  Water is the matrix of life and is generally considered a homogeneous,
uniform, liquid. Recent experiments report that water is instead a two-state
liquid. However, subsequently, these findings were contested. The structure of
water and whether we should think of it as uniform therefore remains an open
question. Here, we report femtosecond elastic second harmonic scattering
(fs-ESHS) of liquid water in comparison to an isotropic liquid (CCl4) and show
that water is indeed a non-uniform liquid, The coherent fs-ESHS intensity was
interpreted, using molecular dynamics simulations, as arising from charge
density fluctuations and consequentially enhanced nanoscale polarizabilities
around transient voids having an average lifetime of 300 fs. Although voids
were also present in CCl4, they were not characterized by hydrogen bond defects
and did not show strong polarizability fluctuations, leading to fs-ESHS of an
isotropic liquid. The voids increased in number at higher temperatures above
room temperature, in agreement with the fs-ESHS results. The measured
nanoscale-femtosecond inhomogeneities therefore do not necessarily relate to
the proposed two state model of water, but instead underscore the elusive
nature of liquid water, and undoubtedly have an impact on any type of
transition that occurs in liquid water.
