1,720,995 research outputs found
Total internal reflection scattering
No full textWe show that the wave front of a total internal reflected beam is perturbed by fluctuations in the region probed by the evanescent wave, and light is scattered both above and below the critical angle. While singly scattered light is related to the two-dimensional Fourier transform at the boundary, multiply scattered light originating from very turbid samples can appear only below the critical angle. We show that the very weak scattered light above the critical angle is due to a double tunnel effect, and it is solely due to single scattering at the surface. Applications are discussed
Descrizione di un procedimento per la misurazione di proprietà di materiali torbidi e relativa apparecchiatura
No full textTransient oscillations in Soret-driven convection in a colloidal suspension
No full textWe present measurements of the transient stage of Soret-driven convective instability. The sample is a diluted colloidal suspension of silica spheres in water with an unusually large negative Soret coefficient ST. A large temperature gradient (heating from above) is rapidly applied over the sample, while a shadowgraph imaging technique provides images of the convective flow. From the processing of the variance of the intensity of the images we are able to recover the time evolution of the overall intensity of the convective flow. A typical evolution of such signal exhibits, after a latency time, a peak followed by some damped oscillations leading to a steady-state value. Both the onset time taup (the temporal position of the first peak), and the oscillation period tauosc show a power law dependence as a function of the solutal Rayleigh number Rs. The exponents found are compared with predictions from existing models
Boosting the voltage of a salinity-gradient-power electrochemical cell by means of complex-forming solutions
No full textWe report experiments on a concentration cell with zinc electrodes and ZnCl2 solutions at different concentrations, separated by a porous diaphragm. The cell is aimed at the conversion of the free energy associated to the concentration difference into electrical energy, for renewable and clean energy applications. Usually, the diffusion of the solute across the diaphragm constitutes a waste of free energy, which impairs the voltage generation of the concentration cell with respect to other well-known techniques that work quasi-reversibly, such as reverse electrodialysis or the “mixing entropy battery.” Quite surprisingly, we find that the voltage produced by our concentration cell is significantly higher than the voltage obtained with the other quasi-reversible techniques. We show that the surplus voltage comes from the active transformation of the mixing free energy into electrical energy performed by the liquid junction, and we show the connection with the negative apparent transference number of the zinc ion. This fortunate consequence of using ZnCl2 solution is ultimately related to the formation of complexes. We present the results of a cell for power production, which has excellent performances with respect to known salinity-difference-power methods
Near field scattering
No full textWe present a series of novel, unconventional optical techniques that are capable of performing measurements of low and ultra-low angle scattered light, and are therefore of potential interest for colloid physics and complex fluids in general. These techniques determine the angular intensity distribution through a spatial frequency spectral analysis of the signal recorded in the near field of the scattered light. Experimental results and a comparative discussion of these techniques are presented
Theoretical thermodynamic analysis of a closed-cycle process for the conversion of heat into electrical energy by means of a distiller and an electrochemical cell
Full text linkWe analyse a device aimed at the conversion of heat into electrical energy,
based on a closed cycle in which a distiller generates two solutions at
different concentrations, and an electrochemical cell consumes the
concentration difference, converting it into electrical current. We first study
an ideal model of such a process. We show that, if the device works at a single
fixed pressure (i.e. with a ``single effect''), then the efficiency of the
conversion of heat into electrical power can approach the efficiency of a
reversible Carnot engine operating between the boiling temperature of the
concentrated solution and that of the pure solvent. When two heat reservoirs
with a higher temperature difference are available, the overall efficiency can
be incremented by employing an arrangement of multiple cells working at
different pressures (``multiple effects''). We find that a given efficiency can
be achieved with a reduced number of effects by using solutions with a high
boiling point elevation.Comment: The following article has been submitted to Journal of Renewable and
Sustainable Energy. After it is published, it will be found at
http://scitation.aip.org/content/aip/journal/jrs
Proof-of-Concept of a Zinc-Silver Battery for the Extraction of Energy from a Concentration Difference
Full text linkThe conversion of heat into current can be obtained by a process with two stages. In the first one, the heat is used for distilling a solution and obtaining two flows with different concentrations. In the second stage, the two flows are sent to an electrochemical cell that produces current by consuming the concentration difference. In this paper, we propose such an electrochemical cell, working with water solutions of zinc chloride. The cell contains two electrodes, made respectively of zinc and silver covered by silver chloride. The operation of the cell is analogous to that of the capacitive mixing and of the “mixing entropy battery”: the electrodes are charged while dipped in the concentrated solution and discharged when dipped in the diluted solution. The cyclic operation allows us to extract a surplus of energy, at the expense of the free energy of the concentration difference. We evaluate the feasibility of such a cell for practical applications and find that a power up to 2 W per m2 of the surface of the electrodes can be achieved
Nondiffusive decay of gradient-driven fluctuations in a free-diffusion process
No full textWe report the results of an experimental study of the static and dynamic properties of long wavelength concentration fluctuations in a mixture of glycerol and water undergoing free diffusion. The shadowgraph method was used to measure both the mean-squared amplitude and the temporal correlation function of the fluctuations for wave vectors so small as to be inaccessible to dynamic light scattering. For a fluid with a stabilizing vertical concentration gradient, the fluctuations are predicted to have a decay rate that increases with decreasing wave vector q, for wave vectors below a cutoff wave vector qC, determined by gravity and the fluid properties. This behavior is caused by buoyant forces acting on the fluctuations. We find that for wave vectors above ~qC, the decay rate does vary in the normal diffusive manner as Dq2, where D is the mass diffusion coefficient. Furthermore, for qqC we find that longer wavelength fluctuations decay more rapidly than do shorter wavelength fluctuations, i.e., the behavior is nondiffusive, as predicted
Use of dynamic schlieren interferometry to study fluctuations during free diffusion
No full textWe used a form of schlieren interferometry to measure the mean-squared amplitude and temporal autocorrelation function of concentration fluctuations driven by the presence of a gradient during the free diffusion of a urea solution into water. By taking and processing sequences of images separated in time by less than the shortest correlation time of interest, we were able to simultaneously measure dynamics at a number of different wave vectors. The technique is conceptually similar to the shadowgraph method, which has been used to make similar measurements, but the schlieren method has the advantage that the transfer function is wave-vector independent rather than oscillatory
Chlorophyll Ring Deformation Modulates Qy Electronic Energy in Chlorophyll-Protein Complexes and Generates Spectral Forms
No full textAbstractThe possibility that the chlorophyll (chl) ring distortions observed in the crystal structures of chl-protein complexes are involved in the transition energy modulation, giving rise to the spectral forms, is investigated. The out-of-plane chl-macrocycle distortions are described using an orthonormal set of deformations, defined by the displacements along the six lowest-frequency, out-of-plane normal coordinates. The total chl-ring deformation is the linear combination of these six deformations. The two higher occupied and the two lower unoccupied chl molecular orbitals, which define the Qy electronic transition, have the same symmetry as four of the six out-of-plane lowest frequency modes. We assume that a deformation along the normal-coordinate having the same symmetry as a given molecular orbital will perturb that orbital and modify its energy. The changes in the chl Qy transition energies are evaluated in the Peridinin-Chl-Protein complex and in light harvesting complex II (LHCII), using crystallographic data. The macrocycle deformations induce a distribution of the chl Qy electronic energy transitions which, for LHCII, is broader for chla than for chlb. This provides the physical mechanism to explain the long-held view that the chla spectral forms in LHCII are both more numerous and cover a wider energy range than those of chlb
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