  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, $\sim\omega^2$) 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 towards 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.
