ALBERT

Description: We have developed and tested a practical device for manually separating pollen from pollen concentrates in sufficient quantity for AMS 14C dating. It is a combination of standard, commercially available equipment handled in a clean room by an individual trained to recognize pollen. A typical example requires about 15–20 h of hand-picking under the microscope. We show the usefulness of this procedure with results on a mid-Holocene segment from a core from Mono Lake. Sediments from this hardwater lake contain pollen and finely disseminated organic matter, but no macrofossils. The pollen dated ca. 1000 yr younger than the bulk sediment. The sediment “date” is most likely affected by incorporation of limestone-derived carbon, and is erroneously old.

Description: We report on the present status of the Lawrence Livermore AMS spectrometer, including sample throughput and progress towards routine 1% measurement capability for 14C, first results on other isotopes and experience with a multisample high-intensity ion source.

Description: Benard convection of a two-component liquid is considered. The liquid displays Soret effects and the boundary temperatures are fiXed to span the solidification temperature of the miXture. Near the lower, heated plate the material is liquid and near the upper cooled plate there is a layer of pure solid solvent; all the solute is rejected during freezing. Linear stability theory is used to determine the effects on the critical conditions for Soret convection in the presence of the solidified layer and the interface between solid and liquid. EXperiments on miXtures of ethyl alcohol and water are performed using interferometry, photography and thermocouple measurements. The measured onset of instability to travelling waves at negative Soret coefficient compares well with those predicted by our linear theory. In the absence of ice the waves develop at finite amplitude to a fiXed-amplitude state. However, when ice is present, these waves fail to persist but evolve to a state of steady finite-amplitude (overturning) convection. These differences are attributed to the presence of the ice and the nonlinear density profile of the basic state, both of which act as sources of non-Boussinesq effects. © 1992, Cambridge University Press. All rights reserved.

Description: A new law for the thinning of surfactant-free lamellae (applicable to metallic and ceramic foams with mobile interfaces) in a cross-section of an arid gas-liquid foam is derived using matched asymptotic analysis. Two limiting cases are identified at small capillary number: the well-known semi-arid foam having unit-order liquid fraction and the arid foam in which it is small. The lamellar thinning rates in both cases exhibit t-2 power-law behaviour at long times even though the foam liquid area fractions have different orders of magnitude in capillary number. At early times, arid foam thinning is slowed because the curvature of the capillary quasi-static interfacial region must decrease in order to accommodate the flow from the films. Therefore, the thinning of lamellae feeding into a given Plateau border is coupled and the dynamics is distinct from that of the semi-arid foam. Approximations of rupture times in arid and semi-arid foams are found by calculating the times for lamellae to thin to a pre-specified thickness. For given initial lamellar thicknesses, and for arid and semi-arid foams that have identical initial lamellar liquid areas, the arid foam ruptures more quickly than the semi-arid foam. On the other hand the rupture of lamellae is significantly delayed in arid foam compared to semi-arid foam if the initial lamellar thickness and capillary number are the same. © 2005 Cambridge University Press.

Description: An insoluble particle, a solid sphere or a spherical bubble, submerged in a liquid and approached by an advancing solidification front, may be captured by the front or rejected. The particle behaviour is determined by an interplay among van der Waals interactions, thermal conductivity differences between the particle and the melt, solid-liquid interfacial energy, the density change caused by the liquid-solid phase transition, and in the case of a bubble, the Marangoni effect at the liquid-gas interface. We calculate the particle velocity and the deformation of the front when the particle is close to the front, using the lubrication approximation, and investigate how the particle speed, relative to the front, depends on the parameters that characterize the described effects. © 2006 Cambridge University Press.

