 2017).
  6. Patterns in Nature – Functions, Variations and Control, oral presentation, (Bayreuth, Germany 2016).
  7. StatPhys 26, poster presentation, Poster award, (Lyon, France 2016).
  8. Active and smart matter, poster presentation, (Syracuse, NY, USA 2016).


  1. Flowing active liquids in a pipe: Hysteretic response of polar flocks to external fields, Alexandre Morin and Denis Bartolo, Phys. Rev. X 8, 021037, arXiv:1803.10782, [PDF], (2018). ---- Show Abstract
    We investigate the response of colloidal focks to external fields. We first show that individual colloidal rollers align with external flows as would a classical spin with magnetic fields. Assembling polar active liquids from colloidal rollers, we experimentally demonstrate their hysteretic response: confined colloidal flocks can proceed against external flows. We theoretically explain this collective robustness, using an active hydrodynamic description, and show how orientational elasticity and confinement protect the direction of collective motion. Finally, we exploit the intrinsic bistability of confined active flows to devise self-sustained microfluidic oscillators.
  2. Sounds and hydrodynamics of polar active fluids, Delphine Geyer, Alexandre Morin and Denis Bartolo, Nature Materials, (2018). ---- Show Abstract
    Spontaneously flowing liquids have been successfully engineered from a variety of biological and synthetic self-propelled units. Together with their orientational order, wave propagation in such active fluids have remained a subject of intense theoretical studies for more than two decades. Until now, this phenomenon has however never been experimentally observed. Here, we establish and exploit the propagation of sound waves in colloidal active materials with broken rotational symmetry. We show that two mixed modes coupling density and velocity fluctuations propagate along all directions in colloidal-roller fluids. We then show how the six materials constants defining the linear hydrodynamics of these active liquids can be measured from their spontaneous fluctuation spectrum, while being out of reach of conventional rheological methods. This active-sound spectroscopy is not specific to synthetic active materials and could provide a quantitative hydrodynamic description of herds, flocks and swarms from inspection of their large-scale fluctuations
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  3. Diffusion, subdiffusion, and localization of active colloids in random post lattices, Alexandre Morin, David Lopes Cardozo, Vijayakumar Chikkadi, and Denis Bartolo, Phys. Rev. E 91, 042611, arXiv:1702.07655, [PDF], (2017). ---- Show Abstract
    Combining experiments and theory, we address the dynamics of self-propelled particles in crowded environments. We first demonstrate that motile colloids cruising at constant speed through random lattices undergo a smooth transition from diffusive, to subdiffusive, to localized dynamics upon increasing the obstacle density. We then elucidate the nature of these transitions by performing extensive simulations constructed from a detailed analysis of the colloid-obstacle interactions. We evidence that repulsion at a distance and hard-core interactions both contribute to slowing down the long-time diffusion of the colloids. In contrast, the localization transition stems solely from excluded-volume interactions and occurs at the void-percolation threshold. Within this critical scenario, equivalent to that of the random Lorentz gas, genuine asymptotic subdiffusion is found only at the critical density where the motile particles explore a fractal maze.
  4. Distortion and destruction of colloidal flocks in disordered environments, Alexandre Morin, Nicolas Desreumaux, Jean-Baptiste Caussin, and Denis Bartolo, Nature Physics, arXiv:1610.04404, [PDF+SI], (2017). ---- Show Abstract
    How do flocks, herds and swarms proceed through disordered environments? This question is not only crucial to animal groups in the wild, but also to virtually all applications of collective robotics, and active materials composed of synthetic motile units. In stark contrast, appart from very rare exceptions, our physical understanding of flocking has been hitherto limited to homogeneous media. Here we explain how collective motion survives to geometrical disorder. To do so, we combine experiments on motile colloids cruising through random microfabricated obstacles, and analytical theory. We explain how disorder and bending elasticity compete to channel the flow of polar flocks along sparse river networks akin those found beyond plastic depinning in driven condensed matter. Further increasing disorder, we demonstrate that collective motion is suppressed in the form of a first-order phase transition generic to all polar active materials.
  5. Collective motion with anticipation: Flocking, spinning, and swarming, Alexandre Morin, Jean-Baptiste Caussin, Christophe Eloy, and Denis Bartolo, Phys. Rev. E 91, 012134, arXiv:1501.02468, [PDF], (2015). ---- Show Abstract
    We investigate the collective dynamics of self-propelled particles able to probe and anticipate the orientation of their neighbors. We show that a simple anticipation strategy hinders the emergence of homogeneous flocking patterns. Yet, anticipation promotes two other forms of self-organization: collective spinning and swarming. In the spinning phase, all particles follow synchronous circular orbits, while in the swarming phase, the population condensates into a single compact swarm that cruises coherently without requiring any cohesive interactions. We quantitatively characterize and rationalize these phases of polar active matter and discuss potential applications to the design of swarming robots.