In this Letter, active control of near-field radiative heat transfer (NFRHT) between two isotropic materials is realized by a coating-twisting method. The two slabs are coated with graphene gratings, and then the NFRHT can be not only enhanced but also weakened, by tuning the twisted angle between the two gratings. The physical mechanism is attributed to the modes coupled by the graphene gratings and the isotropic material, which can vary with the twisted angle. The proposed method is also applicable for other kinds of anisotropic films and may provide a way to realize high-precision nanoscale thermal management, nimble thermal communications, and thermal switch.

We propose a method relying on structural measurements by small-angle scattering to quantitatively follow aggregation of nanoparticles (NPs) in concentrated colloidal assemblies or suspensions up to percolation, regardless of complex structure factors arising due to interactions. As experimental model system, the dispersion of silica NPs in a styrene-butadiene matrix has been analyzed by small-angle X-ray scattering and transmission electron microscopy (TEM), as a function of particle concentration. A reverse Monte Carlo analysis applied to the NP scattering compared favorably with TEM. By combining it with an aggregate recognition algorithm, series of representative real space structures and aggregation number distribution functions have been determined up to high concentrations, taking into account particle polydispersity. Our analysis demonstrates that the formation of large percolating aggregates on the scale of the simulation box (of linear dimension 1/qmin, here micron-sized) can be mapped onto the macroscopic percolation characterized by rheology. Our method is thus capable of determining aggregate structure in dense NP systems with strong – possibly unknown – interactions visible in scattering. It is hoped to be useful in many other colloidal systems, beyond the case of polymer nanocomposites exemplarily studied here.

Amorphous solids display a ductile to brittle transition as the kinetic stability of the quiescent glass is increased, which leads to a material failure controlled by the sudden emergence of a macroscopic shear band in quasi-static protocols. We numerically study how finite deformation rates influence ductile and brittle yielding behaviors using model glasses in two and three spatial dimensions. We find that a finite shear rate systematically enhances the stress overshoot of poorly-annealed systems, without necessarily producing shear bands. For well-annealed systems, the non-equilibrium discontinuous yielding transition is smeared out by finite shear rates and it is accompanied by the emergence of multiple shear bands that have been also reported in metallic glass experiments. We show that the typical size of the bands and the distance between them increases algebraically with the inverse shear rate. We provide a dynamic scaling argument for the corresponding lengthscale, based on the competition between the deformation rate and the propagation time of the shear bands.

Inspired by the path followed by Matthias Ballauff over the past 20 years, the development of thermosensitive core-shell microgel structures is reviewed. Different chemical approaches, from hard nanoparticle cores to double stimuli-responsive microgels have been devised and successfully implemented by many different groups. Some of the rich variety of these systems is presented, as well as some recent progress in structural analysis of such microstructures by small-angle scattering of neutrons or X-rays, including modeling approaches. In the last part, again following early work by the group of Matthias Ballauff, applications with particular emphasis on incorporation of catalytic nanoparticles inside core-shell structures – stabilizing the nanoparticles and granting external control over activity – will be discussed, as well as core-shell microgels at interfaces.

The radial density profile of deuterated poly(N,n-propyl acrylamide) shell monomers within core-shell microgels has been studied by small angle neutron scattering in order to shed light on the origin of their linear thermally-induced swelling. The poly(N-isopropyl methacrylamide) core monomers have been contrast-matched by the H2O/D2O solvent, and the intensity thus provides a direct measurement of the spatial distribution of the shell monomers. Straightforward modelling shows that their structure does not correspond to the expected picture of a well-defined external shell. A multi-shell model solved by a reverse Monte Carlo approach is then applied to extract the monomer density as a function of temperature and of the core crosslinking. It is found that most shell monomers fill the core at high temperatures approaching synthesis conditions of collapsed particles, forming only a dilute corona. As the core monomers tend to swell at lower temperatures, a skeleton of insoluble shell monomers hinders swelling, inducing the progressive linear thermoresponse.