Description: A novel boundary-integral algorithm is used to study the general, three-dimensional motion of neutrally buoyant prolate and oblate spheroids in a low-Reynolds-number Poiseuille flow between parallel plates. Adaptive meshing of the spheroid surface assists in obtaining accurate numerical results for particle - wall gaps as small as 1.3% of the spheroid's major axis. The resistance formulation and lubrication asymptotic forms are then used to obtain results for arbitrarily small particle - wall separations. Spheroids with their major axes shorter than the channel spacing experience oscillating motion when the spheroid's centre is initially located in or near the midplane of the channel. For both two-dimensional and three-dimensional oscillations, the period length decreases with an increase in the initial inclination of the spheroid's major axis with respect to the lower wall. These spheroids experience tumbling motions for centre locations further from the midplane of the channel, with a period length that decreases as the spheroid is located closer to a wall. The transition from two-dimensional oscillating motion to two-dimensional tumbling motion occurs for an initial centre location closer to a wall as the initial inclination of the major axis is increased. For these spheroids, the average translational velocity along the channel length for two-dimensional oscillating motion decreases for an increase in the initial inclination of the major axis, and the average translational velocity for two-dimensional tumbling motion decreases for a decrease in the initial centre location. A prolate spheroid with its major axis 50% longer than the channel spacing and confined to the (x2, x3)-plane (where x2 is the primary flow direction and xfis normal to the walls) cannot experience two-dimensional tumbling; instead, the spheroid becomes wedged between the walls for initial centre locations near the midplane of the channel when the initial inclination of the large spheroid's major axis is steep, and experiences two-dimensional oscillations for initial centre locations near a wall. When this spheroid's major axis is not confined to the (x2, x3plane, it experiences three-dimensional oscillations for initial centre locations in or near the midplane of the channel, and three-dimensional tumbling for initial centre locations near a wall. © 2006 Cambridge University Press.

Description: We examine steady longitudinal freezing of a two-dimensional single-component free liquid film. In the liquid, there are thermocapillary and volume-change flows as a result of temperature gradients along the film and density change upon solidification. We examine these flows, heat transfer, and interfacial shapes using an asymptotic analysis which is valid for thin films with small aspect ratios. These solutions depend sensitively on contact conditions at the tri-junctions. In particular, when the sum of the angles formed in the solid and liquid phases falls below a critical value, the existence of steady solutions is lost and the liquid film cannot be continuous, suggesting breakage of the film owing to freezing. The solutions are relevant to the freezing of foams of metals or ceramics, materials unaffected by surface active agents. © 2008 Cambridge University Press 2008.

Description: We study the dynamics of a mushy layer in directional solidification for the case of a thin near-eutectic mush with a deformable and permeable mush - liquid interface. We examine the onset of convection using linear stability analysis, and the weakly nonlinear growth of liquid inclusions that signal the onset of chimneys. This analysis is compared to past analyses in which the mush - liquid interface is replaced by a rigid impermeable lid. We find qualitative agreement between the two models, but the rigid-lid approximation gives substantially different quantitative behaviour. In linear theory, the rigid-lid approximation leads to an over-estimate of the critical Rayleigh number and wavenumber of the instability. The condition for the onset of oscillatory instability is also changed by a factor of about 5 in composition number C. In the weakly nonlinear theory, the location of the onset of liquid inclusions is near the undisturbed front for the free-boundary analysis, whereas it lies at the centre of the mushy layer when the rigid-lid approximation is used. For hexagonal patterns, the boundary between regions of parameter space in which up and down hexagons are stable, shifts as a result of coupling between the liquid and mush regions. © 2008 Cambridge University Press.

Description: Matched asymptotic expansion methods are used to establish governing equations of Oseen type for a tethered sphere that describes a circular path and a stationary sphere subjected to a rotating fluid in an ‘antisedimentation’tube. The two cases are shown to be significantly different, in contrast to an earlier presentation (Davis & Brenner 1986), because only the latter is subject to the Coriolis force. The evaluation of the force and torque coefficients is much improved, enabling better comparisons to be made with the classical rectilinear trajectory result of Proudman & Pearson (1957). © 1992, Cambridge University Press. All rights reserved.