  Evidence is accumulating for the crucial role of a solid's free electrons in
the dynamics of solid-liquid interfaces. Liquids induce electronic polarization
and drive electric currents as they flow; electronic excitations, in turn,
participate in hydrodynamic friction. Yet, the underlying solid-liquid
interactions have been lacking a direct experimental probe. Here, we study the
energy transfer across liquid-graphene interfaces using ultrafast spectroscopy.
The graphene electrons are heated up quasi-instantaneously by a visible
excitation pulse, and the time evolution of the electronic temperature is then
monitored with a terahertz pulse. We observe that water accelerates the cooling
of the graphene electrons, whereas other polar liquids leave the cooling
dynamics largely unaffected. A quantum theory of solid-liquid heat transfer
accounts for the water-specific cooling enhancement through a resonance between
the graphene surface plasmon mode and the so-called hydrons -- water charge
fluctuations --, particularly the water libration modes, that allows for
efficient energy transfer. Our results provide direct experimental evidence of
a solid-liquid interaction mediated by collective modes and support the
theoretically proposed mechanism for quantum friction. They further reveal a
particularly large thermal boundary conductance for the water-graphene
interface and suggest strategies for enhancing the thermal conductivity in
graphene-based nanostructures.