Unidirectional surface plasmon polaritons (SPPs) at the interface between a gyrotropic medium and a simple medium are studied in a newly recognized frequency regime wherein the SPPs form narrow, beam-like patterns due to quasi-hyperbolic dispersion. The SPP beams are steerable by controlling parameters such as the cyclotron frequency (external magnetic bias) or the frequency of operation. The bulk band structure along different propagation directions is examined to ascertain a common bulk bandgap, valid for all propagation directions, which the SPPs cross. In addition, group velocity and Poynting vector for the SPPs are presented. The case of a finite-thickness gyrotropic slab is also considered, for which we present the Green function and examine the thickness and loss level required to maintain a unidirectional SPP.

The negative thermal expansion material potassium cadmium dicyanoargentate, KCd[Ag(CN) 2 ] 3 , is studied at high pressure using a combination of X-ray single-crystal diffraction, X-ray powder diffraction, infrared and Raman spectroscopy, and density functional theory calculations. In common with the isostructural manganese analogue, KMn[Ag(CN) 2 ] 3 , this material is shown to exhibit very strong negative linear compressibility (NLC) in the crystallographic c direction due to structure hinging. We find increased structural flexibility results in enhanced NLC and NTE properties, but this also leads to two pressure-induced phase transitionsto very large unit cells involving octahedral tilting and shearing of the structurebelow 2 GPa. The presence of potassium cations has an important effect on the mechanical and thermodynamic properties of this family, while the chemical versatility demonstrated here is of considerable interest to tune unusual mechanical properties for application. ■ INTRODUCTION Materials where external stimuli induces an anomalous response, such as expansion upon cooling (negative thermal expansion, NTE 1−3) or on application of hydrostatic pressure (negative linear compressibility, NLC 4,5), have recently received considerable attention. These materials that "break the rules"as a result of specific, and therefore rare, elastic anomalies 6−8 are curious from a fundamental point of view but might also be revolutionary for a range of technologies including interferometric pressure sensors, pressure-controlled or pressure-sensitive electronic devices, and smart actua-tion. 2,4,9−11 The ability to design materials with unusual properties is therefore a key focus of the field: can we understand these elastic anomalies so that we can tune the magnitude and/or range of negative responses? Negative thermal expansion may occur in one, two, or all principal directions without violating thermodynamics. 12,13 Thus, for example, the volume coefficient of thermal expansion (CTE), () V V V T p 1 α = ∂ ∂ , takes large negative values in zirconium tungstate ZrW 2 O 8 and zinc(II) cyanide Zn(CN) 2 over large temperature ranges. 14 Under hydrostatic pressure, however, volume must always reduce and so the volume compressibility (() K V V V p T 1 = − ∂ ∂ , conventionally formulated as the bulk modulus, B = (K V) −1), must be definite and positive. 15 Expansion under pressure can occur in one or two directions, as long as this is coupled to volume reduction. 5,12,16 Therefore, negative linear compressibility (NLC)the expansion in one principal direction of a material under hydrostatic pressureis conceptually and empirically related to linear-NTE. In both cases, anomalous response relies on mechanical anisotropy. The so-called "wine-rack" model is often invoked where mechanical hinging allows linear expansion coupled to volume reduction on increasing pressure or decreasing temperature. 5,8 Attention in this field has focused on a class of materials known as coordination polymers (CPs), in particular due to the very large magnitude of responses found in comparison to conventional oxide-based materials. 7 Examples that show large NTE and NLC include cyanide frameworks such as Ag 3 [Co-(CN) 6 ] (refs 17 and 18), KMn[Ag(CN) 2 ] 3 (refs 19 and 20), Zn[Au(CN) 2 ] 2 (refs 21 and 22), and metal−organic frameworks 23 (MOFs) such as MIL-53 (ref 24), UTSA-16 (ref 25), ZAG-4 (ref 26), and [NH 4 ][Zn(HCOO) 3 ] (ref 27). In many cases, mechanical response can be predicted in these systems by considering the framework as a set of rigid connectors and hinges, sodepending on geometry and topologyNTE or