Cutting Ice: Nanowire Regelation
Original entry: Darren Yang, AP225, Fall 2010
When ice is subjected to high pressure locally, even below its normal melting temperature, that region can under go phase transition to liquid. This phenomenon was previously demonstrated as a thin wire passed through a block of ice when sufficient force is exerted. In this paper, the authors, present a molecular-dynamics study of a nanowire cutting through ice to unravel the molecular level mechanisms responsible for regelation. They 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 hydrophobic one replaces a hydrophilic wire.
The phenomenon of ice melting when subject to high pressure and refreezing once the pressure is lifted is known as regelation. Regelation is often demonstrated experimentally by setting a thin, weighted wire on a block of ice, whereby the wire slowly passes through the ice. However, such experiments show complicated motion of the wire as a function of temperature, driving force, wire diameter, and wire material, due to defects and impurities in the ice and conduction of heat through the wire. Recent studies demonstrated that regelation is a process at the nanoscale and involving physical principles in nucleation, adsorption, diffusion, confinement, and friction. Thus the authors study wire regelation using atomistic molecular-dynamics (MD) simulations. They 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 hydrophobic one replaces a hydrophilic wire.
The authors first show the velocity of a hydrophilic wire as a function of the driving force and different wire radii (Figure 1). As the wire is made thicker, the critical force increases since thicker wire must push through a larger area of ice breaking more hydrogen bonds between water molecules. The authors also look at the scaling relation between the velocity and the force, and they find that they have power law relationship of 0.62 (Figure 1).
The same analysis was also carried out for the hydrophobic wire. Unlike hydrophilic wire, they notice that hydrophobic wire has a clear hysteresis. (i.e. When the force is gradually increased, the depinning is seen at a higher critical force than if the force is decreased.) Thus, they indicated that the depinning transition of the hydrophobic wire is discontinuous.
The author also analyzed water-bonding energy around a hydrophilic wire at two different regimes: stick-slip motion just beyond the depinning and steady sliding at high driving force. They noticed that rapid moving wire moves a longer distance before the ice behind it has had time to heal (Figure 2a 2b). The difference between the hydrophilic and hydrophobic cases, however, is striking. The layer of liquid in front of the hydrophobic wire is much thicker than that seen for the hydrophilic one, as shown in Figure 2c.
When the wire is nonwetting, the liquid will seek to avoid the surface (Figure 3c 3d). In particular, the water will tend to form a finite wire-liquid-solid contact angle, so that all the liquid is in front of the wire in the high-pressure region and there is solid ice behind the wire.
Soft Matter Connection
The dynamics of the liquid water flowing around the wires is highly depended on the surface property of the wire. It is important to note that hydrophilic and hydrophobic surfaces are fundamentally different in their wetting properties. For a hydrophilic wire, even a very small amount of liquid spontaneously covers the surface of the wire since water wets the surface. This thin layer may then flow around the wire, allowing the wire to move through the ice as liquid is produced by pressure melting in front of the wire at the same rate as it is consumed in solidification behind the wire. When the wire is nonwetting the liquid will avoid the surface can created larger distortion in the ice crystal structure.