publications

2023

1. 

   Cell protrusions and contractions generate long-range membrane tension propagation

   Henry De Belly, Shannon Yan, Hudson Borja da Rocha, and 8 more authors

   Cell, 2023

   Abs HTML

   Membrane tension is thought to be a long-range integrator of cell physiology. Membrane tension has been proposed to enable cell polarity during migration through front-back coordination and long-range protrusion competition. These roles necessitate effective tension transmission across the cell. However, conflicting observations have left the field divided as to whether cell membranes support or resist tension propagation. This discrepancy likely originates from the use of exogenous forces that may not accurately mimic endogenous forces. We overcome this complication by leveraging optogenetics to directly control localized actin-based protrusions or actomyosin contractions while simultaneously monitoring the propagation of membrane tension using dual-trap optical tweezers. Surprisingly, actin-driven protrusions and actomyosin contractions both elicit rapid global membrane tension propagation, whereas forces applied to cell membranes alone do not. We present a simple unifying mechanical model in which mechanical forces that engage the actin cortex drive rapid, robust membrane tension propagation through long-range membrane flows.

2022

1. 

   A viscous active shell theory of the cell cortex

   Hudson Borja da Rocha, Jeremy Bleyer, and Hervé Turlier

   Journal of the Mechanics and Physics of Solids, 2022

   Abs HTML

   The cell cortex is a thin layer beneath the plasma membrane that gives animal cells mechanical resistance and drives most of their shape changes, from migration, division to multicellular morphogenesis. It is mainly composed of actin filaments, actin binding proteins, and myosin molecular motors. Constantly stirred by myosin motors and under fast renewal, this material may be well described by viscous and contractile active-gel theories. Here, we assume that the cortex is a thin viscous shell with non-negligible curvature and use asymptotic expansions to find the leading-order equations describing its shape dynamics, starting from constitutive equations for an incompressible viscous active gel. Reducing the three-dimensional equations leads to a Koiter-like shell theory, where both resistance to stretching and bending rates are present. Constitutive equations are completed by a kinematical equation describing the evolution of the cortex thickness with turnover. We show that tension and moment resultants depend not only on the shell deformation rate and motor activity but also on the active turnover of the material, which may also exert either contractile or extensile stress. Using the finite-element method, we implement our theory numerically to study two biological examples of drastic cell shape changes: osmotic shocks and cell division. Our work provides a numerical implementation of thin active viscous layers and a generic theoretical framework to develop shell theories for slender active biological structures.

2. Mean field fracture in disordered solids: Statistics of fluctuations

   Hudson Borja da Rocha, and Lev Truskinovsky

   Journal of the Mechanics and Physics of Solids, 2022

   Abs HTML

   Power law distributed fluctuations are known to accompany terminal failure in disordered brittle solids. The associated intermittent scale-free behavior is of interest from the fundamental point of view as it emerges universally from an intricate interplay of threshold-type nonlinearity, quenched disorder, and long-range interactions. We use the simplest mean-field description of such systems to show that they can be expected to undergo a transition between brittle and quasi-brittle (ductile) responses. While the former is characterized by a power law distribution of avalanches, in the latter, the statistics of avalanches is predominantly Gaussian. The realization of a particular regime depends on the variance of disorder and the effective rigidity represented by a combination of elastic moduli. We argue that the robust criticality, as in the cases of earthquakes and collapsing porous materials, indicates the self-tuning of the system towards the boundary separating brittle and ductile regimes.

2021

1. 

   A plausible mechanism of muscle stabilization in stall conditions

   Hudson Borja da Rocha, and Lev Truskinovsky

   The European Physical Journal Plus, 2021

   HTML

2020

1. 

   Rigidity-Controlled Crossover: From Spinodal to Critical Failure

   Hudson Borja da Rocha, and Lev Truskinovsky

   Phys. Rev. Lett., Jan 2020

   HTML

2019

1. 

   Functionality of Disorder in Muscle Mechanics

   Hudson Borja da Rocha, and Lev Truskinovsky

   Phys. Rev. Lett., Mar 2019

   HTML
2. Equilibrium unzipping at finite temperature

   Hudson Borja da Rocha, and Lev Truskinovsky

   Archive of Applied Mechanics, Mar 2019

   Abs HTML

   We study thermally activated unzipping, which is modeled as a debonding process. The system is modeled as a parallel bundle of elastically interacting breakable units loaded through a series spring. Using equilibrium statistical mechanics, we compute the reversible response of this mechanical system under quasi-static driving. Depending on the stiffness of the series spring, the system exhibits either ductile behavior, characterized by noncooperative debonding, or brittle behavior, with a highly correlated detachment of the whole bundle. We show that the ductile to brittle transition is of the second order and that it can also be controlled by temperature.

2015

1. 

   On the modeling of fiber dispersion in fiber-reinforced elastic materials

   Andrey V. Melnik, Hudson Borja Da Rocha, and Alain Goriely

   International Journal of Non-Linear Mechanics, Mar 2015

   Instabilities and Nonlinearities in Soft Systems: From Fluids to Biomaterials

   Abs

   When an isotropic material is subject to a uniaxial tension, the principal strain transverse to the direction of applied load is always negative. However, in fiber reinforced materials the transverse principal strain can change its sign as the load increases, passing through the zero-points, known as perversions. We investigate how the number of perversions in a material reinforced by two symmetrically aligned families of distributed fibers depends both on the degree of fiber dispersion and the model used for fiber dispersion. Angular integration and three variants of the generalized structure tensor approach are considered and discussed. The study of perversions clearly demonstrates the qualitative difference between these approaches in the case of high dispersion of fibers. The results suggest that this difference is primarily due to the way compressive fibers are modeled.