Μm Sec Research Articles

The microstructure and mechanical properties of unidirectionally solidified Al−Si alloys

Hypereutectic Al−Si alloys with minor additions of Sr were directionally solidified with a temperature gradient of 125°C cm−1 in the liquid. Silicon in the range 14–17 wt%, Sr in the range 0.0–0.5 wt% and specimen traction velocities between 1 and 1500 μm sec−1 were used. The relationship between hardness and traction velocity and spacing in eutectic silicon morphologies is defined and shown to be of the same form as that for yield stress. The possibility of using hardness measurements to be of the same form as that for yield stress. The possibility of using hardness measurements to indicate mechanical properties is discussed. The complex regular silicon structure makes a significant contribution to the hardness of hypereutectic alloys. This makes the relationship between hardness and traction velocity more complex adding difficulties to the use of hardness to measure mechanical properties.

Permeability of ammonia, methylamine and ethylamine in the cyanobacterium,Synechococcus R-2 (Anacystis nidulans) PCC 7942

Permeabilities of ammonia (NH3), methylamine (CH3NH2) and ethylamine (CH3CH2NH2) in the cyanobacterium (cyanophyte)Synechococcus R-2 (Anacystis nidulans) have been measured. Based on net uptake rates of DCMU (dichlorophenyldimethylurea) treated cells, the permeability of ammonia was 6.44±1.22 μm sec−1 (n=13). The permeabilities of methylamine and ethylamine, based on steady-state14C labeling were more than ten times that of ammonia (Pmethylamine=84.6±9.47 μm sec−1 (76),Pethylamine=109±11 μm sec−1 (55)). The apparent permeabilities based on net uptake rates of methylamine and ethylamine uptake were significantly lower, but this effect was partially reversible by ammonia, suggesting that net amine fluxes are rate limited by proton fluxes to an upper limit of about 700 nmol m−2 sec−1. Increasing concentrations of amines in alkaline conditions partially dissipated the pH gradient across the cell membrane, and this property could be used to calculate the relative permeabilities of different amines. The ratio of ethylamine to methylamine permeabilities was not significantly different from that calculated from the direct measurements of permeabilities; ammonia was much less effective in dissipating the pH gradient across the cell membrane than methylamine or ethylamine. An apparent permeability of ammonia of 5.7±0.9 μm sec−1 could be calculated from the permeability ratio of ammonia to methylamine and the experimentally measured permeability of methylamine. The permeability properties of ammonia and methylamine are very different; this poses problems in the interpretation of experiments where14C-methylamine is used as an ammonia analogue.

Trypanosoma cruzi: Surface charge and freeze-fracture of amastigotes

Amastigotes of Trypanosoma cruzi, within vertebrate cells or isolated from the supernatant of vertebrate cell cultures (L-A9 fibroblast or J774G8 macrophage-like cell lines), possess glycoproteins or glycolipids on the cell surface according to the periodic acid-thiosemi-carbazide-silver proteinate technique used in association with electron microscopy. The cell surface of isolated amastigotes is negatively charged, as evaluated by the binding of cationic particles (colloidal iron hydroxyde at pH 1.8 and cationized ferritin at pH 7.2) as well as by direct measurement of cellular electrophoretic mobility. Amastigotes (Y strain) isolated from the spleen of infected mice and amastigotes (Y and CL strains) from the supernatant of cell cultures previously infected with T. cruzi have the same mean electrophoretic mobility (−0.85 μm sec −1 V −1 cm). It is intermediate between the epimastigote and the trypomastigote forms (determined previously). Sialic acid is the important component responsible for the negative surface charge, as determined by the use of neuraminidase. Thus, it is possible to use the mean electrophoretic mobility as an indicator for identifying amastigotes of T. cruzi.

Platelet aggregation in Poiseuille flow: II. Effect of shear rate

Using a double infusion technique described in the previous paper, the effect of shear rate, G, on platelet aggregation was studied in citrated platelet-rich plasma in Poiseuille flow over a range of mean linear flow rates, u 3 from 50 to 1500 μm sec −1 corresponding to G from 2 to 54 sec −1. At 1 μm ADP, aggregates were formed at all u 3 and, except at the lowest flow rate, both the degree of aggregation (net fraction of cells in aggregates, A′) and size of the aggregates increased with distance down the tube. The degree of aggregation was both time and shear rate-dependent. Over the first 10 sec, A′ appeared to be independent of the mean transit time indicating that the collision capture efficiency decreased with increasing G. Thereafter, A′ increased with increasing shear rate leading to the formation of an appreciable number of aggregates >10 cells at G > 24 sec −1 . Small aggregates initially formed near the tube wall and, as they grew in size, migrated toward the axis as their rotation was physically impeded by the wall. In addition, the values of A′ in cPRP from five female donors were significantly greater than those from five male donors.

