No abstract

No abstract

No abstract

Monodisperse distributions of 1.01 mu m and 2.88 mu m polystyrene microspheres in monolayers on the surface of water were used in a study of isothermal-expansion melting in two dimensions. The equation of state, defect structures, and the translational and orientational correlation functions were obtained from digitised particle positions as the particle-number density ranged from the ordered solid to the liquid phase. The 2.88 mu m system showed evidence of defect mediated melting and of an intermediate hexatic phase, in partial accord with theoretical results of other studies. Melting in the 1.01 mu m system appeared to proceed by a weak first-order transition. If so, the difference in melting behaviour of the two samples may reflect differences in defect core creation energies which can be traced to sphere size.

No abstract

A two-dimensional melting transition has been observed in a freely expanding colloidal monolayer lattice formed of 1.01-jum polystyrene microspheres in an aqueous suspension between two parallel optical flats. The correlation functions computed from digitized images of the particle distribution within a fixed sampling area are consistent with a continuous two-step melting transition. However, direct observations of defect creation and evolution reveal that the system may melt via a first-order process.PACS numbers: 64.70.Dv, 61.25.Hq, 82.70.Dd Whether the melting transition in two dimensions (2D) is first order or continuous as proposed by Kosterlitz, Thouless, Halperin, Nelson, and Young (KTHNY) 1 " 3 has been a challenging problem for over a decade. 4 In KTHNY theory, between the solid (S) and the liquid (L) phases, there exists an intermediate hexatic (H) phase which has short-range translational order and quasi-long-range orientational order.Melting occurs in a two-state process governed by dislocation pairs unbinding at the S-H transition and by disclination pairs unbinding at the H-L transition. These two transitions are characterized by separate divergences in the translational and orientational correlation lengths, respectively. Colloidal monolayers are an especially good experimental system for studying this problem because the charged colloidal micorspheres (CCM) in aqueous suspensions are microscopically observable. In particular, a colloidal monolayer system in a small wedge geometry, first introduced by Pansu, Pieranski, and Strzelecki 5 and by Clark, Ackerson, and Hurd, 6 has been successfully employed by Murray and Van Winkle (M-VW) 7 in the study of 2D melting.In this Letter we describe the results of a study of free-expansion melting of 2D CCM solids formed in films of highly uniform thickness. Two-dimensional populations of 1.01-jum-diam polystyrene-sulphated microspheres suspended in water were established between fused-silica optical flats in a parallel-plate film cell. With this arrangement, the particle-number density gradient inherent to the wedge method could be eliminated. Most importantly the free-expansion melting is time dependent, which allows observation of the time evolution of defects as a given sample of the monolayer melts.The films are formed between a 1.6 mm x 3.2 cm diam flat which defines their lower boundary, and a 3.2 mmx 1.27 cm diam flat which is glued to a 3.2 mm x3.2 cm flat. These are fitted in an anodized aluminum cell which is sealed by tightening alignment screws in order to press the two larger flats against a Viton rubber O ring; the space -1.2 cm 3 between the O ring and the periphery of the smaller flat serves as a 3D reservoir. Interferometry reveals that the lower flat flexes slightly concave when the cell is sealed so that the gap thickness decreases -0.011 jum from the cell center out to a ra-dius of 500 /im. This distortion gives rise to an inward directed electrostatic wall-particle compressive force. A three-nested-ring stepped annular channel was plas...

The incorporation of purine nucleotide precursors into Helicobacter pylori and the activities of enzymes involved in nucleotide salvage biosynthetic pathways were investigated by radioactive tracer analysis and nuclear magnetic resonance spectroscopy. The organism took up the nucleobases adenine, guanine and hypoxanthine, and the nucleosides adenosine, guanosine and deoxyadenosine. Any incorporation of deoxyguanosine by the cells was below the detection limits of the methods employed. The activities of adenine-, guanine- and hypoxanthine-phosphoribosyl transferases were established. The bacterium showed high levels of adenosine and guanosine nucleosidase activities and lesser activity for deoxyadenosine; no hydrolysis of deoxyguanosine was detected. Phosphorylase activities were not observed with any of the nucleosides. Phosphotransferase activities with similar rates were demonstrated for adenosine, guanosine and deoxyadenosine; and a weaker activity was detected for deoxyguanosine. No nucleoside kinase activities were observed with any of the nucleosides. The presence of adenylate kinase was established, but no guanylate kinase activity was observed. The study provided evidence for the presence in H. pylori of salvage pathways for the biosynthesis of purine nucleotides.

No abstract