When a macroscopic concentration gradient is present across a binary mixture, long-ranged non-equilibrium concentration fluctuations (NCF) appear as a consequence of the coupling between the gradient and spontaneous equilibrium velocity fluctuations. Long-ranged equilibrium concentration fluctuations (ECF) may be also observed when the mixture is close to a critical point. Here we study the interplay between NCF and critical ECF in a near-critical mixture aniline/cyclohexane in the presence of a vertical concentration gradient. To this aim, we exploit a commercial optical microscope and a simple, custom-made, temperature-controlled cell to obtain simultaneous static and dynamic scattering information on the fluctuations. We first characterise the critical ECF at fixed temperature T above the upper critical solution temperature Tc, in the wide temperature range T−Tc∈[0.1,30]T−Tc∈[0.1,30] °C. In this range, we observe the expected critical scaling behaviour for both the scattering intensity and the mass diffusion coefficient and we determine the critical exponents γγ , νν and ηη , which are found in agreement with the 3D Ising values. We then study the system in the two-phase region (T < Tc). In particular, we characterise the interplay between ECF and NCF when the mixture, initially at a temperature Ti, is rapidly brought to a temperature Tf > Ti. During the transient, a vertical diffusive mass flux is present that causes the onset of NCF, whose amplitude vanishes with time, as the flux goes to zero. We also study the time dependence of the equilibrium scattering intensity Ieq, of the crossover wave vector qco and of the diffusion coefficient D during diffusion and find that all these quantities exhibit an exponential relaxation enslaved to the diffusive kinetics.

Equilibrium and non-equilibrium concentration fluctuations in a critical binary mixture / F. Giavazzi, A. Fornasieri, A. Vailati, R. Cerbino. - In: THE EUROPEAN PHYSICAL JOURNAL. E, SOFT MATTER. - ISSN 1292-8941. - 39(2016), pp. 103.1-103.11. [10.1140/epje/i2016-16103-9]

Equilibrium and non-equilibrium concentration fluctuations in a critical binary mixture

F. Giavazzi
Primo
;
A. Vailati
Penultimo
;
R. Cerbino
Ultimo
2016

Abstract

When a macroscopic concentration gradient is present across a binary mixture, long-ranged non-equilibrium concentration fluctuations (NCF) appear as a consequence of the coupling between the gradient and spontaneous equilibrium velocity fluctuations. Long-ranged equilibrium concentration fluctuations (ECF) may be also observed when the mixture is close to a critical point. Here we study the interplay between NCF and critical ECF in a near-critical mixture aniline/cyclohexane in the presence of a vertical concentration gradient. To this aim, we exploit a commercial optical microscope and a simple, custom-made, temperature-controlled cell to obtain simultaneous static and dynamic scattering information on the fluctuations. We first characterise the critical ECF at fixed temperature T above the upper critical solution temperature Tc, in the wide temperature range T−Tc∈[0.1,30]T−Tc∈[0.1,30] °C. In this range, we observe the expected critical scaling behaviour for both the scattering intensity and the mass diffusion coefficient and we determine the critical exponents γγ , νν and ηη , which are found in agreement with the 3D Ising values. We then study the system in the two-phase region (T < Tc). In particular, we characterise the interplay between ECF and NCF when the mixture, initially at a temperature Ti, is rapidly brought to a temperature Tf > Ti. During the transient, a vertical diffusive mass flux is present that causes the onset of NCF, whose amplitude vanishes with time, as the flux goes to zero. We also study the time dependence of the equilibrium scattering intensity Ieq, of the crossover wave vector qco and of the diffusion coefficient D during diffusion and find that all these quantities exhibit an exponential relaxation enslaved to the diffusive kinetics.
non-equlibrium; fluctuations; critical fluids; differential dynamic microscopy; quantitative microscopy; binary mixtures
Settore FIS/07 - Fisica Applicata(Beni Culturali, Ambientali, Biol.e Medicin)
Settore FIS/03 - Fisica della Materia
   Anisotropies and non equilibrium in soft matter: routes to the self assembly of advanced materials
   ANISOFT
   MINISTERO DELL'ISTRUZIONE E DEL MERITO
   RBFR125H0M_002
2016
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/2434/448304
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