Masahiko Arakawa

Adhesion shear theory of ice friction at low sliding velocities, combined with ice sintering

Adhesion and shear deformation of ice have been traditionally considered to be responsible for ice friction at sliding velocities lower than about 10−2 m/s, but the simple mechanism cannot explain the recent finding that the ice–ice friction coefficient increases with decreasing sliding velocity. This article proposes an improved adhesion shear theory, which takes account of junction growth of asperities at the sliding ice interface due to sintering. At lower sliding velocities and higher homologous temperatures, contacts of ice asperities develop resulting in the increase of friction force.

Mechanical strength of polycrystalline ice under uniaxial compression

Abstract Mechanical strength of polycrystalline ice Ih was investigated by uniaxial compression tests at wide ranges of temperature, −10 to −173°C, and of strain rate, 4×10−4 to 4×10−6 s−1. A systematic change of the deformation type from brittle fracture to ductile deformation was observed to take place at a critical strain rate and temperature. A systematic increase of the strength was also found with decreasing temperature and increasing strain rate. In both the ductile and brittle regions, a similar relation was found to hold: e =Aσ n exp (−E/RT) where e is the strain rate, σ is the peak stress for ductile deformation and failure stress for brittle fracture, R is the gas constant and T is the absolute temperature. The apparent activation energy E and exponent n are 48 kJ/mol and 6.5 in the brittle region and 64 kJ/mol and 3.4 in the ductile region, respectively. The maximum stress (σmax: peak and failure stress) has a good correlation with the strain (emax) at that stress irrespective of the temperature and strain rate. An empirical equation, emax=0.41+0.015σmax, where the unit of σmax is MPa, can be applied to both the ductile and brittle regions.

MEASUREMENTS OF RESTITUTION COEFFICIENTS OF ICE AT LOW TEMPERATURES

Abstract Measurements of the restitution coefficient (e) of a smooth water ice sphere (radius = 1.5 cm) are made in a wide range of impact velocities (1≤υi≤700cms−1) and temperatures (113≤T≤269K). The impact velocity dependence of e is different in the quasi-elastic and inelastic regimes separated by a critical velocity (υc) at which fracture deformation occurs at the impact point of ice samples. In the quasi-elastic regime (υi≤υc), the value of e is almost constant (0.88) and ice samples show no fracture deformations. In the inelastic regime (υi>υc), e decreases with increasing υi and ice samples have fracture patterns. The velocity dependence of e is fitted as e(υ i ) = ( υ i υ c ) − log ( υ i υ c ). vc is shown to increase with decreasing temperature from 25cms−1 (269K) to 180cms−1 (113–215K).

Rapid Growth of Asteroids Owing to Very Sticky Interstellar Organic Grains

We experimentally found interstellar grains covered with organic matter in an asteroid belt, and more importantly, the organic matter played an essential role in the formation of the asteroids. The sticking threshold velocityof 5 m s-1 of the millimeter-sized organic grains was several orders of magnitude higher than those of the coexisting silicate and ice grains. This indicated a very rapid coagulation of the very sticky organic grain aggregates and the formation of planetesimals in the asteroid region, covering even the early stage of the turbulent solar nebula. In contrast, there was no coagulation of the silicate and ice grains in the terrestrial and Jovian regions, respectively.

Effective viscosity of partially melted ice in the ammonia-water system

The steady-state deformation of partially melted ice in the ammonia-water system was studied by means of a concentric cylinder viscometer in shear stresses, 10 kPa∼0.1 MPa, temperatures, 180∼210 K and NH3 contents, 4.0∼8.4%. The flow law found was of a non-Newtonian power-law type; the stress exponent was 4.0±0.1. The activation energy at constant melt fractions was 33.7±0.8 kJ/mol, which was close to that of viscosity for aqueous ammonia solutions. However, the effective viscosity of partially melted ice estimated at 0.1 MPa was 107 ∼ 1010 Pa s, which is about ten orders of magnitudes larger and smaller than that of the ammonia-water mixture in the liquid and solid phases (below a peritectic point, 176 K), respectively.

Ultrastructural localization of protein kinase C β-subspecies in the axon terminal of rat neuromuscular junction

Ultrastructural localization of protein kinase C (PKC) beta-subspecies in neuromuscular junctions of the rat lumbrical muscle was investigated by the immunoperoxidase and immunofluorescence methods. By light microscopy, PKC beta-like immunoreactivity (PKC beta-LIR) was found in the axon terminal expansions as well as in the preterminal axons. By confocal laser scanning microscopy, the staining for PKC beta-like immunoreactivity was more intense in the presynaptic regions just in contact with the acetylcholine receptor stained by FITC-alpha-bungarotoxin. By electron microscopy, PKC beta-like immunoreactivity was distributed non-uniformly in the terminal expansions. In the terminal expansions, PKC beta-like immunoreactivity was accumulated in the presynaptic regions in contact with the post-synaptic folds. This accumulation was approximately 0.1-0.2 microns in diameter, which comprised a part of the presynaptic plasma membrane and a group of synaptic vesicles adjacent to it. Weak immunoreactivity was also found diffusely in the axoplasmic matrix. The discrete presynaptic accumulation of PKC beta-subspecies may represent the strategical localization specialized for the effective regulation of neurotransmitter release.

