Ice–water interface stays slippery
Ice and snow have exceptionally low friction coefficients, making them good for skiing, skating and sledging, but dangerous for drivers on icy winter roads. Now, however, after studying these materials for more than 150 years, scientists think they may finally have the answer as to why ice and snow are so slippery.
The low friction coefficients have long been attributed by some to the formation of a thin layer of liquid water between the ice and the sliding object – caused, paradoxically, by frictional heating slightly melting the ice. But this hypothesis raises many unanswered questions, says Lydéric Bocquet of the Physics Laboratory at the École Normale Supérieure (ENS) in Paris. Water is a bad lubricant compared to oil, and the thickness and properties of the proposed interfacial water layer have not been measured. Indeed, its very existence has been under debate.
In the latest work, a team led by Bocquet and his ENS colleague Alessandro Siria have used a new instrument – dubbed a stroke-probe tribometer – to measure the properties of this interfacial water layer. Their work shows that the liquid film does indeed exist, but it is much thinner than expected – just a few hundred nanometres to a micron deep. Its viscoelastic properties also resemble those of polymers or polyelectrolytes in oil, rather than simple water.
Tuning fork technique
Bocquet, Siria and colleagues studied “interfacial water” using a modified double-mode Tuning Fork Atomic Force Microscope (TF-AFM). The instrument they developed comprises a millimetre-sized probe ball glued to a macroscopic tuning fork. Although the fork is very similar to a piano tuning fork, it can be excited by a very low frequency vibration, typically several hundred hertz. The system can be accurately modelled as a stiff mass-spring resonator with a quality factor of around 2500.
When the researchers brought the vibrating ball at the end of the fork in contact with the surface of a centimetre-sized block of ice (using a piezo element with an integrated motion sensor of nanometric resolution), the lateral stroke of the ball slid across the ice with a fixed amplitude and velocity. The frequency of the system then changes, but so does its quality factor.
The ENS team used the frequency offset to measure the elastic properties of the contact surface, and the change in quality factor to evaluate the dissipation processes occurring there. Together, the two measurements gave the layer’s interfacial viscosity.
The researchers say the instrument allows them to “listen” to the forces between the probe and the ice with remarkable precision. Indeed, despite being centimetres in size, the instrument is sensitive enough to let them probe ice contact and friction properties at the nanometre scale. “The system allows us to access several vibration frequencies, offering us the possibility to simultaneously probe the tribology of the contact – ‘how it rubs’ – by moving the ball in a lateral direction, and its rheology – ‘how it flows’ – by moving the ball in a perpendicular direction,” Bocquet explains.
The experiments confirm the super-slippery nature of the interfacial ice, but they also – for the first time – confirm that friction generates a film of liquid water when the probe ball is set in motion. This film is, however, much thinner than previous theoretical calculations have suggested, and it is also as viscous as oil, with a viscosity roughly two orders of magnitude larger than that of water. The researchers also showed that the film’s viscosity depends on the shear velocity – a behaviour known as shear thinning.
According to Bocquet, one interpretation for this unexpected behaviour is that surface ice does not completely transform into liquid water when an object glides across it. Instead, it may enter a mixed “granité-like” (crushed ice and water) state. This mixed film might lubricate the contact between the solid ice and the ball and prevents any direct contact between the two surfaces.
Further experiments by the ENS team show that making the probe hydrophobic reduces friction even further by modifying the interfacial viscosity. This “waxing” process is practised empirically by skiers, but the reason why it made skis glide better was not previously understood.
A new theory would provide a better understanding of sliding on ice and might also help, conversely, to find ways of increasing friction, which is essential to avoid slipping on icy roads
The team’s result means that existing theoretical descriptions for interfacial ice need an overhaul, Bocquet told Physics World. A new theory would provide a better understanding of sliding on ice, which would come in useful in developing winter sports equipment or self-healing, ultra-low-friction lubricants for industrial applications. It might also help, conversely, to find ways of increasing friction, which is essential to avoid slipping on icy roads.
Angelos Michaelides of University College London, UK, who was not involved in the research, says that the ENS study is very exciting. “I am not aware of such a nice and elegant set of measurements on the friction of the quasi-liquid layer and think it is an extremely interesting new perspective on this age-old story,” he says (Phys. Rev. X 9 041025).
Belle Dumé