The temperature dependence of the femtosecond elastic second harmonic scattering (fs-ESHS) response of bulk light and heavy water and their electrolyte solutions is presented. We observe clear temperature dependent changes in the hydrogen (H)-bond network of water that show a decrease in the orientational order of water with increasing temperature. Although DO has a more structured H-bond network (giving rise to more fs-ESHS intensity), the relative temperature dependence is larger in HO. The changes are interpreted in terms of the symmetry of H-bonds and are indicators of nuclear quantum effects. Increasing the temperature in electrolyte solutions decreases the influence of the total electrostatic field from ions on the water-water correlations, as expected from Debye-Hückel theory, since the Debye length becomes longer. The effects are, however, 1.9 times (6.3 times) larger than those predicted for HO (DO). Since fs-ESHS responses can be computed from known molecular coordinates, our observations provide a unique opportunity to refine quantum mechanical models of water.
Water is the liquid of life thanks to its three-dimensional adaptive hydrogen (H)-bond network. Confinement of this network may lead to dramatic structural changes influencing chemical and physical transformations. Although confinement effects occur on a <1 nm length scale, the upper length scale limit is unknown. Here, we investigate 3D-confinement over lengths scales ranging from 58−140 nm. By confining water in zwitterionic liposomes of different sizes and measuring the change in Hbond network conformation using second harmonic scattering (SHS), we determined long-range confinement effects in light and heavy water. D 2 O displays no detectable 3D-confinement effects <58 nm (<3 × 10 6 D 2 O molecules). H 2 O is distinctly different. The vesicle enclosed inner H-bond network has a different conformation compared to the outside network and the SHS response scales with the volume of the confining space. H 2 O displays confinement effects over distances >100 nm (>2 × 10 7 H 2 O molecules).
Water is the matrix of life and serves as a solvent for numerous physical and chemical processes. The origins of the nature of inhomogeneities that exist in liquid water and the time scales over which they occur remains an open question. Here, we report femtosecond elastic second harmonic scattering (fs-ESHS) of liquid water in comparison to an isotropic liquid (CCl4) and show that water is indeed a nonuniform liquid. The coherent fs-ESHS intensity was interpreted, using molecular dynamics simulations, as arising from charge density fluctuations with enhanced nanoscale polarizabilities around transient voids having an average lifetime of 300 fs. Although voids were also present in CCl4, they were not characterized by hydrogen bond defects and did not show strong polarizability fluctuations, leading to fs-ESHS of an isotropic liquid. The voids increased in number at higher temperatures above room temperature, in agreement with the fs-ESHS results.
Experimental plasma discharges in linear PANTA device are studied by Mach probe measurements, providing floating potential, ion saturation current, and parallel flow velocity time evolution, at different radii of the device. Spectral analysis indicates that drift waves and D'Angelo modes exist simultaneously in the plasma. A discrimination study shows they are located at different positions in radius and frequency. Plasma turbulence is one of the central issues of modern plasma research. Extensive studies have been conducted and it is argued that the family of drift wave (DW) turbulence, which is driven by scalar gradients such as density, temperature, and pressure, plays an important role to understand confinement of fusion plasmas [1]. In addition to this, plasmas also support flows, which give rise to instability driven by vector fields [3,4]. For instance, Kelvin-Helmholtz instability [2] and/or D'Angelo mode (D'AM) [3][4][5][6] can arise from the inhomogeneity of the velocity in the perpendicular and the parallel direction to the magnetic fields, respectively, and impact the evolution of the system [7]. Since plasmas have several gradients, of both scalar and vector fields, it is likely that different types of modes/instabilities coexist and interact. As DWs and D'AMs play different roles on transport and structural formation [4], it is important to clarify the mechanism how/where these fluctuations are excited in plasmas. In this paper, we report an experimental observation of the simultaneous excitation of DWs driven by the density gradient and D'AMs driven by the parallel velocity gradients. Fluctuations are directly measured and power spectra are used to differentiate the modes.Fluctuation of parallel velocity is a key quantity to discriminate DWs and D'AMs (see Table 1). It is well known that parallel velocity perturbations are weak in the case of DW, with respect to density perturbations [1]:author's e-mail: nathan.dupertuis@alumni.epfl.ch In the case of D'AM [5,6], the relation becomes:For most unstable mode k c s = k y ρ s v /2. Then the relation reduces to ṽ c sThe coefficient between the normalized amplitudes is order of unity, slightly higher than 1, so we get ṽ /c s 2 ≥ |ñ/n 0 | 2 .The difference can be used to discriminate the modes. We note that, while the relation is derived from the linear response, a similar relation holds even for the nonlinear state, as indicated by numerical simulation [8].An experiment was conducted to determine if and how DWs and D'AMs could coexist in linear plasma devices. Several plasma discharges were realized in PANTA device in Kyushu University [9], with Argon gas pressure of 3.0 m Torr and B = 0.09 T, at parallel position z = 1625 mm. I is
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