Macroscopic scale

Abstract: Carbon nanotubes weave their way into a range of imaginative macroscopic applications. The creation of continuous yarns made out of carbon nanotubes would enable macroscopic nanotube devices and structures to be constructed1,2. Here we show that carbon nanotubes can be self-assembled into yarns of up to 30 cm in length simply by being drawn out from superaligned arrays of carbon nanotubes, and that the strength and conductivity of these yarns can be enhanced by heating them at high temperatures. Our findings should help to translate the remarkable mechanical, electrical and thermal properties of carbon nanotubes to a macroscopic scale.

Abstract: Quasi-static shear deformation of granular media is examined by performing numerical simulations on polydisperse systems of elastic spheres in a periodic cell. Results of axisymmetric compression test simulations are reported for both a dense and a loose system. At the macroscopic scale, the simulated stress–strain–dilation responses are in excellent qualitative agreement with the mechanical behaviour of sand observed in real experiments. Numerical simulation permits a detailed examination of the evolution of internal variables associated with the micromechanical processes occurring at the particle scale. In this context, the evolution of the induced structural anisotropy, the percentage of sliding contacts, the average coordination number and the normal and tangential contact force contributions to the stress tensor are presented and discussed. Further simulations are reported which illustrate the effect of interparticle friction on both the macrocopic and microscopic mechanical behaviour. Finally, the v...

Abstract: Publisher Summary This chapter focuses on variational and related methods for the overall properties of composites. A wide range of phenomena that are observable macroscopically are governed by partial differential equations that are linear and self-adjoint. This chapter is concerned with such phenomena for materials, such as fiber-reinforced composites or polycrystals, whose properties vary in a complicated fashion from point to point over a small, “microscopic” length scale, while they appear “on average” (that is, relative to the larger, macroscopic scale) to be uniform. This chapter treats the elastic behavior of composites, and emphasizes that a number of other properties (conductivity, viscosity of a suspension, etc.) are described by the same equations. Extensions to viscoelastic and thermoelastic behavior are presented, for both of which the variational characterization given is believed to be new. Problems, such as the resistance to flow of viscous fluid through a fixed bed of particles are mentioned, and a model problem that involves diffusion is presented in some detail. This displays the same difficulty in relation to divergence of an integral and is one problem of this type that has so far been approached variationally. Methods related to the Hashin–Shtrikman variational principle are also described in the chapter.

Abstract: Publisher Summary This chapter discusses several composites or granular or porous materials that display inhomogeneity on a macroscopic scale. In such materials, there are small, yet much larger than atomic, regions exhibiting macroscopic homogeneity. Different regions may display quite different properties. The chapter discusses the dc and ac electrical properties of composite media including the basic theory and results for dc electrical properties. It also presents a discussion of the effective-medium approximation, electrostatic resonances, exact bounds, and analytical properties and describes a number of static physical properties of composites. These properties include electrical conductivity and dielectric behavior near a percolation threshold, magnetotransport, thermoelectricity, superconductivity, and duality in two-dimensional composites. Finally, the chapter discusses the nonlinear properties and flicker noise in composites.

Abstract: A key goal of nanotechnology is the development of artificial machines capable of converting molecular movement into macroscopic work. Although conversion of light into shape changes has been reported and compared to artificial muscles, real applications require work against an external load. Here, we describe the design, synthesis and operation of spring-like materials capable of converting light energy into mechanical work at the macroscopic scale. These versatile materials consist of molecular switches embedded in liquid-crystalline polymer springs. In these springs, molecular movement is converted and amplified into controlled and reversible twisting motions. The springs display complex motion, which includes winding, unwinding and helix inversion, as dictated by their initial shape. Importantly, they can produce work by moving a macroscopic object and mimicking mechanical movements, such as those used by plant tendrils to help the plant access sunlight. These functional materials have potential applications in micromechanical systems, soft robotics and artificial muscles