Spontaneous rolling on a 90 degree incline

TL;DR

This paper demonstrates that an elastic sphere can spontaneously roll on a vertical elastic substrate due to contact asymmetry and crack-like contact dynamics, challenging traditional views on vertical locomotion.

Contribution

It introduces a novel phenomenon of spontaneous rolling on vertical elastic surfaces, explained through contact asymmetry and crack propagation models.

Findings

Spontaneous rolling occurs on elastic vertical surfaces under specific conditions.

Contact asymmetry and crack-like contact dynamics drive the rolling motion.

The study provides a new understanding of soft-on-soft contact interactions.

Abstract

On perfectly vertical surfaces, rolling is conventionally deemed impossible without external torque. While various species like geckos and spiders exhibit vertical locomotion, they cannot achieve rolling; instead, they fall. In this study, we demonstrate the spontaneous rolling of an elastic polyacrylamide sphere on an elastic polydimethylsiloxane (PDMS) substrate held vertically at a 90 degree incline, given specific elasticity values for the materials. Our experiments uncover a slow rolling motion induced by a dynamically changing contact diameter and a unique contact asymmetry. The advancing edge behaves like a closing crack, while the receding edge acts as an opening crack. Utilizing adhesion hysteresis theories and crack propagation models, we explain how this contact asymmetry generates the necessary torque and friction to maintain rolling, preventing either pinning or falling. The findings challenge the traditional understanding of vertical surface interactions and open new avenues for exploring soft-on-soft contact systems. This novel phenomenon has potential implications for designing advanced materials and understanding biological locomotion on vertical surfaces.