1,721,044 research outputs found
Scaling of the spatial power spectrum of excitations at the onset of solutal convection in a nanofluid far from equilibrium
No full textWe investigate pattern formation in the very early stages of solutal convective instabilities in a suspension of highly thermophilic nanoparticles heated from above. The processing of shadowgraph images allows us to recover the spatial power spectrum of the excitations at the onset. Remarkably, the power spectra obtained at large solutal Rayleigh numbers 2.56×10<6Ras<4.53×108 scale onto a single curve without adjustable parameters. The critical wave number exhibits power-law scaling with exponent 1/4 as a function of Ras, in excellent agreement with recent theoretical predictions
Near-field scattering techniques : novel instrumentation and results from time and spatially resolved investigations of soft matter systems
No full textRecently the investigation of non-stationary systems exhibiting spatial and dynamic heterogeneities has propelled the development of innovative optical techniques providing the missing link between the scattering and imaging approaches. The novel techniques are characterized by the fact that the scattered radiation is measured close to the sample. They allow to recover information on the structure and dynamics of the system under investigation equivalent to that available with scattering techniques, with the great advantage that they simultaneously allow to achieve a local characterization of the sample, which is lost in traditional scattering measurements
Nonequilibrium fluctuations during diffusion in liquid layers
Full text linkTheoretical analysis and experiments have provided compelling evidence of the presence of long-range nonequilibrium concentration fluctuations during diffusion processes in fluids. In this paper, we investigate the dependence of the features of the fluctuations from the dimensionality of the system. In three-dimensional fluids the amplitude of nonequilibrium fluctuations can become several orders of magnitude larger than that of equilibrium fluctuations. Notwithstanding that, the amplitude of nonequilibrium fluctuations remains small with respect to the concentration difference driving the diffusion process. By extending the theory to two-dimensional systems, such as liquid monolayers and bilayers, we show that the amplitude of the fluctuations becomes much stronger than in three-dimensional systems. We investigate the properties of the fronts of diffusion and show that they have a self-affine structure characterized by a Hurst exponent H=1. We discuss the implications of these results for diffusion in liquid crystals and in cellular membranes of living organisms
Soret driven convection in a colloidal solution heated from above at very large solutal Rayleigh number
No full textConvection in a colloidal suspension with a large negative separation ratio ? is studied experimentally by heating from above. Shadowgraph observations at very large solutal Rayleigh numbers R~ are reported as a function of time. Fast relaxation oscillations are reported for the root mean square value of the shadowgraph intensity. While pure fluids exhibit a transition to turbulent convection for Rayleigh number R?106, stable spoke-pattern planform with up and down columnar flows are observed up to R~?1.9×109. It is suggested that the surprising stability of the planform against turbulence is due to nonlinear focusing arising from the concentration dependence of the diffusion coefficient
Bistable heat transfer in a nanofluid
No full textHeat convection in water can be suppressed by adding a small amount of highly thermophilic nanoparticles (NP). We show that such suppression is not effective when a suspension with uniform concentration of NP is suddenly heated from below. At Rayleigh numbers smaller than a sample dependent threshold Ra* we observe transient oscillatory convection. Unexpectedly, the duration of convection diverges at Ra* Above Ra* oscillatory convection becomes permanent and the heat transferred exhibits bistability. Our results are explained only partially and qualitatively by existing theories
Decay of ozone in water : a review
No full textScientific publications on ozone decay kinetics in water report very wide result variations, depending on a multiplicity of factors, such as ozone concentration, pH, temperature, alkalinity, fluid-dynamic conditions, presence of UV radiations, and concentration of organic and inorganic carbon. This paper intends to provide an engineering-oriented review that summarizes and compares the main results reported in the recent literature so that the condensed information can guide the reader in getting operational indications to assist applications and research
Tunable heat transfer with smart nanofluids
No full textStrongly thermophilic nanofluids are able to transfer either small or large quantities of heat when subjected to a stable temperature difference. We investigate the bistability diagram of the heat transferred by this class of nanofluids. We show that bistability can be exploited to obtain a controlled switching between a conductive and a convective regime of heat transfer, so as to achieve a controlled modulation of the heat flux
Fast-onset Soret-driven convection in a colloidal suspension heated from above
No full textConvection in a colloidal suspension with a large negative separation ratio b.Psi is studied experimentally by heating the sample from above. Shadowgraph observations at large solutal Rayleigh numbers (R)tilde are reported as a function of time. Stable spoke-pattern planforms with up and down columnar flows are observed at (R)tilde ap10 6 . The convective flow starts in a time much smaller than that needed for the concentration gradient to reach steady state. Fast relaxation oscillations are reported for the rms value of the shadowgraph intensity. The oscillation period is in good agreement with the predictions of a recent work on self-oscillation behaviour induced by the Soret effect. However, in contrast with the prediction of the theory, the oscillations are damped and the rms attains a steady state value. Moreover, at very high stresses, the oscillations are overdamped
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