Light-Driven Dynamic Adhesion on Photosensitized Nematic Liquid Crystalline Elastomers.

In liquid crystal elastomers (LCEs), the internal mechanical loss increases around the nematic-isotropic phase transition and remains high all through the nematic phase, originating from the internal orientational relaxation related to the so-called "soft elasticity". Because the viscoelastic dissipation of the materials affects their adhesion properties, the nematic-isotropic phase transition can cause dramatic changes in the adhesion strength. Although the phase transitions can generally be induced by heat, here, we demonstrate the light-driven transition in dynamic adhesion in dye-doped nematic LCE. The special dye is chosen to efficiently generate local heat on light absorption. The adhesion strength is lowered with fine tunability depending on the light power, which governs the effective local temperature and through that the viscoelastic damping of the system. We demonstrate the light-assisted dynamic control of adhesion in a 90°-peel test and in pick-and-release of objects, which may lead to the development of stimuli-responsive adhesive systems with fine spatio-temporal controls.ERC H2020 Advanced grant No: 786659

The influence of elastic thickness non-uniformity on viscoelastic crustal response to magma emplacement: application to the Kutcharo caldera, eastern Hokkaido, Japan

An elastic layer plays an important role in deformation of the crust. At active volcanoes, its thickness would be effectively thinned by a higher geothermal gradient, particularly in a region beneath which magmatic activity is relatively high. This study examines the influence of elastic thickness non-uniformity on viscoelastic crustal deformation by magma emplacement. A 3-D linear Maxwell viscoelastic model is employed, in which an elastic layer underlain by a viscoelastic layer with a spatially uniform viscosity is thinned to be hi in the volcano centre, compared with hi + Δh in the peripheral regions, and a sill-like magma emplacement occurs in the upper layer beneath the centre. It is found that the post-emplacement viscoelastic subsidence is diminished or enhanced by the elastic thickness non-uniformity, depending on whether or not the horizontal width of the magma emplacement (ωs) is greater than the horizontal width (ωe) over which the elastic layer is thinner. The available signature of the non-uniformity is explored by comparison with a model that has a spatially uniform elastic thickness (UET) of hi. If an apparent viscosity (ηa) of the UET model is adjusted so that the difference in post-emplacement subsidence is minimized at the deformation centre, the non-uniformity appears in the overall deformation field as a displacement anomaly over the perimeter of the sill in which viscoelastic subsidence is greater for the non-uniform model. The anomaly is, however, by no more than the magnitude of ∼15 per cent of the maximal syn-emplacement uplift, though ηa is necessarily modified to be ∼0.2–10 times the non-uniform model viscosity (ηc). If ωe is larger than a few times ωs, a weak signature is no longer expected in the deformation field, and ηa is not significantly deviated from ηc. Since the signature appears so faintly in a displacement field, the InSAR data in the Kutcharo caldera for a period from 1993 August 13 to 1998 June 9 do not allow us to capture the non-uniformity. However, it can be concluded that if ωe beneath the caldera is comparable with or greater than the topographic caldera diameter (ωc) as implied by the spatial variation of the geothermal gradient, the non-uniformity has no significant influence. Otherwise, if ωe < ωc, the non-uniformity influences the estimation of the crustal viscosity, but does not affect the overall deformation field. The elastic thickness non-uniformity can be theoretically captured in the deformation field, but in practice, its influence, particularly on estimating crustal viscosity, cannot be properly inferred without other geophysical data such as the geothermal gradient in and around the caldera

Internal constraints and arrested relaxation in main-chain nematic elastomers

Abstract: Nematic liquid crystal elastomers (N-LCE) exhibit intriguing mechanical properties, such as reversible actuation and soft elasticity, which manifests as a wide plateau of low nearly-constant stress upon stretching. N-LCE also have a characteristically slow stress relaxation, which sometimes prevents their shape recovery. To understand how the inherent nematic order retards and arrests the equilibration, here we examine hysteretic stress-strain characteristics in a series of specifically designed main-chain N-LCE, investigating both macroscopic mechanical properties and the microscopic nematic director distribution under applied strains. The hysteretic features are attributed to the dynamics of thermodynamically unfavoured hairpins, the sharp folds on anisotropic polymer strands, the creation and transition of which are restricted by the nematic order. These findings provide a new avenue for tuning the hysteretic nature of N-LCE at both macro- and microscopic levels via different designs of polymer networks, toward materials with highly nonlinear mechanical properties and shape-memory applications

Zigzag line defects and manipulation of colloids in a nematic liquid crystal in microwrinkle grooves

Spatially confined liquid crystals exhibit non-uniform alignment, often accompanied by self-organised topological defects of non-trivial shape in response to imposed boundary conditions and geometry. Here we show that a nematic liquid crystal, when confined in a sinusoidal microwrinkle groove, exhibits a new periodic arrangement of twist deformations and a zigzag line defect. This periodic ordering results from the inherent liquid crystal elastic anisotropy and the antagonistic boundary conditions at the flat liquid crystal–air and the curved liquid crystal–groove interfaces. The periodic structure can be tuned by controlling the groove geometry and the molecular chirality, which demonstrates the importance of boundary conditions and introduced asymmetry for the engineering of topological defects. Moreover, the kinks in the zigzag defects can trap small particles, which may afford a new method for manipulation of colloids. Our system, which uses easily fabricated microwrinkle grooves, provides a new microfabrication method based on the arrangement of controllable defects

Rapid resolution of femoral head osteonecrosis after rotational acetabular osteotomy

The natural history of osteonecrosis of the femoral head is generally thought to be one of progressive deterioration if no intervention is undertaken. However, it is unknown whether surgical intervention is beneficial for patients with a small region of osteonecrosis. We observed rapid improvement of MRI findings after rotational acetabular osteotomy (RAO) was performed in a young patient with osteonecrosis of the femoral head. The band-like low signal area on T2-weighted images almost resolved by six months after surgery. He returned to work as an electrician by six months after surgery. Early surgical intervention such as RAO that alters the mechanical force acting on the necrotic region of the femoral head may accelerate the recovery of osteonecrosis and the improvement of symptoms

Clues from joint inversion of tsunami and geodetic data of the 2011 Tohoku-oki earthquake

The 2011 Tohoku-oki (Mw 9.1) earthquake is so far the best-observed megathrust rupture, which allowed the collection of unprecedented offshore data. The joint inversion of tsunami waveforms (DART buoys, bottom pressure sensors, coastal wave gauges, and GPS-buoys) and static geodetic data (onshore GPS, seafloor displacements obtained by a GPS/acoustic combination technique), allows us to retrieve the slip distribution on a non-planar fault. We show that the inclusion of near-source data is necessary to image the details of slip pattern (maximum slip ~48 m, up to ~35 m close to the Japan trench), which generated the large and shallow seafloor coseismic deformations and the devastating inundation of the Japanese coast. We investigate the relation between the spatial distribution of previously inferred interseismic coupling and coseismic slip and we highlight the importance of seafloor geodetic measurements to constrain the interseismic coupling, which is one of the key-elements for long-term earthquake and tsunami hazard assessment