Actuating and sensing properties of nanoporous gold

We show that variations in the relative humidity give rise to reversible macroscopic dimensional changes in nanoporous gold exposed to ambient air. The macroscopic strain is the consequence of changes in the nanoporous gold surface stress. We have measured reversible strain amplitudes up to 0.02% in response to a 15% change in relative humidity. The direct conversion into mechanical work of the energy involved in the liquid to vapor phase transition of water is attractive for environmentally friendly short-stroke actuator and sensor applications.

Adhesion along metal-polymer interfaces during plastic deformation

In this paper a numerical study is presented that concentrates on the influence of the interface roughness that develops during plastic deformation of a metal, on the work of adhesion and on the change of interface energy upon contact with a glassy polymer. The polymer coating is described with a constitutive law that mimics the behavior of Poly-Ethylene Terephthalate. It includes an elastic part, a yield stress, softening and hardening with increasing strains. For the interface between the metal and the polymer a mixed-mode (mode I and II) stress-separation law is applied that defines the interface energy and an interaction length scale. At the onset of deformation the surface of the substrate has a self-affine roughness characterized by the so-called Hurst exponent, a correlation length and an rms roughness amplitude, that evolves as a function of increasing strain. The findings are the following: the interface energy decreases until the strain at yield of the polymer coating. Interestingly, after yielding as the polymer starts to soften macroscopically, the decreasing average stress levels result in partial recovery of the interface energy at the interface. At higher strains, when macroscopic hardening develops the recovery of the interface stops and the interface energy decreases. The effect of coating thickness is discussed as well as the physical relevance of various model parameters.

Rigidity percolation in dispersions with a structured viscoelastic matrix

This paper deals with rigidity percolation in composite materials consisting of a dispersion of mineral particles in a microstructured viscoelastic matrix. The viscoelastic matrix in this specific case is a hydrocarbon refinery residue. In a set of model random composites the mean interparticle surface-to-surface distance was controlled, changing particle volume fraction phi and particle number density independently. This was achieved by mixing two sets of monodisperse particles with widely differing radii (0.35 microm and 17.5 microm) with the matrix. A scaling exponent of 3.9 +/- 0.6 for the storage modulus G' vs phi- phi(c) was observed above a threshold phi(c) , in good agreement with theoretical values for rigidity percolation. It is found that at the rigidity-percolation threshold the pore structure, as characterized by the mean surface-to-surface distance for the filler, rather than the filler volume fraction, is similar for different types of composites. This behavior is explained from the internal structure of the viscoelastic matrix, which consists of fractal solid aggregates dissolved in a viscous medium; the effective radius of these aggregates and the mean surface-to-surface distance together determine whether or not the aggregates are capable of providing rigidity to the composite. The explanation is further supported by a qualitative comparison with effective-medium calculations. These indicate that the observed breakdown of time-temperature superposition near phi(c) is due to the appearance of a time scale characteristic for the mechanical interplay between the viscous binder phase and the purely elastic solid particles.

Sliding friction dynamics of hard single asperities on soft substrates

The sliding friction of hard, micron-sized single asperities sliding on soft polyester films was studied. Transitions from steady sliding to so-called "stick-slip" or nonstationary motion occur for decreasing driving speed, decreasing driving spring stiffness, increasing normal load, decreasing tip radius, and decreasing crosslink density. Normal displacements of the tip during sliding were studied in some detail. It is argued these play an important role in the dynamics of the system, being the dominant factor in determining the contact area between asperity and substrate. A rather simple model is proposed that is related to rate-and-state descriptions of stick-slip phenomena. In this particular description the normal displacement plays a part analogous to that of the state parameter. In a limited comparison of experiment and numerical results we find qualitative agreement on all measured trends.