Deeply-cooled water under strong confinement: Neutron scattering investigations and the liquid-liquid critical point hypothesis

We present an overview of recent experimental investigations into the properties of strongly-confined water below the bulk freezing temperature. Under strong confinement, the crystallization of water is completely suppressed and the behavior of the confined liquid state can be measured at temperatures and pressures that are inaccessible to the bulk liquid. We focus on two phenomena that have recently been discovered in strongly confined water: the density minimum and the fragile-to-strong dynamic crossover. All experimental results seem to indicate that confined water undergoes a unique kind of transition below the bulk homogeneous nucleation limit. Much of the recent work on deeply-cooled water under strong confinement has been motivated by the liquid-liquid critical point (LLCP) hypothesis. We discuss this hypothesis in the context of the various experimental findings.