Emergence of Debye Scaling in the Density of States of Liquids under Nanoconfinement

YX Yu and S Jin and X Fan and M Sarter and DH Yu and L Hong and M Baggioli, ACS NANO, 18, 24829-24841 (2024).

DOI: 10.1021/acsnano.4c04729

In the realm of nanoscience, the dynamic behaviors of liquids at scales beyond the conventional structural relaxation time, tau, unfold a fascinating blend of solid-like characteristics, including the propagation of collective shear waves and the emergence of elasticity. However, in classical bulk liquids, where tau is typically of the order of 1 ps or less, this solid-like behavior remains elusive in the low- frequency region of the density of states (DOS). Here, we provide evidence for the emergent solid-like nature of liquids at short distances through inelastic neutron scattering measurements of the low- frequency DOS in liquid water and glycerol confined within graphene oxide membranes. In particular, upon increasing the strength of confinement, we observe a transition from a liquid-like DOS (linear in the frequency omega) to a solid-like behavior (Debye law, similar to omega(2)) in the range of 1-4 meV. Molecular dynamics simulations confirm these findings and reveal additional solid-like features, including propagating collective shear waves and a reduction in the self-diffusion constant. Finally, we show that the onset of solid-like dynamics is pushed toward low frequency along with the slowing-down of the relaxation processes upon confinement. This nanoconfinement-induced transition, aligning with k-gap theory, underscores the potential of leveraging liquid nanoconfinement in advancing nanoscale science and technology, building more connections between fluid dynamics and materials engineering.

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