Causal nature of the acoustic response, for any materials or structures, dictates an inequality that relates the absorption spectrum of the sample to its thickness. We present a general recipe for constructing sound-absorbing structures that can attain near-equality for the causal relation with very high absorption performance; such structures are denoted "optimal." Our strategy involves using carefully designed acoustic metamaterials as backing to a thin layer of conventional sound absorbing material, e.g., acoustic sponge. By using this design approach, we have realized a 12 cm-thick structure that exhibits broadband, near-perfect flat absorption spectrum starting at around 400 Hz. From the causal relation, the calculated minimum sample thickness is 11.5 cm for the observed absorption spectrum. We present the theory that underlies such absorption performance, involving the evanescent waves and their interaction with a dissipative medium, and show the excellent agreement with the experiment.

We designed a metamaterial field rotator that can rotate electromagnetic wave fronts. Our starting point was the transformation-media concept. Effective medium theories and full simulations facilitated the actual design process. We created at a very simple structure comprising of an array of identical aluminum metal plates. We made and measured a sample and we experimentally demonstrated the field rotation effect as well as the broadband functionality at microwave frequencies.

Electrorheological fluids constitute a type of colloids that can vary their rheological characteristics upon the application of an electric field. The recently discovered giant electrorheological (GER) effect breaks the upper bound of the traditional ER effect, but a microscopic explanation is still lacking. By using molecular dynamics to simulate the urea-silicone oil mixture trapped in a nanocontact between two polarizable particles, we demonstrate that the electric field can induce the formation of aligned (urea) dipolar filaments that bridge the two boundaries of the nanoscale confinement. This phenomenon is explainable on the basis of a 3D to 1D crossover in urea molecules' microgeometry, realized through the confinement effect provided by the oil chains. The resulting electrical energy density yields an excellent account of the observed GER yield stress variation as a function of the electric field. Electrorheological (ER) fluids [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15] are a type of colloidal dispersions which can vary their rheological characteristics through the application of an external electric field. The traditional ER mechanism is based on induced polarizations arising from the dielectric constant contrast between the solid particles and the fluid [6,12]. The recent discovery of the giant electrorheological (GER) effect [7][8][9][10][11][12], in urea-coated barium titanyl-oxalate nanoparticles ½NH 2 CONH 2 @BaTiOðC 2 O 4 Þ 2 , or BTRU for short, dispersed in silicone oil, has shown that the theoretical upper bound of the ER effect is no longer applicable to this new type of materials. Instead, a phenomenological model of the GER mechanism, based on aligned urea molecular dipoles in the small contact regions of the nanoparticles, yielded an adequate account of the observed effect [7,9,12]. However, a microscopic picture of how this can occur has so far eluded persistent efforts. Moreover, as the GER effect is highly sensitive to whether the dispersing oil can wet the solid particles [10,11], in contrast to the traditional ER fluids, a natural question is how this observation can be integrated into a coherent GER mechanism. In view of the fact that the GER effect has now been reproduced in many different material systems and therefore is becoming a much more general effect [14,15], answers to the above questions would not only be timely, but may also shed light on how to devise general strategies for harnessing and controlling the large electric energy stored in molecular dipoles.In this work we use molecular dynamics (MD) simulations to show that in a mixture of urea molecules with silicone oil chains confined between two bounding surfaces (denoted as substrates below) of a nanoscale contact, aligned urea molecular dipoles can form filaments snaking through the pores of the oil film to bridge the substrates. The required electric field for aligning the urea dipoles is found to be lowered by a factor of 2 to 3 in the presence of the oil chains, compared to that without the oil chains. More...

Freshwater flux and energy consumption are two important benchmarks for the membrane desalination process. Here, we show that nanoporous carbon composite membranes, which comprise a layer of porous carbon fibre structures grown on a porous ceramic substrate, can exhibit 100% desalination and a freshwater flux that is 3-20 times higher than existing polymeric membranes. Thermal accounting experiments demonstrated that the carbon composite membrane saved over 80% of the latent heat consumption. Theoretical calculations combined with molecular dynamics simulations revealed the unique microscopic process occurring in the membrane. When the salt solution is stopped at the openings to the nanoscale porous channels and forms a meniscus, the vapour can rapidly transport across the nanoscale gap to condense on the permeate side. This process is driven by the chemical potential gradient and aided by the unique smoothness of the carbon surface. The high thermal conductivity of the carbon composite membrane ensures that most of the latent heat is recovered.

