Time-resolved spectroscopy and scaling behavior in LiCl/H2O near the liquid-glass transition

Abstract

Time-domain light scattering experiments on ps-/xs time scales have been conducted on a concentrated aqueous LiCl solution. Analysis of mechanical and electrical modulus spectra confirms some results of mode-coupling theory: simple scaling for a peaks and property-independent power laws. This is made without any assumptions about the data which are not derived from theory. A new characteristic time scale, reflecting the short-and intermediate-range ordering dynamics, is also observed.PACS numbers: 64.70.Pf, 65.70.+y, 78.47.+p During the past decade the study of the liquid-glass transition has taken a new impetus, sparked by the emergence of new theoretical and experimental approaches [ll. Of great interest is the mode-coupling theory (MCT) [2][3][4][5] which has generated a new way of thinking about this difficult problem. Traditionally, the emphasis in the analysis of susceptibility data has been on the different relaxation peaks. MCT shifts the focus to the wings and to the minimum between the a and ft relaxations, for which it makes quantitative predictions. Their experimental verification requires measurements over many decades in time scale. Large bandwidth electrical susceptibility measurements have been possible for decades, but mechanical measurements in the MHz-GHz range (relevant to MCT) have been difficult. We present here impulsive stimulated light scattering (ISS) data from an aqueous LiCl solution, as well as a novel analysis of mechanical and electrical susceptibility data for supercooled liquids.Descriptions of the theoretical underpinnings of the ISS technique and its experimental implementations are given elsewhere [6,7]. Briefly, two X L =1064 nm, 100 ps, parallel polarized laser pulses crossed at an angle 6g were used for transient grating excitation of both longitudinal acoustic waves and the thermal diffusion mode at the grating wave vector q = s (47rA£)sin(0£/2). The timedependent diffraction of a variably delayed 532-nm probe pulse was then measured. We used seven scattering angles between 1° and 60°, yielding acoustic wavelengths of 62.8, 30.8, 14.1, 7.53, 3.96, 1.94, and 1.07 pm and frequencies from 25 MHz to 4 GHz. The electrolyte LiCl/H 2 0 13 mol% was studied for 70 < T < 300 K. This concentration is situated between the equilibrium (12.5 mol%) and the lower nonequilibrium (14.5 mol%) eutectic concentrations [8]. The calorimetric glass transition temperature is T g = 135 K.Typical ISS data are displayed in Fig. 1. All exhibit damped acoustic oscillations (the transient response to sudden heating) and a steady-state density modulation (caused by thermal expansion at grating maxima and compression at nulls) which decays due to thermal diffusion. The trends in acoustic frequency and attenuation rate are as expected and observed in other glassforming materials [9]. Between 165 and 200 K the steady-state response shows additional temporal features. There is a gradual rise in signal, which can be thought of as time-dependent thermal expansion and which reflects slow components of structu...

Nonlinear hydrodynamics of a hard-sphere fluid near the glass transition

“…In some cases, the β and the α regimes are separated by a region of so-called von Schweidler relaxation which has a power law form. The stretched exponential behavior in the α regime and the von Schweidler relaxation are seen in dielectric measurements 14 and in light 15 and neutron scattering 16 experiments. The parameters which describe the decay of S(q, t) are known to be q-dependent, but the nature of this dependence has not been studied in detail.…”

Revisiting impulsive stimulated thermal scattering in supercooled liquids: Relaxation of specific heat and thermal expansion

“…The intriguing behavior of glass-forming liquids is attracting continued interest from many researchers [2][3][4][5][6][7][8][9]. By virtue of its ability to simultaneously probe multiple relaxation processes (thermal expansion, acoustic and even orientational response [10,11]), the use of impulsive stimulated scattering (ISS) in a periodical grating geometry has been successful in obtaining new insights from the thermoelastic response (measured via the accompaning coherent diffraction of a probe laser beam) to impulsive photothermal excitation of different glassformers [11][12][13][14][15][16][17]. Standard thermo-mechanical modelling, based on the assumption of frequency independent or non-relaxing heat capacity and thermal expansion coefficient, have been shown not to be adequate to characterize the dynamics triggered in an ISS experiment, especially for viscous systems.…”

Numerical study of long-time dynamics and ergodic-nonergodic transitions in dense simple fluids

“…This glass transition is seen in a wide variety of fluids from complex molecular liquids [5][6][7][8][9][10][11][12][13] to simple colloidal suspensions [14][15][16][17].…”

“…As one approaches this glassy state, the dense fluid can show several features unique to this transition: the relaxation dynamics slow, the structure arrests, and a disordered nonequilibrium state emerges [1][2][3][4]. This glass transition is seen in a wide variety of fluids from complex molecular liquids [5][6][7][8][9][10][11][12][13] to simple colloidal suspensions [14][15][16][17].…”

“…Remarkably, many dynamic quantities which appear to diverge at the glass transition-e.g., viscosity, diffusion constant, and relaxation time-can be collapsed to universal functions when temperature is scaled by T g (or density scaled by η g ) [19]. Likewise, the two-step decay of the densitydensity correlation function detailed above appears on supercooling for so wide a range of materials-from macroscopic colloidal suspensions described by Brownian dynamics [14][15][16][17] to complex molecular or polymer fluids whose behavior is controlled by atomic-level forces [5][6][7][8][9][10][11][12][13]-that it is considered the very hallmark of the glass transition.…”