The effects of cationised ferritin and native ferritin upon the filtration coefficient of single frog capillaries. Evidence that proteins in the endothelial cell coat influence permeability

To investigate whether certain macromolecules reduce capillary permeability by binding to the surface coat of endothelial cells, the effects of cationised ferritin (CF) upon the filtration coefficient ( L p ) of individually perfused frog mesenteric capillaries were compared with those of native ferritin (NF). With perfusate CF concentrations between 0.1 g 100 ml −1 and 2.5 g 100 ml −1, L p was reduced to approximately 30% of its value for the same vessel perfused with protein-free Ringer solution. Electron micrographs of the perfused capillaries revealed that over this range of perfusate concentrations, CF was concentrated uniformly in the endothelial cell coat, occupying 8.5% of its volume. Neither the effect of cationised ferritin upon L p nor its concentration in the cell coat varied significantly over this range of perfusate concentrations. When perfusate concentration of CF was reduced to 0.01 g 100 ml −1, CF no longer reduced L p and its concentration in the cell coat fell below 2%. Native ferritin, which is excluded from the cell coat, did not reduce L p at a perfusate concentration of 0.1 g 100 ml −1. At a concentration of 2.5 g 100 ml −1, NF reduced L p in a few very permeable vessels ( L p > 60 × 10 −3 μm sec −1 cm H 2O −1) but had no significant effect on vessels with lower and more normal values of L p . The effects of CF upon L p can be described in terms of the Kozeny equation if a major proportion of the hydraulic resistance through the capillary wall is attributed to a fiber protein matrix.

Morphology and ecology of Oltmannsiella virida, sp. nov. (Chlorophyceae: Volvocales)

Abstract A new quadriflagellate green alga is described from temperate coastal waters. It occurs primarily as four-celled colonies, and its ultrastructure and general morphology place it in the Dunaliellaceae (order Volvocales, class Chlorophyceae). Oltmannsiella virida sp. nov. grows well in salinities from 10‰ to full-strength coastal water, and at temperatures from 15–25°C. The swimming rate of colonies varies from c. 225–325 μm sec−1 (c. 0·8–1·2 m h−1), which is similar to other marine flagellates.

Magnetic properties of a directionally solidified eutectic alloy containing Co17Sm2 fibres

An eutectic alloy containing 44% by volume Co17Sm2 fibres has been located in the Co-Sm-Sn system and directionally solidified at speeds of between 1.7 and 550 μm sec−1. The spacing λ, between the fibres and their diameter, d, as a function of the growth rate, R, show the following relations: λ2 R = 42 μm3 sec−1 d 2 R = 10 μm3 sec−1. Fibre diameters ranging from 1.6 to 0.14μm have been obtained and coercive forces of up to 2.3 kOe, for the finest fibres, measured. The behaviour of the coercive force with changing diameter is compared with that in a Bi-MnBi eutectic and in powders made from the same compounds. The results can be interpreted by means of a simple theoretical analysis which shows that the attainment of high coercive forces in powder is restricted by the microscopic surface-defect density introduced during preparation, whilst in eutectics it is limited by the difficulties of growing fine regular structures at high speed.

The Microrheology of Colloidal Dispersions. III. Concentrated Emulsions

The flow behavior of 50 and 70 vol% transparent 2-μm emulsions of amyl acetate in aqueous glycerol is described. The systems exhibited shear thinning behavior as well as large normal stress differences, the latter an order of magnitude greater than the tangential shear stresses. Particle motions were studied in 100-μm tubes at mean flow rates from 40 to 1500 μm sec−1 using 2-μm resin tracer spheres. The velocity profiles were blunted at all flow rates and at both concentrations with a region of apparent plug flow in the tube center whose radius Re decreased with increasing flow rate and decreasing concentration. Large radial displacements of tracer spheres were observed at R>Re, and even when R<Re small displacements were detected. The distribution of tracer particles across the median plane of the tube was non-uniform with a markedly higher concentration in the plug; this effect increased with distance downstream from the tube entrance and with decreasing flow rate.

Bulk-like InSb films by hot-wire zone crystallization

Films of InSb have been prepared by the zone crystallization in helium of vacuum-deposited layers on glass substrates using a simple hot-wire apparatus. Regions as large as 25 mm × 25 mm can be processed, and multiple zone passes can be used to provide additional purification of the InSb. The improved film quality is demonstrated by a film 9 μm thick prepared with four zone passes at 10 μm sec growth rate in a temperature gradient of 400 °C cm . The film is n-type with 7 × 10 15 carriers/cm 3. Its Hall mobility is 72 000 cm 2 V sec at 295 °K and 163 000 cm 2 V sec at 77 °K.