Increasing 'ease of sliding' also increases friction: when is a lubricant effective?

Area of Science:

• Physics
• Materials Science
• Rheology

Background:

• Granular materials exhibit complex frictional behaviors.
• Understanding friction in granular media is crucial for various applications, from engineering to geophysics.
• Previous models often simplify the interplay of factors governing granular friction.

Purpose of the Study:

• To experimentally investigate the effective Coulomb friction of a granular medium on a shearing plate.
• To analyze the influence of granular medium depth and driving speed on friction and sliding regimes.
• To introduce and utilize an order parameter to quantify the fraction of time spent slipping.

Main Methods:

• Experimental setup involving a spring-driven plate shearing through a granular medium of varying depth.
• Controlled motor speed to achieve different driving velocities.
• Introduction and measurement of an order parameter representing the slip fraction.

Main Results:

• At low speeds, friction initially decreases then increases with depth, with minimal slipping.
• At intermediate speeds, both friction and slipping increase significantly with depth.
• At higher speeds, minimum friction occurs at one layer, with friction increasing with depth, while sliding is maintained.

Conclusions:

• The ease of slipping in granular media is not solely determined by friction.
• A complex interplay exists between driving velocity, friction, and granular medium depth.
• The findings necessitate more sophisticated models to capture granular material dynamics.