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Soft amorphous materials are viscoelastic solids ubiquitously found around us, from clays and cementitious pastes to emulsions and physical gels encountered in food or biomedical engineering. Under an external deformation, these materials undergo a noteworthy transition from a solid to a liquid state that reshapes the material microstructure. This yielding transition was the main theme of a workshop held from January 9 to 13, 2023 at the Lorentz Center in Leiden. The manuscript presented here offers a critical perspective on the subject, synthesizing insights from the various brainstorming sessions and informal discussions that unfolded during this week of vibrant exchange of ideas. The result of these exchanges takes the form of a series of open questions that represent outstanding experimental, numerical, and theoretical challenges to be tackled in the near future., Comment: 21 pages, 7 figures, perspective on the workshop 'Yield stress and fluidization in brittle and ductile amorphous systems' held from January 9 to 13, 2023 at the Lorentz Center in Leiden

To understand cell migration, it is crucial to gain knowledge on how cells exert and integrate forces on/from their environment. A quantity of prime interest for biophysicists interested in cell movements modeling is the intracellular stresses. Up to now, three different methods have been proposed to calculate it, they are all in the regime of the thin plate approximation. Two are based on solving the mechanical equilibrium equation inside the cell material (Monolayer Stress Microscopy, and Bayesian Inference Stress Microscopy) and one is based on the continuity of displacement at the cell/substrate interface (Intracellular Stress Microscopy). We show here using 3D FEM modeling that these techniques do not calculate the same quantities (as was previously assumed), the first techniques calculate the sum of the active and resistive stresses within the cell, whereas the last one only calculate the resistive component. Combining these techniques should in principle permit to get access to the active stress alone., Comment: 7 pages, 5 figures. arXiv admin note: substantial text overlap with arXiv:2105.05792

Animal cells are active, contractile objects. While bioassays address the molecular characterization of cell contractility, the mechanical characterization of the active forces in cells remains challenging. Here by confronting theoretical analysis and experiments, we calculated both the resistive and the active components of the intracellular stresses that build up following cell adhesion. We obtained a linear relationship between the divergence of the resistive stress and the traction forces, which we show is the consequence of the cell adhering and applying forces on the surface only through very localized adhesion points (whose size is inferior to our best resolution, of 400 nm). This entails that there is no measurable forces outside of these active point sources, and also that the resistive and active stresses inside cells are proportional., Comment: 5 pages, 4 figures

We present an in-depth investigation of a fully automated Fourier-based analysis to determine the cell size and the width of its distribution in 3D biological tissues. The results are thoroughly tested using generated images, and we offer valuable criteria for image acquisition settings to optimize accuracy. We demonstrate that the most important parameter is the number of cells in the field of view, and we show that accurate measurements can already be made on volume only containing 3x3x3 cells. The resolution in $z$ is also not so important and a reduced number of in-depth images, of order of one per cell, already provides a measure of the mean cell size with less than 5\% error. The technique thus appears to be a very promising tool for very fast live local volume cell measurement in 3D tissues \textit{in vivo} while strongly limiting photobleaching and phototoxicity issues., Comment: 9 pages, 9 figures

We have performed surface tension measurements on carbopol gels of different concentrations and yield stresses. Our setup, based on the force exerted by a capillary bridge on two parallel plates, allows to measure an effective surface tension of the complex fluid and to investigate the influence of flow history. More precisely the effective surface tension measured after stretching the bridge is always higher than after compressing it. The difference between the two values is due to the existence of a yield stress in the fluid. The experimental observations are successfully reproduced with a simple elasto-plastic model. The shape of successive stretching-compression cycles can be described by taking into account the yield stress and the elasticity of the gel. We show that the surface tension $\gamma_{LV}$ of yield stress fluids is the mean of the effective surface tension values only if the elastic modulus is high compared to the yield stress. This work highlights that thermodynamical quantities measurements are challenged by the fluid out-of-equilibrium state implied by jamming, even at small scales where the shape of the bridge is driven by surface energy. Therefore setups allowing deformation in opposite directions are relevant for measurements on yield stress fluids., Comment: 12 pages, 16 figures in Soft Matter 2015

Apparent tissue surface tension allows the quantification of cell-cell cohesion and was reported to be a powerful indicator for the cellular rearrangements that take place during embryonic development or for cancer progression. The measurement is realized with a parallel compression plate tensiometer using the capillary laws. Although it was introduced more than a decade ago, it is based on various geometrical or physical approximations. Surprisingly, these approximations have never been tested. Using a novel tensiometer, we compare the two currently used methods to measure tissue surface tension and propose a third one, based on a local polynomial fit (LPF) of the profile of compressed droplets or cell aggregates. We show the importance of measuring the contact angle between the plate and the drop/aggregate to obtain real accurate measurement of surface tension when applying existing methods. We can suspect that many reported values of surface tension are greatly affected because of not handling this parameter properly. We show then the benefit of using the newly introduced LPF method, which is not dependent on this parameter. These findings are confirmed by generating numerically compressed droplet profiles and testing the robustness and the sensitivity to errors of the different methods., Comment: 14 pages, 3 figures