arXiv subjects

A. Sapelkin

Publications and source records attributed to A. Sapelkin.

5 recordsLinked to original sources

arXiv2017

Crossover between liquid-like and gas-like behaviour in CH4 at 400 K

We report experimental evidence for a crossover between a liquid-like state and a gas-like state in fluid methane (CH4). This crossover is observed in all of our experiments, up to 397 K temperature; 2.1 times the critical temperature of methane. The crossover has been characterized with both Raman spectroscopy and X-ray diffraction in a number of separate experiments, and confirmed to be reversible. We associate this crossover with the Frenkel line - a recently hypothesized crossover in dynamic properties of fluids extending to arbitrarily high pressure and temperature, dividing the phase diagram into separate regions where the fluid possesses liquid-like and gas-like properties. On the liquid-like side the Raman-active vibration increases in frequency linearly as pressure is increased, as expected due to the repulsive interaction between adjacent molecules. On the gas-like side this competes with the attractive Van der Waals potential leading the vibration frequency to decrease as pressure is increased.

cond-mat.stat-mech↗

arXiv2015

The Significance of Bundling Effects on Carbon Nanotubes' Response to Hydrostatic Compression

The study of the G-mode pressure coefficients of carbon nanotubes, reflecting the stiff sp2 bond pressure dependence, is essential to the understanding of their extraordinary mechanical properties as well as fundamental mechanics. However, it is hindered by the availability of carbon nanotubes samples only as bundles or isolated with surfactants. Octadecylamine functionalized carbon nanotubes are mostly of a single diameter and can be stably dispersed in 1, 2-dichloroethane and chloroform without surfactants. Here we perform high pressure Raman spectroscopy on these tubes and obtain their experimental G-mode pressure coefficients for individual tubes and bundles. The G-plus pressure coefficient for bundles is only about half of that for individual tubes in 1, 2-dichloroethane and is about two-thirds in chloroform. The G-minus pressure coefficient for bundles is about one-third of G-plus in 1, 2-dichloroethane and about the same in chloroform. These results for the first time provide unambiguous experimental evidence of the significant effect of bundling on carbon nanotubes' G-mode pressure coefficients, identifying it as one of the major reasons for the lack of consensus on what the values should be in the literature.

cond-mat.mtrl-sci↗

arXiv2014

Measurement of bitumen viscosity in the room-temperature drop experiment: student education, public outreach and modern science in one

Slow flow of the viscous liquid is a thought-provoking experiment that challenges students, academics and public to think about some fundamental questions in modern science. In the Queensland demonstration, the world-longest running experiment earning the Ig Nobel prize, one drop of pitch takes about 10 years to fall, leading to problems of actually observing the drops. Here, we describe our recent demonstration of slowly-flowing bitumen where appreciable flow is observed on the time scale of months. The experiment is free from dissipative heating effects and has the potential to improve the accuracy of measurement. Bitumen viscosity was calculated by undergraduate students during the summer project. The worldwide access to the running experiment is provided by webcams uploading the images to a dedicated website, enhancing student education experience and promotion of science. This demonstration serves as an attractive student education exercise and stimulates the discussion of fundamental concepts and hotly debated ideas in modern physics research: difference between solids and liquids, the nature of liquid-glass transition, emergence of long time scales in a physical process, and the conflict between human intuition and physical reality.

cond-mat.soft↗

arXiv2012

Pressure coefficients of Raman modes of carbon nanotubes resolved by chirality: Environmental effect on graphene sheet

Studies of the mechanical properties of single-walled carbon nanotubes are hindered by the availability only of ensembles of tubes with a range of diameters. Tunable Raman excitation spectroscopy picks out identifiable tubes. Under high pressure, the radial breathing mode shows a strong environmental effect shown here to be largely independent of the nature of the environment . For the G-mode, the pressure coefficient varies with diameter consistent with the thick-wall tube model. However, results show an unexpectedly strong environmental effect on the pressure coefficients. Reappraisal of data for graphene and graphite gives the G-mode Grueuneisen parameter gamma = 1.34 and the shear deformation parameter beta = 1.34.

cond-mat.mes-hall↗

arXiv2012

Raman excitation spectroscopy of carbon nanotubes: effects of pressure medium and pressure

Raman excitation and emission spectra for the radial breathing mode (RBM) are reported, together with a preliminary analysis. From the position of the peaks on the two-dimensional plot of excitation resonance energy against Raman shift, the chiral indices (m, n) for each peak are identified. Peaks shift from their positions in air when different pressure media are added - water, hexane, sulphuric acid - and when the nanotubes are unbundled in water with surfactant and sonication. The shift is about 2 - 3 cm-1 in RBM frequency, but unexpectedly large in resonance energy, being spread over up to 100meV for a given peak. This contrasts with the effect of pressure. The shift of the peaks of semiconducting nanotubes in water under pressure is orthogonal to the shift from air to water. This permits the separation of the effects of the pressure medium and the pressure, and will enable the true pressure coefficients of the RBM and the other Raman peaks for each (m, n) to be established unambiguously.

cond-mat.mes-hall↗