Abstract:For liquid rubidium the Stokes-Einstein (SE) relation is well fulfilled near the melting point with an effective hydrodynamic diameter, which agrees well with a value from structural investigations. A wealth of thermodynamic and microscopic data exists for a wide range of temperatures for liquid rubidium and hence it represents a good test bed to challenge the SE relation with rising temperature from an experimental point of view. We performed classical molecular dynamics simulations to complement the existing… Show more
“…Interestingly, this change is close to a temperature , which was derived as a thermodynamic stability limit of the solid crystalline state of aluminium 31 , and beyond which no superheated metastable solid aluminium may survive.. That temperature agrees well with the increase in the amplitude , which is a measure for the decay of density fluctuations on next-neighbor distances. The amplitude can be related directly to an average relaxation time for the density fluctuations 33 and hence evidences a slowing down of the dynamics. Figure 3 The relaxation times from the KWW-fits to the simulation and X-ray data are plotted against the inverse temperature on a logarithmic scale.…”
The characteristic property of a liquid, discriminating it from a solid, is its fluidity, which can be expressed by a velocity field. The reaction of the velocity field on forces is enshrined in the transport parameter viscosity. In contrast, a solid reacts to forces elastically through a displacement field, the particles are trapped in their potential minimum. The flow in a liquid needs enough thermal energy to overcome the changing potential barriers, which is supported through a continuous rearrangement of surrounding particles. Cooling a liquid will decrease the fluidity of a particle and the mobility of the neighbouring particles, resulting in an increase of the viscosity until the system comes to an arrest. This process with a concomitant slowing down of collective particle rearrangements might already start deep inside the liquid state. The idea of the potential energy landscape provides an attractive picture for these dramatic changes. However, despite the appealing idea there is a scarcity of quantitative assessments, in particular, when it comes to experimental studies. Here we present results on a monatomic liquid metal through a combination of ab initio molecular dynamics, neutron spectroscopy and inelastic x-ray scattering. We investigated the collective dynamics of liquid aluminium to reveal the changes in dynamics when the high temperature liquid is cooled towards solidification. The results demonstrate the main signatures of the energy landscape picture, a reduction in the internal atomic structural energy, a transition to a stretched relaxation process and a deviation from the high-temperature Arrhenius behavior of the relaxation time. All changes occur in the same temperature range at about $$1.4 \cdot T_{melting}$$
1.4
·
T
melting
, which can be regarded as the temperature when the liquid aluminium enters the landscape influenced phase and enters a more viscous liquid state towards solidification. The similarity in dynamics with other monatomic liquid metals suggests a universal dynamic crossover above the melting point.
“…Interestingly, this change is close to a temperature , which was derived as a thermodynamic stability limit of the solid crystalline state of aluminium 31 , and beyond which no superheated metastable solid aluminium may survive.. That temperature agrees well with the increase in the amplitude , which is a measure for the decay of density fluctuations on next-neighbor distances. The amplitude can be related directly to an average relaxation time for the density fluctuations 33 and hence evidences a slowing down of the dynamics. Figure 3 The relaxation times from the KWW-fits to the simulation and X-ray data are plotted against the inverse temperature on a logarithmic scale.…”
The characteristic property of a liquid, discriminating it from a solid, is its fluidity, which can be expressed by a velocity field. The reaction of the velocity field on forces is enshrined in the transport parameter viscosity. In contrast, a solid reacts to forces elastically through a displacement field, the particles are trapped in their potential minimum. The flow in a liquid needs enough thermal energy to overcome the changing potential barriers, which is supported through a continuous rearrangement of surrounding particles. Cooling a liquid will decrease the fluidity of a particle and the mobility of the neighbouring particles, resulting in an increase of the viscosity until the system comes to an arrest. This process with a concomitant slowing down of collective particle rearrangements might already start deep inside the liquid state. The idea of the potential energy landscape provides an attractive picture for these dramatic changes. However, despite the appealing idea there is a scarcity of quantitative assessments, in particular, when it comes to experimental studies. Here we present results on a monatomic liquid metal through a combination of ab initio molecular dynamics, neutron spectroscopy and inelastic x-ray scattering. We investigated the collective dynamics of liquid aluminium to reveal the changes in dynamics when the high temperature liquid is cooled towards solidification. The results demonstrate the main signatures of the energy landscape picture, a reduction in the internal atomic structural energy, a transition to a stretched relaxation process and a deviation from the high-temperature Arrhenius behavior of the relaxation time. All changes occur in the same temperature range at about $$1.4 \cdot T_{melting}$$
1.4
·
T
melting
, which can be regarded as the temperature when the liquid aluminium enters the landscape influenced phase and enters a more viscous liquid state towards solidification. The similarity in dynamics with other monatomic liquid metals suggests a universal dynamic crossover above the melting point.
“…For monatomic liquid metals changes with temperature were found in the structural relaxation dynamics on an atomistic length scale for rubidium, lead, and aluminium [8][9][10]. The amplitude S(Q 0 , ω = 0) of the collective dynamics at the structure factor maximum Q 0 showed in all three cases a change in the slope in a similar temperature range above the * franz.demmel@stfc.ac.uk melting temperature [11]. Furthermore, for all three metals an increase of the generalized longitudinal viscosity was demonstrated which occurs in a temperature range of 1.3-1.5 T m upon cooling, suggesting that the liquid metals become more viscous around this temperature range.…”
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“…These changes are based on fluctuations on an atomic length scale. However, recently the scrutiny of macroscopic transport coefficients of liquid rubidium and their product, the Stokes-Einstein relation, suggested that the change in dynamics also shows some fingerprints in the macroscopic transport parameters [56]. Furthermore the evolution of excitations with rising temperature indicated a change in collective particle dynamics in that temperature range [57].…”
Density fluctuations of liquid and 20 K undercooled gallium have been studied by neutron spectroscopy. The decay of density fluctuations has been recorded at the structure factor maximum over a wide temperature range up to twice the melting temperature. The amplitude of the scattering function falls off with rising temperature in a nonlinear way with a changing slope around 1.5 T melting . The derived generalized longitudinal viscosity shows an upturn with decreasing temperature in the same temperature range. This increase in viscosity can be understood that liquid gallium transforms from a more fluid liquid metal to a more viscous liquid metal in that temperature range upon cooling. The change in the amplitude shows a remarkable agreement with results from liquid aluminium, lead and rubidium. This study suggests a universal crossover in dynamics of liquid monatomic metals, despite the many peculiar properties of gallium.
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