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Register a new userBulk viscosity of CO$_2$ from Rayleigh-Brillouin lightscattering spectroscopy at 532 nm
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Rayleigh-Brillouin scattering spectra of CO$_2$ were measured at pressures ranging from 0.5 to 4~bar, and temperatures from 257 to 355~K using green laser light (wavelength 532~nm, scattering angle of 55.7$^circ$). These spectra were compared to two lineshape models, which take the bulk viscosity as a parameter. One model applies to the kinetic regime, i.e. low pressures, while the second model uses the continuum, hydrodynamic approach and takes the rotational relaxation time as a parameter, which translates into the bulk viscosity. We do not find a significant dependence of the bulk viscosity with pressure or temperature. At pressures where both models apply we find a consistent value of the ratio of bulk viscosity over shear viscosity $eta_b/eta_s = 0.41 pm 0.10$. This value is four orders of magnitude smaller than the common value that is based on the damping of ultrasound, and signifies that in light scattering only relaxation of rotational modes matters, while vibrational modes remain frozen.
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Spontaneous Rayleigh-Brillouin scattering (RBS) experiments have been performed in air for pressures in the range 0.25 - 3 bar and temperatures in the range 273 - 333 K. The functional behaviour of the RB-spectral profile as a function of experimental parameters, such as the incident wavelength, scattering angle, pressure and temperature is analyzed, as well as the dependence on thermodynamic properties of the gas, as the shear viscosity, the thermal conductivity, the internal heat capacity and the bulk viscosity. Measurements are performed in a scattering geometry detecting at a scattering angle $theta=55.7^circ$ and an incident wavelength of $lambda_i=532.22$ nm, at which the Brillouin features become more pronounced than in a right angles geometry and for ultraviolet light. For pressure conditions of 1 - 3 bar the RB-spectra, measured at high signal-to-noise ratio, are compared to Tenti-S6 model calculations and values for the bulk viscosity of air are extracted. Values of $eta_b$ are found to exhibit a linear dependence on temperature over the measurement interval in the range $1.0 - 2.0 times 10^{-5}$ Pa$cdot$s. A temperature dependent value is deduced from a collection of experiments to yield: $eta_{rm b} = (0.86 times 10^{-5}) + 1.29 times 10^{-7} cdot (T - 250)$. These results are implemented in model calculations that were verified for the low pressure conditions ($p < 1$ bar) relevant for the Earths atmosphere. As a result we demonstrate that the RB-scattering spectral profiles for air under sub-atmospheric conditions can be generated via the Tenti-S6 model, for given gas-phase and detection conditions ($p$, $T$, $lambda_i$, and $theta$), and for values for the gas transport coefficients.
High signal-to-noise and high-resolution light scattering spectra are measured for nitrous oxide (N$_2$O) gas at an incident wavelength of 403.00 nm, at 90$^circ$ scattering, at room temperature and at gas pressures in the range $0.5-4$ bar. The resulting Rayleigh-Brillouin light scattering spectra are compared to a number of models describing in an approximate manner the collisional dynamics and energy transfer in this gaseous medium of this polyatomic molecular species. The Tenti-S6 model, based on macroscopic gas transport coefficients, reproduces the scattering profiles in the entire pressure range at less than 2% deviation at a similar level as does the alternative kinetic Grads 6-moment model, which is based on the internal collisional relaxation as a decisive parameter. A hydrodynamic model fails to reproduce experimental spectra for the low pressures of 0.5-1 bar, but yields very good agreement ($< 1$%) in the pressure range $2-4$ bar. While these three models have a different physical basis the internal molecular relaxation derived can for all three be described in terms of a bulk viscosity of $eta_b sim (6 pm 2) times 10^{-5}$ Pa$cdot$s. A rough-sphere model, previously shown to be effective to describe light scattering in SF$_6$ gas, is not found to be suitable, likely in view of the non-sphericity and asymmetry of the N-N-O structured linear polyatomic molecule.
We investigate CO$_2$-driven diffusiophoresis of colloidal particles and bacterial cells in a Hele-Shaw geometry. Combining experiments and a model, we understand the characteristic length and time scales of CO$_2$-driven diffusiophoresis in relation to system dimensions and CO$_2$ diffusivity. Directional migration of wild-type V. cholerae and a mutant lacking flagella, as well as S. aureus and P. aeruginosa, near a dissolving CO$_2$ source shows that diffusiophoresis of bacteria is achieved independent of cell shape and Gram stain. Long-time experiments suggest possible applications for bacterial diffusiophoresis to cleaning systems or anti-biofouling surfaces.
(abridged) Context: Turbulent diffusion of large-scale flows and magnetic fields play major roles in many astrophysical systems. Aims: Our goal is to compute turbulent viscosity and magnetic diffusivity, relevant for diffusing large-scale flows and magnetic fields, respectively, and their ratio, the turbulent magnetic Prandtl number, ${rm Pm}_{rm t}$, for isotropically forced homogeneous turbulence. Methods: We use simulations of forced turbulence in fully periodic cubes composed of isothermal gas with an imposed large-scale sinusoidal shear flow. Turbulent viscosity is computed either from the resulting Reynolds stress or from the decay rate of the large-scale flow. Turbulent magnetic diffusivity is computed using the test-field method. The scale dependence of the coefficients is studied by varying the wavenumber of the imposed sinusoidal shear and test fields. Results: We find that turbulent viscosity and magnetic diffusivity are in general of the same order of magnitude. Furthermore, the turbulent viscosity depends on the fluid Reynolds number (${rm Re}$) and scale separation ratio of turbulence. The scale dependence of the turbulent viscosity is found to be well approximated by a Lorentzian. The results for the turbulent transport coefficients appear to converge at sufficiently high values of ${rm Re}$ and the scale separation ratio. However, a weak decreasing trend is found even at the largest values of ${rm Re}$. The turbulent magnetic Prandtl number converges to a value that is slightly below unity for large ${rm Re}$ whereas for small ${rm Re}$, we find values between 0.5 and 0.6. Conclusions: The turbulent magnetic diffusivity is in general consistently higher than the turbulent viscosity. The actual value of ${rm Pm}_{rm t}$ found from the simulations ($approx0.9ldots0.95$) at large ${rm Re}$ and scale separation ratio is higher than any of the analytic predictions.
We give theoretical analyses of the Magneto-Rayleigh-Taylor instability driven by a rotating magnetic field. Both slab and liner configurations with finite thicknesses are dealt with in the WKB and the non-WKB approximations. Results show that instabilities for all modes (combinations of wave vectors) are alleviated. We further discuss the potential application of the alternant/nested configurations of a theta and a Z pinch to the Theta-Z Liner Inertia Fusion (Theta-Z-LIF) concept.
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