Ricard Alert
The Physics of Living Matter
RESEARCH
Guidance mechanisms for cell migration
We study how cells direct their motion to follow environmental signals. For example, we study how cells move along gradients in substrate friction — a new type of directed cell migration that we call frictiotaxis. This type of directed motion is possible even when cells do not adhere to the substrate, like is the case for amoeboid migration.
Collective cell migration in bacterial colonies
I study how groups of bacteria migrate collectively. In one example, we model colonies of the soil bacterium Myxococcus xanthus as active fluids to understand collective behaviors such as the formation of layered aggregates and fruiting bodies, and the propagation of waves on the surface of the colony. In another example, I study how propagating fronts of chemotactic bacteria manage to remain stable even when moving through porous media.
PUBLICATIONS
Active droplets
Many biological and reconstituted systems behave as active droplets. Examples include biomolecular condensates, the mitotic spindle, cellular tissues, bacterial biofilms, and synthetic vesicles containing active nematic films. Because of their internal driving, active droplets often break symmetry and change shape spontaneously. To understand these phenomena, I study the morphological stability of active droplets.
Phase transitions in active matter
We use active colloids as controlled model systems to study phase transitions in active matter. For example, self-propelled particles can condense even if they repel each other. This phase transition, known as motility-induced phase separation, has become a paradigmatic example of active-matter phenomena. Another landmark phase transition in active matter is the flocking transition, whereby active agents like birds align and move collectively. We study how interparticle torques enable new mechanisms for these phase transitions in self-propelled Janus colloids.
PUBLICATIONS
Mechanics of biofilm growth and morphogenesis
Biofilms are surface-adhered communities of bacterial cells embedded in a matrix of secreted polymers. As cells grow and proliferate, mechanical stress builds up in the biofilm. I study how the mechanics of biofilm growth governs biofilm morphogenesis. In particular, we study how wrinkle patterns emerge from the spatiotemporal dynamics of stress accumulation in growing biofilms. I also study the statistics of stress fluctuations in growing biofilms.
Image from J. Yan, C. Fei, S. Mao, A. Moreau, N.S. Wingreen, A. Košmrlj, H.A. Stone, and B.L. Bassler. Mechanical instability and interfacial energy drive biofilm morphogenesis. eLife 8, e43920 (2019).
PUBLICATIONS
Active turbulence
Active fluids are driven internally by their microscopic components, and hence they can flow spontaneously without applying external forces. Examples include suspensions of bacteria, cytoskeletal components, self-propelled particles, and even epithelial tissues. All these systems exhibit turbulent-like chaotic flows driven by activity. I study the statistical properties of active turbulence, trying to understand how they differ from those of classic inertial turbulence.
PUBLICATIONS
Collective cell migration and tissue morphology
Collective cell migration is a key driver of embryonic development, wound healing, and some types of cancer invasion. I study how the collective migration of cells in epithelial monolayers impacts tissue morphology. We propose active polar fluid models of tissue spreading, and we analyze experiments that measure the traction forces exerted by cells on a deformable substrate. Using this approach, we address collective mechanical phenomena in tissues, such as their wetting behavior, morphological instabilities, collective durotaxis, or structural transitions.
Image from X. Trepat, M.R. Wasserman, T.E. Angelini, E. Millet, D.A. Weitz, J.P. Butler, and J.J. Fredberg. Physical forces during collective cell migration. Nat. Phys. 5, 426 (2009).​​
PUBLICATIONS
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R. Alert and X. Trepat. Living cells on the move. Phys. Today (2021)
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R. Alert, C. Blanch-Mercader, and J. Casademunt. Active Fingering Instability in Tissue Spreading. PRL (2019)
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R. Alert and J. Casademunt. Role of Substrate Stiffness in Tissue Spreading: Wetting Transition and Tissue Durotaxis. Langmuir (2019)
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C. Pérez-González*, R. Alert*, C. Blanch-Mercader, M. Gómez-González, T. Kolodziej, E. Bazellières, J. Casademunt, and X. Trepat. Active wetting of epithelial tissues. Nat. Phys. (2019)
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B. Smeets, R. Alert, J. Pešek, I. Pagonabarraga, H. Ramon, and R. Vincent. Emergent structures and dynamics of cell colonies by contact inhibition of locomotion. PNAS (2016)
Mechanics of active gels
Biological materials such as the actomyosin cytoskeleton, the mitotic spindle, and epithelial tissues are active gels, i.e. viscoelastic media driven internally by non-equilibrium molecular processes. I study how these molecular processes control the mechanical properties of active gels. In particular, we derived the hydrodynamic equations of active gels from the dynamics of their crosslinker proteins. This way, we unveiled how the breaking of detailed balance at the molecular scale gives rise to active stresses and tunes otherwise passive properties such as the gel's viscosity.
PUBLICATIONS​
Phase transitions in colloidal crystals
Crystals made of colloidal particles can be imaged with single-particle resolution, and interparticle interactions can be externally tuned. Leveraging these features, I use colloidal crystals as an experimental model system to study fundamental aspects of phase transitions. We combine theory, simulations, and experiments to study the structural transitions of a two-dimensional magnetic colloidal crystal assembled on a micropatterned substrate. The interplay between the pinning imposed by the substrate and the intrinsic anisotropy of the magnetic dipolar interactions gives rise to a new type of phase transition with unprecedented equilibrium and kinetic properties. This new transition has properties in between those of first- and second-order transitions.
PUBLICATIONS
Membrane-cortex adhesion and bleb nucleation
The cell membrane (red) is attached to the underlying actin cortex (green) by specific linker proteins. We proposed a mechanochemical model for membrane-cortex adhesion that couples the binding kinetics of the linker proteins to membrane deformations and cortical tension. With this model, we predicted the conditions for membrane detachment, and we interpreted micropipette aspiration experiments. We also study the effect of membrane-cortex adhesion on membrane fluctuations, providing predictions for fluctuation spectroscopy experiments.
A local detachment of the membrane leads to blister-like protrusions called blebs. We studied bleb nucleation, i.e. the spontaneous formation of blebs, which we predict to be governed by a rupture process called membrane peeling. We quantified the kinetics of bleb nucleation by means of stochastic simulations.
Image from G.T. Charras. A short history of blebbing. J. Microsc. 231, 466 (2008).