Partial squeeze film levitation modulates fingertip friction

Area of Science:

• Physics
• Tribology
• Biophysics

Background:

• Surfaces excited by ultrasonic transverse waves exhibit reduced friction when touched.
• Understanding the physical mechanisms behind this friction reduction is crucial for designing advanced tactile interfaces.

Purpose of the Study:

• To investigate the phenomenon of reduced friction on ultrasonically vibrated glass plates.
• To elucidate the role of skin-surface interactions and air films in dynamic friction modulation.

Main Methods:

• Utilized frustrated total internal reflection (TIR) to visualize skin asperities in contact with a vibrating glass plate.
• Employed stroboscopic imaging to analyze the time evolution of the interfacial gap during vibration.

Main Results:

• Observed that interfacial load is supported by both elastic compression of skin asperities and a trapped air squeeze film.
• Found that the interfacial gap's time evolution is partially out of phase with the plate's vibration.
• Demonstrated that dynamic levitation occurs, reducing the number of contacting asperities and thus friction.

Conclusions:

• Dynamic levitation, facilitated by air squeeze films, explains friction reduction on vibrating surfaces.
• This mechanism provides a pathway for designing interfaces that can actively control friction with soft materials.
• The findings have implications for haptics, robotics, and understanding tactile sensation.