Eur. Phys. J. E (2014) 37: 105
https://doi.org/10.1140/epje/i2014-14105-3

Regular Article

Static versus dynamic analysis of the influence of gravity on concentration non-equilibrium fluctuations

¹ Laboratoire des Fluides Complexes et leurs Réservoirs - UMR5150, Université de Pau et des Pays de l’Adour, 64600, Anglet, France
² Department of Physics, University of Fribourg, CH-1700, Fribourg, Switzerland

^* e-mail: fabrizio.croccolo@univ-pau.fr

Received: 26 June 2014
Revised: 2 September 2014
Accepted: 14 October 2014
Published online: 6 November 2014

Abstract

In a binary fluid mixture subject to gravity and a stabilizing concentration gradient, concentration non-equilibrium fluctuations are long-ranged. While the gradient leads to an enhancement of the respective equilibrium fluctuations, the effect of gravity is a damping of fluctuations larger than a “characteristic” size. This damping is visible both in the fluctuation power spectrum probed by static and the temporal correlation function probed by dynamic light scattering. One aspect of the “characteristic” size can be appreciated by the dynamic analysis; in fact at the corresponding “characteristic” wave vector q ^* one can observe a maximum of the fluctuation time constant indicating the more persistent fluctuation of the system. Also in the static analysis a “characteristic” size can be extracted from the crossover wave vector. According to common theoretical concepts, the result should be the same in both cases. In the present work we provide evidence for a systematic difference in the experimentally observed “characteristic” size as obtained by static and dynamic measurements. Our observation thus points out the need for a more refined theory of non-equilibrium concentration fluctuations.