We report on the response of semi-dilute shear-banding wormlike micelles to time-dependent flow protocols such as step stress and shear startup. We use global rheology coupled to direct optical visualisations in Taylor-Couette flow to provide a detailed 2D description of the spatio-temporal dynamics of the shear banding flow under imposed stress. We compare these dynamics to their extensively studied counterpart under imposed shear rate. We show that, for a given asymptotic state, the onset of banding is delayed and the transients are longer when the stress is used as control parameter. The way the banding structure is built up strongly depends on the control parameter. In the two cases, secondary flows develop on top of the shear-banding instability: after shear startup, the interface between bands is formed in the bulk of the system and migrates inwards to its stationary position in the gap of the Taylor-Couette device, before being destabilized due to elastic instability; under creep flow, the interface between bands forms at the inner wall and becomes wavy while it is still migrating outwards to its steady position, leading to permanent rearrangements of the flow structure until steady state is reached, potentially explaining longer transients. We also outline the possible mechanical signature of the onset of shear banding in terms of shape of the bulk mechanical signals, which is the subject of the second part of this work [Briole et al., submitted].

Understanding the complex structure of polymerblends filled with nanoparticles (NPs) is key to design theirmacroscopic properties. Here, the spatial distribution of hydrogenated (H) and deuterated (D) polymer chains asymmetric in mass is studied by small-angle neutron scattering. Depending on the chain mass, a qualitatively new large-scale organization of poly(vinyl acetate) chains beyond the random-phase approximation is evidenced in nanocomposites with attractive polymer−silica interactions. The silica is found to systematically induce bulk segregation. Only with long H-chains, a strong scattering signature is observed in the q range of the NP size: it is the sign of interfacial isotopic enrichment, that is, of contrasted polymer shells close to the NP surface. A quantitative model describing both the bulk segregation and the interfacial gradient (over ca. 10−20 nm depending on the NP size) is developed, showing that both are of comparable strength. In all cases, NP surfaces trap thepolymer blend in a non-equilibrium state, with preferential adsorption around NPs only if the chain length and isotopic preference toward the surface combine their entropic and enthalpic driving forces. This structural evidence for interfacial polymer gradients willopen the road for quantitative understanding of the dynamics of many-chain nanocomposite systems.

We present a theoretical study of near-field radiative thermal rectification combining phase-transition and high-infrared-transmittance materials. The phase-transition material vanadium dioxide (VO2), with a metal–insulator transition near 341 K, is utilized under a reasonable temperature. Four types of high-infrared-transmittance materials, including potassium bromide, sodium chloride, polyethylene, andmagnesium fluoride, are introduced as thin film substrates under a VO2 grating on one side of the near-field rectifier. We explore the effects of various high-infrared-transmittance thin-film substrates and relevant geometric parameters on the thermal rectification of the device. The results show that thermal rectification can be greatly enhanced by using a one-dimensional VO2 grating backed with a high-infraredtransmittance thin-film substrate. With the introduction of a high-infrared-transmittance substrate, the rectification ratio is dramatically boosted due to the enhancement of the substrate transmittance. This work predicts a remarkable rectification ratio as high as 161—greater than the recently reported peak values for comparable near-field radiative thermal rectification. The results outlined herein will shed light on the rapidly expanding fields of nanoscale thermal harvesting, conversion, and management.