Cutting Ice: Nanowire Regelation

Even below its normal melting temperature, ice melts when subjected to high pressure and refreezes once the pressure is lifted. A classic demonstration of this regelation phenomenon is the passing of a thin wire through a block of ice when sufficient force is exerted. Here we present a molecular-dynamics study of a nanowire cutting through ice to unravel the molecular level mechanisms responsible for regelation. In particular, we show that the transition from a stationary to a moving wire due to increased driving force changes from symmetric and continuous to asymmetric and discontinuous as a hydrophilic wire is replaced by a hydrophobic one. This is explained at the molecular level in terms of the wetting properties of the wire.

Figure 1

(a) Velocity of a hydrophilic wire as a function of driving force for different radii. The inset shows the data on a logarithmic scale. (b) Velocity of a hydrophobic wire (9.0 Å radius) as a function of driving force.

Figure 2

(a) Average bonding energy (in the scale of the energy of ideal H bonds) of water molecules around a hydrophilic wire (9.0 Å radius) in the stick-slip regime. (b) Bonding in the sliding regime. (c) Bonding around a hydrophobic wire in the sliding regime. The direction of wire movement is downwards in each case. The vertical stripes are artifacts of the lattice structure of the surrounding ice.

Figure 3

(a) Schematic: The liquid flows easily around a wetting wire and only a thin layer of liquid builds. (b) Snapshot of a wetting wire, corresponding to (a). (c) Schematic: The liquid is slow to pass a nonwetting wire, leading to a thick liquid layer and formation of voids behind the wire. Once the liquid passes the wire, it rapidly fills the voids partially depleting the liquid layer. (d) Snapshots of a nonwetting wire, corresponding to (c). (e) Water velocity field around a wetting wire (in wire reference frame): the velocity is highest at the sides of the wire [red (dark gray), $0.1\AA /\mathrm{ps}$]. (f) Water velocity field around a nonwetting wire: the velocity is highest on top of the wire.