Ice is a common scene during winters, and for winter lovers, it provides a chance for sliding and playing winter games. The ease by which people or things slide on ice has continued to attract significant research attention. This can be attributed to water films induced by fractional melting responsible for lowering the dynamic frictional coefficient. Unlike common materials that exist as solids at room temperature, it is difficult to control and characterize the surface conditions of ice prepared by freezing water. This makes it even harder to determine the actual friction coefficients of high-speed sliders on ice. Even though it is widely accepted that the ice-slider friction is largely influenced by melted film, how the film is generated and evolves with time has been elusive.
Previously, the contact area between the ice and slider has been investigated via several techniques such as X-ray computer tomography to obtain more insights into the temporal evolution of the ice-slider contact area. Nevertheless, despite the good progress, in situ observation of ice-slider contact area has remained rare in literature. On this account, Professor Ho-Young Kim from Seoul National University together with Changho Yun, Jin-Woo Choi, Hyungseok Kim, and Dongjo Kim proposed an experimental technique, based on the optical principle of total internal reflection, for in situ visualization of the real contact area between ice asperities and high-speed sliding surfaces. Their aim was to directly quantify the temporal evolutions of the contact area, film thickness, and the resulting friction force. The research work is currently published in the International Journal of Heat and Mass Transfer.
In their approach, the system model comprised a hemispherical ice specimen and a flat slider for easy observation and measurement of the contacting region. A hydrodynamic theory was developed to predict the contact area, melted film thickness, and the resulting friction force as a function of the time coupled with geometrical and physical properties of the agents. Results showed that the analytical model led to the scaling laws obtained for the contact area, liquid film thickness, and consequent friction force, which verified the experimental observations. For the experiments with flat ice surfaces, the temporal evolutions of the multiple contact spots were visualized between the ice and the high-speed slider. Additionally, the strong correlations between the instantaneous friction force and contact area allowed accurate estimation of the melted film thickness.
In summary, the study presented a new technique for visualizing and quantifying the temporal evolutions of contact area, film thickness, and resulting friction force. Based on the results, it was evident that detailed geometric information of asperities of slider and ice surface is essential in accurate prediction of the friction force. In a statement to Advances in Engineering, Professor Ho-Young Kim, the lead author noted that the theoretical framework and experimental technique could be extended to investigate various natural situations like rock failure and fault slips in geology, as well as winter sport games.
Yun, C., Choi, J., Kim, H., Kim, D., & Kim, H. (2020). Sliding on ice: Real contact area, melted film thickness, and friction force. International Journal of Heat and Mass Transfer, 160, 120166.