Eur. Phys. J. E (2016) 39: 40
https://doi.org/10.1140/epje/i2016-16040-7

Regular Article

Characteristic length scales of the secondary relaxations in glass-forming glycerol

¹ JCNS-SNS, Biology and Soft-matter Division, Neutron Sciences Directorate, Oak Ridge National Laboratory (ORNL), Bethel Valley Road, PO BOX 2008 MS6473, 37831, Oak Ridge, TN, USA
² Chemical and Engineering Materials Division, Neutron Sciences Directorate, Oak Ridge National Laboratory (ORNL), PO BOX 2008 MS6473, 37831-6473, Oak Ridge, TN, USA

^* e-mail: s.gupta@fz-juelich.de

Received: 22 August 2015
Accepted: 26 February 2016
Published online: 29 March 2016

Abstract

We investigate the secondary relaxations and their link to the main structural relaxation in glass-forming liquids using glycerol as a model system. We analyze the incoherent neutron scattering signal dependence on the scattering momentum transfer, Q , in order to obtain the characteristic length scale for different secondary relaxations. Such a capability of neutron scattering makes it somewhat unique and highly complementary to the traditional techniques of glass physics, such as light scattering and broadband dielectric spectroscopy, which provide information on the time scale, but not the length scales, of relaxation processes. The choice of suitable neutron scattering techniques depends on the time scale of the relaxation of interest. We use neutron backscattering to identify the characteristic length scale of 0.7 Å for the faster secondary relaxation described in the framework of the mode-coupling theory (MCT). Neutron spin-echo is employed to probe the slower secondary relaxation of the excess wing type at a low temperature ( ∼ 1.13T _g . The characteristic length scale for this excess wing dynamics is approximately 4.7 Å. Besides the Q -dependence, the direct coupling of neutron scattering signal to density fluctuation makes this technique indispensable for measuring the length scale of the microscopic relaxation dynamics.