Spontaneous rolling of a soft sphere on a vertical soft substrate

Area of Science:

• Soft Matter Physics
• Tribology
• Biophysics

Background:

• Vertical locomotion is typically achieved through adhesion or friction, not rolling.
• Existing models do not account for spontaneous rolling on vertical surfaces without external forces.

Purpose of the Study:

• To investigate the spontaneous rolling of soft spheres on vertical surfaces.
• To elucidate the underlying mechanisms of this novel phenomenon.

Main Methods:

• Experimental setup with a soft polyacrylamide sphere and a soft polydimethylsiloxane (PDMS) substrate at a 90° incline.
• Analysis of contact dynamics, including contact diameter changes and asymmetry.
• Application of adhesion hysteresis theories and crack propagation models.

Main Results:

• Spontaneous, slow rolling motion was observed under specific material elasticity conditions.
• A unique contact asymmetry, akin to closing and opening cracks, was identified.
• This asymmetry generates sufficient torque and friction to sustain rolling, preventing falling or pinning.

Conclusions:

• The study demonstrates spontaneous rolling of soft spheres on vertical surfaces, challenging conventional understanding.
• The findings highlight the importance of soft-on-soft contact mechanics and adhesion hysteresis.
• Potential applications in advanced material design and understanding biological locomotion.