Extensile Stresses Research Articles

Molecular-scale substrate anisotropy, crowding and division drive collective behaviours in cell monolayers.

The ability of cells to reorganize in response to external stimuli is important in areas ranging from morphogenesis to tissue engineering. While nematic order is common in biological tissues, it typically only extends to small regions of cells interacting via steric repulsion. On isotropic substrates, elongated cells can co-align due to steric effects, forming ordered but randomly oriented finite-size domains. However, we have discovered that flat substrates with nematic order can induce global nematic alignment of dense, spindle-like cells, thereby influencing cell organization and collective motion and driving alignment on the scale of the entire tissue. Remarkably, single cells are not sensitive to the substrate's anisotropy. Rather, the emergence of global nematic order is a collective phenomenon that requires both steric effects and molecular-scale anisotropy of the substrate. To quantify the rich set of behaviours afforded by this system, we analyse velocity, positional and orientational correlations for several thousand cells over days. The establishment of global order is facilitated by enhanced cell division along the substrate's nematic axis, and associated extensile stresses that restructure the cells' actomyosin networks. Our work provides a new understanding of the dynamics of cellular remodelling and organization among weakly interacting cells.

Some contributions to mechanical behaviors of lacustrine deposit in Bogotá, Colombia

This paper presents the mechanical behaviors of the lacustrine deposit, a representative soil in Bogotá, Colombia. Initially, the physical characterization of the deposit is performed via laboratory tests (grain size distribution, scanning electron microscopy, Atterberg limits and water content). This characterization intends to explain the special characteristics of the mechanical behaviors of this soil. Then, various triaxial tests are carried out with controlled loading path, strain rate change, relaxation, extensile stress, and cyclic loading. The test results reveal the shape of the yield curve for Bogotá soil (in a natural state), and also show that an increasing effect of the strain rate depends on the liquid limit. This effect is also preserved with extensile stresses (which are poorly studied in soil mechanics). Finally, other effects, such as the loss of structure in the reconstituted samples and the effect of shear modulus at low strains, are studied for Bogotá soil.

Geometry and Mechanics of Microdomains in Growing Bacterial Colonies

Bacterial colonies are abundant on living and nonliving surfaces and are known to mediate a broad range of processes in ecology, medicine, and industry. Although extensively researched, from single cells to demographic scales, a comprehensive biomechanical picture, highlighting the cell-to-colony dynamics, is still lacking. Here, using molecular dynamics simulations and continuous modeling, we investigate the geometrical and mechanical properties of a bacterial colony growing on a substrate with a free boundary and demonstrate that such an expanding colony self-organizes into a "mosaic" of microdomains consisting of highly aligned cells. The emergence of microdomains is mediated by two competing forces: the steric forces between neighboring cells, which favor cell alignment, and the extensile stresses due to cell growth that tend to reduce the local orientational order and thereby distort the system. This interplay results in an exponential distribution of the domain areas and sets a characteristic length scale proportional to the square root of the ratio between the system orientational stiffness and the magnitude of the extensile active stress. Our theoretical predictions are finally compared with experiments with freely growing E. coli microcolonies, finding quantitative agreement.

Self-Driven Droplet Powered By Active Nematics.

Active matter defines a class of far-away-from-equilibrium systems comprising self-driven microparticles. Their anomalous physical properties could be applied in areas such as mixing or separation, micropumps, and self-healing materials. To realize such applications, a thorough understanding of the physical mechanisms as well as the development of methods to manipulate various active systems is required. Using a coarse-grained active liquid crystal model, we designed and investigated a single self-driven droplet which encapsulated a dense suspension comprising nonmotile but mobile active particles that generate extensile stresses. We showed that such droplets can be driven into motion and can have tunable mobilities owing to their internal collective motion, which is characterized by induced active flows and motile disclination defects. Furthermore, it was illustrated that the interplay among the internal directional flows, liquid crystalline structures, droplet size, and surface tension resulted in different types of locomotion and rotation.

Hydrodynamic instabilities in active cholesteric liquid crystals

.We describe the basic properties and consequences of introducing active stresses, with principal direction along the local director, in cholesteric liquid crystals. The helical ground state is found to be linearly unstable to extensile stresses, without threshold in the limit of infinite system size, whereas contractile stresses are hydrodynamically screened by the cholesteric elasticity to give a finite threshold. This is confirmed numerically and the non-linear consequences of instability, in both extensile and contractile cases, are studied. We also consider the stresses associated to defects in the cholesteric pitch (lambda lines) and show how the geometry near to the defect generates threshold-less flows reminiscent of those for defects in active nematics. At large extensile activity lambda lines are spontaneously created and can form steady-state patterns sustained by constant active flows.Graphical abstract

Excess Pore Pressures Around Underground Structures Following Earthquake Induced Liquefaction

Underground structures located in liquefiable soil deposits are susceptible to floatation following an earthquake event due to their lower unit weight relative to the surrounding saturated soil. This inherent buoyancy may cause lightweight structures to float when the soil liquefies. Centrifuge tests have been carried out to study the excess pore pressure generation and dissipation in liquefiable soils. In these tests, near full liquefaction conditions were attained within a few cycles of the earthquake loading. In the case of high hydraulic conductivity sands, significant dissipation could take place even during the earthquake loading which inhibits full liquefaction from occurring. In the case of excess pore pressure generation and dissipation around a floating structure, the cyclic response of the structure may lead to the reduction in excess pore pressure near the face of the structure as compared to the far field. This reduction in excess pore pressure is due to shear-induced dilation and suction pressures arising from extensile stresses at the soil-structure interface. Given the lower excess pore pressure around the structure; the soil around the structure retains a portion of this shear strength which in turn can discourage significant uplift of the underground structure.

A drop of active matter

AbstractWe study theoretically the hydrodynamics of a fluid drop containing oriented filaments endowed with active contractile or extensile stresses and placed on a solid surface. The active stresses alter qualitatively the wetting properties of the drop, leading to new spreading laws and novel static drop shapes. Candidate systems for testing our predictions include cytoskeletal extracts with motors and ATP, suspensions of bacteria or pulsatile cells, or fluids laden with artificial self-propelled colloids.