Shear-induced damped oscillations in an epithelium depend on actomyosin contraction and E-cadherin cell adhesion.

Shear forces between cells occur during global changes in multicellular organization during morphogenesis and tissue growth, yet how cells sense shear forces and propagate a response across a tissue is unknown. We found that applying exogenous shear at the midline of an epithelium induced a local, short-term deformation near the shear plane, and a long-term collective oscillatory movement across the epithelium that spread from the shear-plane and gradually dampened. Inhibiting actomyosin contraction or E-cadherin trans-cell adhesion blocked oscillations, whereas stabilizing actin filaments prolonged oscillations. Combining these data with a model of epithelium mechanics supports a mechanism involving the generation of a shear-induced mechanical event at the shear plane which is then relayed across the epithelium by actomyosin contraction linked through E-cadherin. This causes an imbalance of forces in the epithelium, which is gradually dissipated through oscillatory cell movements and actin filament turnover to restore the force balance across the epithelium

Nonchaotic Nonlinear Motion Visualized in Complex Nanostructures by Stereographic 4D Electron Microscopy

Direct electron imaging with sufficient time resolution is a powerful tool for visualizing the three-dimensional (3D) mechanical motion and resolving the four-dimensional (4D) trajectories of many different components of a nanomachine, e.g., a NEMS device. Here, we report a nanoscale nonchaotic motion of a nano- and microstructured NiTi shape memory alloy in 4D electron microscopy. A huge amplitude oscillatory mechanical motion following laser heating is observed repetitively, likened to a 3D motion of a conductor’s baton. By time-resolved 4D stereographic reconstruction of the motion, prominent vibrational frequencies (3.0, 3.8, 6.8, and 14.5 MHz) are fully characterized, showing evidence of nonlinear behavior. Moreover, it is found that a stress (fluence)−strain (displacement) profile shows nonlinear elasticity. The observed resonances of the nanostructure are reminiscent of classical molecular quasi-periodic behavior, but here both the amplitude and frequency of the motion are visualized using ultrafast electron microscopy

Elasto-buoyant heavy spheres: a unique way to test non-linear elasticity

Extra-large deformations in ultra-soft elastic materials are ubiquitous, yet systematic studies and methods to understand the mechanics of such huge strains are lacking. Here we investigate this complex problem systematically with a simple experiment: by introducing a heavy bead of radius $a$ in an incompressible ultra-soft elastic medium. We find a scaling law for the penetration depth ($\delta$) of the bead inside the softest gels as $\delta \sim a^{3/2}$. While this result is inconsistent with an ideal neo-Hookean model of elastic deformation, according to which the displacement fields must diverge, it is vindicated by an original asymptotic analytic model developed in this article. This model demonstrates that the observed relationship is precisely at the demarcating boundary of what would be required for the field variables to either diverge or converge. This correspondence between a unique mathematical prediction and the experimental observation ushers in new insights into the behavior of the deformations of strongly non-linear materials

Mitigation of Frame Acceleration Induced by a Buried Charge

In this thesis, methods to mitigate acceleration delivered to the frame of a vehicle with an attached v-shaped hull are investigated. The frame of a vehicle represents an alternative location for crew seating, as opposed to seats being secured to the floorboard. Mitigation techniques were investigated for three test setups: aluminum frame with a downwardly convex aluminum hull, steel frame with a downwardly convex steel hull, and a steel frame with a downwardly concave steel hull. Accelerations of the frame were measured using piezoelectric accelerometers placed at three different locations on the frame. These acceleration measurements were verified against video recorded by high speed cameras. Each test was intended to reduce peak accelerations experienced by the frame, and to reduce the width of the acceleration envelope at large g levels. Mitigation techniques focused on reducing the initial hull-frame interactions, while damping subsequent responses of the system. Mitigation systems and hull orientation were compared for their ability to reduce blast effects experienced by the frame

From Statistical Correlations to Stochasticity and Size Effects in Sub-Micron Crystal Plasticity

Metals in small volumes display a strong dependence on initial conditions, which translates into size effects and stochastic mechanical responses. In the context of crystal plasticity, this amounts to the role of pre-existing dislocation configurations that may emerge due to prior processing. Here, we study a minimal but realistic model of uniaxial compression of sub-micron finite volumes. We show how the statistical correlations of pre-existing dislocation configurations may influence the mechanical response in multi-slip crystal plasticity, in connection to the finite volume size and the initial dislocation density. In addition, spatial dislocation correlations display evidence that plasticity is strongly influenced by the formation of walls composed of bound dislocation dipoles

Polymer nematic liquid crystals :: disclination structure and interaction/

Disclinations and inversion walls in the director field of nematic thermotropic liquid crystal polymers (TLCP\u27s) are imaged at high resolution using the lamellar decoration technique of Wood and Thomas. The utility of the lamellar decoration technique has also been greatly extended by using an etch and replication technique to image the director in sections of a bulk sample. The interaction of disclinations in the presence of an applied magnetic or extensional flow field which tends to align the director is studied. At intermediate field strengths, where the disclination separation is comparable to the characteristic length for director field distortions, pairwise interaction no longer dominates, but clusters of disclinations are observed to form. The applied field suppresses fluctuations of the disclination positions which increase the topological dipole moment of a cluster while enhancing those that preserve zero dipole. The applied field tends to minimize the long range distortional energy which is proportional to the square of the dipole moment. As a result, quadrupolar (Lehmann) clusters of disclinations are commonly observed in either magnetically or extensionally flow aligned samples. Many-bodied interaction of disclinations in the absence of an applied field is studied via computer simulation, and the results agree favorably with recent experimental work. The structure of the cores of disclinations has been observed. The core size is on the order of a few molecular lengths, and its structure depends upon polymer architecture. A rigid polymer splays more within the core of a $\pi$ wedge disclination, and an excess of chain ends aggregates. A semi-flexible polymer, in contrast, bends more within the core. Hairpins are not thought to be abundant in the semi-flexible TLCP. The morphology of the TLCP semi-crystalline state has also been examined. It is observed to depend upon the relative primary crystal nucleation and crystal growth rates. The lamellar decoration morphology, where the crystalline lamellae are everywhere perpendicular to the director of the precursor nematic, is favored by rapid nucleation and slow growth. This morphology is unique to TLCP\u27s, but other morphologies are possible. Spherulites have also been grown from the nematic melt. Their growth is favored by slow nucleation and rapid growth

Viscoelastic and Viscoplastic Materials

This book introduces numerous selected advanced topics in viscoelastic and viscoplastic materials. The book effectively blends theoretical, numerical, modeling and experimental aspects of viscoelastic and viscoplastic materials that are usually encountered in many research areas such as chemical, mechanical and petroleum engineering. The book consists of 14 chapters that can serve as an important reference for researchers and engineers working in the field of viscoelastic and viscoplastic materials