We report moment distribution results from a laboratory experiment, similar in character to an isolated strike-slip earthquake fault, consisting of sheared elastic plates separated by a narrow gap filled with a two-dimensional granular medium. Local measurement of strain displacements of the plates at 203 spatial points located adjacent to the gap allows direct determination of the event moments and their spatial and temporal distributions. We show that events consist of spatially coherent, larger motions and spatially extended (noncoherent), smaller events. The noncoherent events have a probability distribution of event moment consistent with an M(-3/2) power law scaling with Poisson-distributed recurrence times. Coherent events have a log-normal moment distribution and mean temporal recurrence. As the applied normal pressure increases, there are more coherent events and their log-normal distribution broadens and shifts to larger average moment.

Liquid 3 He in 98%-porous aerogel simultaneously possesses properties of a disordered p-wave superfluid and an elastic solid. We have observed two sound modes of superfluid 3 He in aerogel. The first (fast) sound is a compression wave, and the second (slow) sound is the out-of-phase oscillation of the superfluid and the normal component that is coupled to the elastic aerogel matrix. A measurement of the slow sound velocity allows an accurate determination of the superfluid fraction.[S0031-9007(99)08991-7] PACS numbers: 67.57. De, 62.65. + k, 67.57.Jj In superfluid hydrodynamics, the presence of two interpenetrating "fluids," the normal and superfluid components, leads to a low velocity mode-second sound-in which the two components move independently [1]. Apart from being a manifestation of the special properties of superfluidity, it provides a valuable measurement tool with which the density of the superfluid component, r s , can be directly determined [2].Liquid 3 He in highly porous aerogel glass is the first realization of disordered superfluid 3 He [3,4]. In contrast to superfluid 4 He in aerogel [5], its zero-temperature superfluid fraction, r s ͞r, is substantially suppressed from unity. Moreover, by changing the pressure [6] or the microscopic structure of the aerogel [7] one can suppress r s ͞r and T c to zero. Direct measurements of the superfluid fraction with a torsional pendulum exhibited interference with sound modes (including those reported in this Letter) of 3 He in aerogel [3,7,8]. We describe sound modes that we observed and the use of acoustic spectroscopy as an alternative technique for the accurate determination of r s ͞r.Our resonator consisted of a cylindrical aerogel sample (length L 1.59 cm, radius R 0.48 cm) grown inside a stainless steel tube. Brass diaphragms of 0.3 mm thickness (to which piezoceramic speaker and microphone transducers were attached) were pressed against the aerogel. At room temperature the gap between the aerogel sample and the tube was small, but the aerogel could freely slide within the tube. Sound spectra were recorded as the quadrature response of the microphone to the oscillations of the speaker while the drive frequency was swept continuously. The temperature, T , was determined with a melting curve thermometer.The aerogel sample is a conglomeration of ϳ50 Å silica particles that form a disordered network of strands with a distribution of interstrand spacing between ϳ50 Å and ϳ1000 Å, porosity f 0.98, density r a r SiO 2 ͑1 2 f͒ 0.04 g͞cm 3 , and with a longitudinal sound velocity c a ϳ 50 m͞s. In liquid 3 He, the viscous penetration depth, d y , at a frequency, f, iswhere h is the viscosity of 3 He, r n is the density of the normal component, and r s 1 r n r fr bulk is the density of 3 He in aerogel. At the bulk superfluid transition temperature, T bulk c , and frequencies below 10 kHz, d y $ 0.1 mm. Hence, in aerogel the normal component is always viscously clamped to the silica skeleton. If the latter were rigid, only the superfluid component would move an...

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Primarily used as an encapsulant and soft adhesive, Sylgard 184 is an engineered, high-performance silicone polymer that has applications spanning microfluidics, microelectromechanical systems, mechanobiology, and protecting electronic and non-electronic devices and equipment. Despite its ubiquity, there are improvements to be considered, namely, decreasing its gel point at room temperature, understanding volatile gas products upon aging, and determining how material properties change over its lifespan. In this work, these aspects were investigated by incorporating well-defined compounds (the Ashby–Karstedt catalyst and tetrakis (dimethylsiloxy) silane) into Sylgard 184 to make modified formulations. As a result of these additions, the curing time at room temperature was accelerated, which allowed for Sylgard 184 to be useful within a much shorter time frame. Additionally, long-term thermal accelerated aging was performed on Sylgard 184 and its modifications in order to create predictive lifetime models for its volatile gas generation and material properties.

The theory for thermoacoustic mixture separation is extended to include the effects of a nonzero concentration gradient. New data are presented, which are in excellent agreement with this theory. The maximum concentration gradient which may be achieved in a binary mixture of gases through this separation process is intrinsically limited by the fractional pressure amplitude, by the tidal displacement, and by the size of the thermal diffusion ratio. Ordinary diffusion further detracts from the attainable final concentration gradient and can become the dominant remixing process as the cross section of the duct is increased. Rayleigh streaming also works against thermoacoustic separation, and an estimate of the molar flux from streaming is given.