Octopus-like Reaching Motion: A Perspective Inspired by Whipping

TL;DR

This study explores whip-like passive dynamics in water to replicate octopus reaching motions, revealing the importance of the surrounding medium and differentiating passive whipping from active biological movement.

Contribution

It demonstrates that water-based whip dynamics can mimic octopus arm bending, but active control distinguishes biological reaching from passive whip behavior.

Findings

Water whipping can reproduce curvature propagation similar to octopus arms.

Passive whip dynamics alone do not produce the bell-shaped bend-point velocity profile.

Surrounding medium significantly influences the formation of octopus-like reaching motion.

Abstract

The stereotypical reaching motion of the octopus arm has drawn growing attention for its efficient control of a highly deformable body. Previous studies suggest that its characteristic bend propagation may share underlying principles with the dynamics of a whip. This work investigates whether whip-like passive dynamics in water can reproduce the kinematic features observed in biological reaching and their similarities and differences. Platform-based whipping tests were performed in water and air while systematically varying material stiffness and driving speed. Image-based quantification revealed that the Ecoflex Gel 2 arm driven at 150 rpm (motor speed) reproduced curvature propagation similar to that observed in octopus reaching. However, its bend-point velocity decreased monotonically rather than exhibiting the biological bell-shaped profile, confirming that the octopus reaching movement is not merely a passive whipping behavior. The absence of propagation in air further highlights the critical role of the surrounding medium in forming octopus-like reaching motion. This study provides a new perspective for understand biological reaching movement, and offers a potential platform for future hydrodynamic research.