Impact cratering of granular mixture targets made of H2O Ice–CO2 Ice–pyrophylite

Abstract Experiments related to impacts onto three-component targets which could simulate cometary nucleus or planetary regolith cemented by ices are presented here. The impact velocities are from 133 to 632 m s −1 . The components are powdered mineral (pyrophylite), H 2 O ice, and CO 2 ice mixed 1:1:0.74 by mass. The porosity of fresh samples is about 0.48. Two types of the samples were studied: nonheated samples and samples heated by thermal radiation. Within the samples a layered structure was formed. The cratering pattern strongly depended on the history of the samples. The craters formed in nonheated targets had regular shapes. The volume was easy to be determined and it was proportional to impact energy E . The crater depth scales as E 0.5 . Impacts on the thermally stratified target led to ejection of a large amount of material from the loose sub-crustal layer. For some particular interval of impact velocity a cratering pattern can demonstrate unusual properties: small hole through the rigid crust and considerable mass transfer (radially, outward of the impact point) within sub-crustal layer.

Shock wave and fracture propagation in water ice by high velocity impact

In order to clarify the elementary processes of impact disruption, we conducted impact experiments with water ice at an impact velocity of 3.6 km/s and observed shock wave and fracture propagation in it by means of ultra-high speed photography. We observed that a region in which HEL (Hugoniot elastic limit) followed the elastic precursor wave, expanded with a velocity of 3–2.5 km/s until the pressure fell below 240 MPa. Below that pressure, a damage region appeared 0.8–3 µs after the passage of precursor wave. In this region, dynamic shear strength of water ice was estimated to be 21 MPa. Below 80 MPa, the several radial cracks proceeded toward the rear surface and broke the sample before the tensile fracture caused by reflection waves from an antipodal point became visible. Therefore, the main mechanism to cause the largest fragment is the radial crack growth rather than a spallation at the rear.

Measurements of ejection velocities in collisional disruption of ice spheres

Impact experiments are performed on ice spheres to measure the velocity field of ejected ice fragments and the conditions under which the fragments would reaccumulate during accretion in the outer solar system are considered. A single-stage light gas gun set in a cold room at −18°C and an image-converter camera running at 2 × 105-1 × 104 frames per second with a xenon flash lamp are used for observing the collisional phenomena. Spherical projectiles of ice (mp = 1.5 g) collide head-on with spherical targets (Mt = 1.5, 12, 172 g) at 150–690 m s−1. The ejection velocity is observed to vary with the initial position and ranges from 3 to 110 of the impact velocity (Vi). The ejection velocity of fragments at the rear side of the target (Ve) varies with distance from the impact point according to a power law relation, Ve = Va(1D)−n, where Va is the antipodal velocity, l and D are the distance and the target diameter, and n = 1.5–2.0. Va depends on the specific energy (Q) at a constant mass ratio (mpMt = 0.13) and the empirical dependence is written as Va = 0.35 × Q0.52. The ejection velocity of fine fragments formed by the jetting process near the impact point is determined to be 1.7–2.9 times as large as the impact velocity irrespective of the target size and the impact velocity.

Shock pressure attenuation in water ice at a pressure below 1 GPa

Shock pressure attenuation in water ice was studied at an impact pressure below 1 GPa and a temperature of 255 K. The observed shock wave showed a multiple shock wave structure: A precursor wave was followed by a main wave, which had a longer rise time and higher amplitude. The Hugoniot elastic limit (HEL) of water ice was measured to be in the range from 0.1 to 0.3 GPa when associated with precursor waves traveling at 3.86 km/s. The peak amplitude of the main wave Pm was observed to decrease with its propagation x from 3 to 60 mm (from 0.4 to 8 times as large as a projectile radius) in two series of experiments in which initial shock pressures Pi at the impact point were 0.60 and 0.87 GPa. The Pm was described as the power law relation Pm/Pi = (x/2.6 mm)−89. The precursor wave disappears as the Pm attenuated to a pressure <0.1 GPa. The measured wave profiles were used to calculate the loading path of water ice in shock compression between the HEL and 0.6 GPa. The loading path obtained by Lagrangian analysis was closely consistent with previous Hugoniot data regarding water ice.