Long-range stress transmission guides endothelial gap formation.

In endothelial gap formation, local tractions exerted by the cell upon its basal adhesions are thought to exceed balancing tensile stresses exerted across the cell-cell junction, thus causing the junction to rupture. To test this idea, we mapped evolving tractions, intercellular stresses, and corresponding growth of paracellular gaps in response to agonist challenge. Contrary to expectation, we found little to no relationship between local tensile stresses and gap formation. Instead, we discovered that intercellular stresses were aligned into striking multi-cellular domains punctuated by defects in stress alignment. Surprisingly, gaps emerged preferentially not at stress hotspots, as predicted, but rather at stress defects. This unexpected behavior is captured by a minimal model of the cell layer as a jammed assembly of cohesive particles undergoing plastic rearrangements under tension. Together, experiments and model suggest a new physical picture in which gap formation, and its consequent effect on endothelial permeability, is determined not by a local stress imbalance at a cell-cell junction but rather by emergence of non-local, cooperative stress reorganization across the cellular collective.

Self-assembly and cooperative dynamics of a model colloidal gel network.

We study the assembly into a gel network of colloidal particles, via effective interactions that yield local rigidity and make dilute network structures mechanically stable. The self-assembly process can be described by a Flory-Huggins theory, until a network of chains forms, whose mesh size is on the order of, or smaller than, the persistence length of the chains. The localization of the particles in the network, akin to some extent to caging in dense glasses, is determined by the network topology, and the network restructuring, which takes place via bond breaking and recombination, is characterized by highly cooperative dynamics. We use NVE and NVT molecular dynamics as well as Langevin dynamics and find a qualitatively similar time dependence of time correlations and of the dynamical susceptibility of the restructuring gel. This confirms that the cooperative dynamics emerge from the mesoscale organization of the network.

Microscopic picture of cooperative processes in restructuring gel networks.

Colloidal gel networks are disordered elastic solids that can form even in extremely dilute particle suspensions. With interaction strengths comparable to the thermal energy, their stress-bearing network can locally restructure via breaking and reforming interparticle bonds. This allows for yielding, self-healing, and adaptive mechanics under deformation. Designing such features requires controlling stress transmission through the complex structure of the gel and this is challenging because the link between local restructuring and overall response of the network is still missing. Here, we use a space resolved analysis of dynamical processes and numerical simulations of a model gel to gain insight into this link. We show that consequences of local bond breaking propagate along the gel network over distances larger than the average mesh size. This provides the missing microscopic explanation for why nonlocal constitutive relations are necessary to rationalize the nontrivial mechanical response of colloidal gels.

Effect of quenched size polydispersity on the fluid-solid transition in charged colloidal suspensions.

We study the effect of quenched size polydispersity on the phase behavior of charged colloidal suspensions using free-energy calculations in Monte Carlo simulations. The colloids are assumed to interact with a hard-core repulsive Yukawa (screened-Coulomb) interaction with constant surface potential, so that the particles are polydisperse both in size and charge. In addition, we take the size distribution to be fixed in both the fluid and crystal phase (no size fractionation is allowed). We study the fluid-solid transition for various screening lengths and surface potentials, finding that upon increasing the size polydispersity the freezing transition shifts toward higher packing fractions and the density discontinuity between the two coexisting phases diminishes. Our results provide support for a terminal polydispersity above which the freezing transition disappears.

Critical depletion.

Depletion interactions and the critical Casimir effect are usually regarded as distinct phenomena in colloidal suspensions. By experimentally investigating how the Asakura-Oosawa picture, appropriate for a weakly correlated depletant, is modified when critical correlations develop within the depletion agent, we conversely show that the former merges continuously into the latter, leading to a distinctive scaling behavior solely dictated by the depletant correlation length. A model based on density functional theory provides a microscopic understanding of the phenomenon and properly accounts for the observed trends.

Enhancement of depletion forces by electrostatic depletant repulsion.

