Competing hydrogen-bond orders drive water's anomalous surface tension

Water's surface tension shows a nonlinear temperature dependence, including a reentrant increase in the supercooled regime - a longstanding puzzle in physical chemistry. Using molecular dynamics simulations, we uncover a structural mechanism linking microscopic ordering to macroscopic interfacial behaviour. Surface tension arises from the interplay between ρ-states, characterised by O-H alignment under surface symmetry breaking, and tetrahedral S-states stabilised in the subsurface by negative pressure. Water's surface tension γ is governed by the interplay of their anisotropies: at intermediate temperatures, ρ-state anisotropy saturates while S-states remain weakly anisotropic, slowing the growth of γ. Upon deeper supercooling, however, S-states acquire orientational order, amplifying anisotropy and producing the reentrant rise. This unified framework explains both inflection points of γ(T) and establishes a structural-mechanical link between local hydrogen-bond motifs and interfacial stress, with implications for nucleation, cryopreservation, and ferroelectric-like ordering, and extending beyond water to other network-forming liquids.