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

  • Fabio Giavazzi
  • Alessandro Fornasieri
  • Alberto Vailati
  • Roberto Cerbino
Regular Article
Part of the following topical collections:
  1. Non-isothermal transport in complex fluids

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 \( T-T_{c}\in [0.1,30]\)°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 \( \gamma\), \( \nu\) and \( \eta\), 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.

Graphical abstract

Keywords

Topical Issue: Non-isothermal transport in complex fluids 

Copyright information

© EDP Sciences, SIF, Springer-Verlag Berlin Heidelberg 2016

Authors and Affiliations

  • Fabio Giavazzi
    • 1
  • Alessandro Fornasieri
    • 1
  • Alberto Vailati
    • 2
  • Roberto Cerbino
    • 1
  1. 1.Dipartimento di Biotecnologie Mediche e Medicina TraslazionaleUniversità degli Studi di MilanoSegrateItaly
  2. 2.Dipartimento di FisicaUniversità degli Studi di MilanoMilanoItaly

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