A large variety of engaging phenomena stems from the occurrence of short-ranged attractive depletion forces. Yet, so far, most experimental studies have been interpreted on the basis of the simple Asakura-Oosawa model, where the depletion agent can be regarded as ideal. Here, conversely, we focus on a system where strong electrostatic coupling is present in the suspension. Specifically, from measurements of equilibrium sedimentation profiles, we obtain an equation of states for a colloidal system where depletion forces are tuned by the addition of a surfactant. At fixed colloid volume fraction, colloidal aggregation takes place when the surfactant concentration reaches a critical value which rises for increasing ionic strength. Screening repulsive electrostatic interactions inhibits the depletion mechanism and weakens the effective colloid-colloid attraction. The metastable coexistence curve displays the universal scaling behavior predicted for short-ranged potentials. The experimental data are compared with the theoretical predictions of a simple model which includes only electrostatic interactions. The effective depletion force on the colloids is evaluated by using the hypernetted-chain equation of liquid state theory at different salt concentrations. This model provides a convincing interpretation of the observed enhancement of the depletion mechanism by Coulomb repulsion.

Elastically driven intermittent microscopic dynamics in soft solids.

Soft solids with tunable mechanical response are at the core of new material technologies, but a crucial limit for applications is their progressive aging over time, which dramatically affects their functionalities. The generally accepted paradigm is that such aging is gradual and its origin is in slower than exponential microscopic dynamics, akin to the ones in supercooled liquids or glasses. Nevertheless, time- and space-resolved measurements have provided contrasting evidence: dynamics faster than exponential, intermittency and abrupt structural changes. Here we use 3D computer simulations of a microscopic model to reveal that the timescales governing stress relaxation, respectively, through thermal fluctuations and elastic recovery are key for the aging dynamics. When thermal fluctuations are too weak, stress heterogeneities frozen-in upon solidification can still partially relax through elastically driven fluctuations. Such fluctuations are intermittent, because of strong correlations that persist over the timescale of experiments or simulations, leading to faster than exponential dynamics.

Dynamical heterogeneity in the supercooled liquid state of the phase change material GeTe.

A contending technology for nonvolatile memories of the next generation is based on a remarkable property of chalcogenide alloys known as phase change materials, namely their ability to undergo a fast and reversible transition between the amorphous and crystalline phases upon heating. The fast crystallization has been ascribed to the persistence of a high atomic mobility in the supercooled liquid phase, down to temperatures close to the glass transition. In this work we unravel the atomistic, structural origin of this feature in the supercooled liquid state of GeTe, a prototypical phase change compound, by means of molecular dynamic simulations. To this end, we employed an interatomic potential based on a neural network framework, which allows simulating thousands of atoms for tens of ns by keeping an accuracy close to that of the underlying first-principles framework. Our findings demonstrate that the high atomic mobility is related to the presence of clusters of slow and fast moving atoms. The latter contain a large fraction of chains of homopolar Ge-Ge bonds, which at low temperatures have a tendency to move by discontinuous cage-jump rearrangements. This structural fingerprint of dynamical heterogeneity provides an explanation of the breakdown of the Stokes-Einstein relation in GeTe, which is the ultimate origin of the fast crystallization of phase change materials exploited in the devices.

When depletion goes critical.

Depletion interactions in correlated fluids are investigated both theoretically and experimentally. A formally exact derivation of a general expression for depletion interactions is presented and then specialized to the case of critical correlations in the depletant by employing a long wavelength approximate analysis. A scaling expression is obtained in the critical region, suggesting a close connection to the critical Casimir effect. As a result we are able to compute the full scaling function of the critical Casimir effect in terms of the known scaling form of the depletant equation of state. These predictions are experimentally tested in a colloidal suspension with a micellar solution as depletion agent. Colloids are seen to aggregate reversibly when the micellar concentration exceeds a temperature dependent value which becomes remarkably small as the temperature approaches the lower consolution point of the micellar suspension. Continuity between the standard depletion picture at low temperature and the Casimir effect in the critical region is demonstrated by identifying several approximate scaling laws which compare favorably with the theoretical analysis. The transition line is seen to lie close to the curve of maximum susceptibility of the depletant. A model, analyzed within mean field approximation, is shown to reproduce the main qualitative features of the phenomenon.