Physics of Living Matter
Ricard Alert's group
RESEARCH
Stochastic tissue growth
Tissues grow by cell proliferation, which is a stochastic process. We study how the variability at the cell scale yields robust growth at the tissue scale. In one example, we ask how spatial disorder in cell growth affects tissue buckling. In another example, we study the feedback between cell proliferation and pressure in tissues.
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.
PUBLICATIONS
Collective migration in bacterial colonies
We 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, we 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, we study the morphological stability of active droplets.
Phase transitions in active matter
We use active colloids to study phase transitions in active matter. For example, self-propelled particles can align and move collectively, and they can condense even if they repel each other — two paradigmatic phenomena known as flocking and motility-induced phase separation, respectively. We study new mechanisms for these phase transitions in self-propelled Janus colloids. We also develop the statistical mechanics of systems with non-reciprocal interactions, which do not obey Newton's action-reaction law.
PUBLICATIONS​
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. We 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.
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. We study the statistical properties of active turbulence, trying to understand how they differ from those of classic inertial turbulence.
PUBLICATIONS​
Collective cell migration in tissues
Collective cell migration is a key driver of embryonic development, wound healing, and some types of cancer invasion. We study how cells migrate collectively and how this process shapes tissues. We propose active polar fluid models for the spreading and directed migration of cell clusters, and we study the traction forces that underlie cell migration. Using this approach, we address collective phenomena such as the active wetting of living tissues, fingering instabilities, and durotaxis.
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 and fluctuations 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. We 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. Thus, we analyze 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, we use colloidal crystals 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. In this system, we found a new type of phase transition with properties in between those of first- and second-order transitions.
PUBLICATIONS
Membrane-cortex adhesion and blebbing
The cell membrane (red) is attached to the underlying actin cortex (green) by linker proteins. We model membrane-cortex adhesion by coupling the linkers' binding kinetics to membrane deformations and cortical tension. With this model, we predicted a membrane detachment transition. We also studied a local membrane detachment propagates through a peeling process to form a large blister-like protrusion called bleb.
Image from G.T. Charras. A short history of blebbing. J. Microsc. 231, 466 (2008).