Effectively characterizing the behavior of deformable objects has wide applicability but remains challenging. We present a new rotation-invariant deformation representation and a novel reconstruction algorithm to accurately reconstruct the positions and local rotations simultaneously. Meshes can be very efficiently reconstructed from our representation by matrix predecomposition, while, at the same time, hard or soft constraints can be flexibly specified with only positions of handles needed. Our approach is thus particularly suitable for constrained deformations guided by examples, providing significant benefits over state-of-the-art methods. Based on this, we further propose novel data-driven approaches to mesh deformation and non-rigid registration of deformable objects. Both problems are formulated consistently as finding an optimized model in the shape space that satisfies boundary constraints, either specified by the user, or according to the scan. By effectively exploiting the knowledge in the shape space, our method produces realistic deformation results in real-time and produces high quality registrations from a template model to a single noisy scan captured using a low-quality depth camera, outperforming state-of-the-art methods.

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Combining high-speed photography with electric current measurement, we investigate the electrocoalescence of Pickering emulsion droplets. Under a high enough electric field, the originally stable droplets coalesce via two distinct approaches: normal coalescence and abnormal coalescence. In the normal coalescence, a liquid bridge grows continuously and merges two droplets together, similar to the classical picture. In the abnormal coalescence, however, the bridge fails to grow indefinitely; instead, it breaks up spontaneously due to the geometric constraint from particle shells. Such connecting-thenbreaking cycles repeat multiple times, until a stable connection is established. In depth analysis indicates that the defect size in particle shells determines the exact merging behaviors: when the defect size is larger than a critical size around the particle diameter, normal coalescence will show up, while abnormal coalescence will appear for coatings with smaller defects. DOI: 10.1103/PhysRevLett.110.064502 PACS numbers: 47.55.df, 47.57.Às When two droplets come into contact, they naturally coalesce to minimize the surface energy, a phenomenon extensively studied since the 19th century [1][2][3][4][5]. A quite recent study reveals that coalescence starts from the regime controlled by inertial, viscous, and surface-tension forces [6], which is followed by either a viscous regime [7] or an inertial regime [8]. However, the coalescence of special droplets-Pickering emulsion droplets-remains poorly understood. Stabilized by colloidal particles instead of surfactant molecules, Pickering emulsions are composed of particle-coated droplets [9,10], as shown in Fig. 1(a). Because of the highly controllable permeability, mechanical strength, and biocompatibility [11][12][13], Pickering emulsions have been actively studied in the last decade, and may find broad applications in important areas such as oil recovery [14] and drug delivery [15]. The wellcontrolled coalescence in Pickering emulsions can also facilitate material mixing and benefit the field of chemical and biochemical assays [16]. More interestingly, the existence of an extra structure-particle shell-may bring fundamentally different merging physics and enrich the classical coalescence research. Consequently, there is great scientific and practical significance to clarifying the coalescence of Pickering emulsion droplets.If the surface is poorly coated, droplets can coalesce spontaneously and form supracolloidal structures [17,18]. Complex dynamics and structure of particles are observed during coalescence, due to the combined effects of charge, surface tension, and liquid flow [19]. Numerical simulation further reveals that the repulsion between particles, the particles' ability to attach to both droplet surfaces, and the stability of the liquid film between droplets are crucial for coalescence behaviors [20]. However, if the surface is coated by closely packed particles, coalescence rarely occurs. Inspired by the strong influence of electric fields, which can d...

The lack of a first-principles derivation has made the hydrodynamic boundary condition a classical issue for the past century. The fact that the fluid can have interfacial structures adds additional complications and ambiguities to the problem. Here we report the use of molecular dynamics to identify from equilibrium thermal fluctuations the hydrodynamic modes in a fluid confined by solid walls, thereby extending the application of the fluctuation-dissipation theorem to yield not only the accurate location of the hydrodynamic boundary at the molecular scale, but also the relevant parameter value(s) for the description of the macroscopic boundary condition. We present molecular dynamics results on two examples to illustrate the application of this approach-one on the hydrophilic case and one on the hydrophobic case. It is shown that the use of the orthogonality condition of the modes can uniquely locate the hydrodynamic boundary to be inside the fluid in both cases, separated from the molecular solid-liquid interface by a small distance Δ that is a few molecules in size. The eigenvalue equation of the hydrodynamic modes directly yields the slip length, which is about equal to Δ in the hydrophilic case but is larger than Δ in the hydrophobic case. From the decay time we also obtain the bulk viscosity which is in good agreement with the value obtained from dynamic simulations. To complete the picture, we derive the Green-Kubo relation for a finite fluid system and show that the boundary fluctuations decouple from the bulk only in the infinite-fluid-channel limit; and in that limit we recover the interfacial fluctuation-dissipation theorem first presented by Bocquet and Barrat. The coupling between the bulk and the boundary fluctuations provides both the justification and the reason for the effectiveness of the present approach, which promises broad utility for probing the hydrodynamic boundary conditions relevant to structured or elastic interfaces, as well as two-phase immiscible flows.