Michael Bretz

Sound-producing sand avalanches

Sound-producing sand grains constitute one of natures more puzzling and least understood physical phenomena. They occur naturally in two distinct types: booming and squeaking sands. Although both varieties of sand produce unexpectedly pure acoustic emissions when sheared, they differ in their frequency range and duration of emission, as well as the environment in which they tend to be found. Large-scale slumping events on dry booming dunes can produce acoustic emissions that can be heard up to 10 km away and which resemble hums, moans, drums, thunder, foghorns or the drone of low-flying propeller aircraft. These analogies emphasize the uniqueness of the phenomenon and the clarity of the produced sound. Although reports of these sands have existed in the literature for over one thousand years, a satisfactory explanation for either type of acoustic emission is still unavailable.

Broad distribution of stick-slip events in Slowly Sheared Granular Media: Table-top production of a Gutenberg-Richter-like distribution

We monitor the stick-slip displacements of a very slowly driven moveable perforated top plate which interacts via shearing with a packing of identical glass beads confined in a tray. When driven at a constant stress rate, the distributions of large event displacements and energies triggered by the stick-slip instabilities exhibit power law responses reminiscent of the Gutenberg-Richter law for earthquakes. Small events are quasi-size independent, signaling crossover from single-bead transport to collective behavior.

Water droplet avalanches

We analyze the statistics of water droplet avalanches in a continuously driven system. Distributions are obtained for avalanche size, lifetime, and time between successive avalanches, along with power spectra and return maps. For low flow rates and different water viscosities, we observe a power-law scaling in the size and lifetime distributions of water droplet avalanches, indicating that a state with no characteristic time and length scales was reached. Higher flow rates resulted in an exponential behavior with characteristic scales

Specific heat of para-hydrogen monolayers on graphite

The specific heat of para-hydrogen films has been measured from 1 to 20 K for areal densities between 0.01 and 0.055 atoms/Å2 of the partial monolayer adsorbed on grafoil. The results are compared with the specific heat of4He films at equivalent densities. It is argued that a nonideal gas regime and liquefaction are present in both systems and that solidification of thep-H2 films has been suppressed to below the temperature range of the experiment. Implications regarding 2D superfluidity in hydrogen monolayers are discussed briefly.

Structural imaging of a thick-walled carbon microtubule

We analyze the structure of a thick-walled carbon microtubule based on direct electron beam imaging of the graphitic cylinders comprising the fiber. Clearly resolved six-fold symmetry of basal planes overlying the fiber core indicate zero overall fiber helicity and alignment of individual cylinders. Sidewall measurements calibrated from the (1010) core fringes show uniform spacings of about 0.375 nm, which are larger than those reported for other microtubules or for crystalline graphite (0.335 nm). Short zones of local 30 helicity are observed along the fiber. Structural transitions which alter the helicity are characterized by extra atomic planes and other defects, including nested sub-tubules. We discuss implications for the fiber’s growth and electrical properties.

Thermal resistivity of layered 4He films on ZYX graphite below 2 K

Abstract Thermal resistance and vapor pressure isotherms were taken near superfluid onset for ultra-thin helium films adsorbed on a ZYX graphite wafer between 1–2 K and 3–7 atomic layers. Our data are consistent with previous graphite onsets and are compatible with a current model of film droplet formation. Overlap of thermal resistance curves at 1.19 and 139 K is believed to be associated with discrete layering effects of 2D superfluid film properties.

Microcalorimetry study of the monolayer4He ordering transition on single crystal graphite

We have developed a low-temperature, ac microcalorimeter for exploring adsorption on a single 2.25 mm2 graphite leaf that is capable of 10 picoJoule/degree resolution. The microcalorimeter was used to determine the phase diagram and heat capacity critical exponents α of monolayer4He films at the commensurate ordering transition. After in situ baking at 600 K, we reproduced the narrow-ordered phase region (≃1% in coverage) reported by Campbell and Bretz for HOPG, but find quasilogarithmic, rather than power law, heat capacity divergences. We argue that the disappearance of the Potts-like exponent in the heat capacity is attributable to the geometry of nucleation along cleavage edge planes present on single crystal graphite surfaces.