Octopus-like Reaching Motion: A Perspective Inspired by Whipping

TL;DR

This paper investigates whether whip-like passive dynamics in water can reproduce the curvature propagation observed in octopus arm reaching. It combines underwater manual whipping with a reverse-engineered robotic rig (a tapered silicone arm on a PLA+ frame) and a quantitative image-analysis pipeline to extract curvature evolution and bend-point velocity. The key finding is that an Ecoflex Gel 2 arm driven at 150 rpm yields curvature propagation most similar to biological reaching, but the bend-point velocity decays monotonically rather than showing the biological bell-shaped profile, suggesting the motion is not purely passive whipping. The results also show no propagation in air, underscoring the critical role of the surrounding fluid and fluid–structure interactions; collectively, the work provides a reproducible platform for hydrodynamics studies of reaching movements and highlights environmental mediation as a dominant factor.

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.

Figures (8)

• Figure 1: (a) Multiple exposures of a bullwhip exhibiting propagating waves during motion, kinematically similar to an octopus arm. noel2018grip (b) Curvature pattern of real octopus reaching behavior ($s/L_0$ denotes the normalized arc length along the arm, with $s$ being the arc-length coordinate and $L_0$ the undeformed arm length).
• Figure 2: Underwater manual whipping motion.
• Figure 3: (a) The trajectory of the midline shift during the octopus's reaching motion. (b) The trajectory of the midline shift during underwater manual whipping motion. (c) Position and orientation extraction in Tracker. (d) Mechanisms generation in MotionGen. (e) SolidWorks Model. (f) Physical prototype of the mechanism.
• Figure 4: (a) Segmentation on original image. (b) Extracted mask. (c) Original midline with a branch. (d) Final midline.
• Figure 5: Ecoflex Gel 2 underwater performance at 150 rpm.