Relationship between the size of a camphor-driven rotor and its angular velocity

Abstract

We consider a rotor made of two camphor disks glued below the ends of a plastic stripe. The disks are floating on a water surface and the plastic stripe does not touch the surface. The system can rotate around a vertical axis located at the center of the stripe. The disks dissipate camphor molecules. The driving momentum comes from the nonuniformity of surface tension resulting from inhomogeneous surface concentration of camphor molecules around the disks. We investigate the stationary angular velocity as a function of rotor radius ℓ. For large ℓ the angular velocity decreases for increasing ℓ. At a specific value of ℓ the angular velocity reaches its maximum and, for short ℓ it rapidly decreases. Such behaviour is confirmed by a simple numerical model. The model also predicts that there is a critical rotor size below which it does not rotate. Within the introduced model we analyze the type of this bifurcation.

A hybrid camphor–camphene wax material for studies on self-propelled motion

“…15 Experiments performed with pills of pure camphor show a longer time window of self-motion. 20 Both abovementioned techniques generate objects with highly inhomogeneous distribution of camphor. The inhomogeneity of pills results from the structure of camphor grains and vacant spaces that randomly appear when a pill maker is filled with camphor, before pressure is applied or to the structure of the porous material.…”

Realization of Self‐Rotating Droplets Based on Liquid Metal

“…Koyano et al. [ 25 ] fabricated a rotor that is made up of two camphor disks, and the maximum angular velocity reached 20 rad s −1 . The driving force originates from the surface tension gradient induced by the dissolution of camphor.…”

“…Spontaneous rotational droplets without any external forces are capable of taking energy from their environment and converting it into directed motion. In previous works, far‐from‐equilibrium systems exhibited a variety of temporal and spatial patterns, [ 25 ] such as the spiral vortex of a bacterial suspension droplet. [ 29 ] In these systems, the confining surfaces play crucial roles in the dynamics, transport, and order of internal active matter.…”

“…The rotation caused by a comma shape is well known from the phenomenon that comma‐shaped camphor fragments can spontaneously rotate in water. [ 33–35 ] The asymmetric shape creates additional torque on the camphor fragments and promotes continuous rotation. [ 33–35 ] In Figure 1a, the droplet rotation is analogous to the case of camphor fragments.…”

“…[ 33–35 ] The asymmetric shape creates additional torque on the camphor fragments and promotes continuous rotation. [ 33–35 ] In Figure 1a, the droplet rotation is analogous to the case of camphor fragments. The asymmetric shape causes the nonequilibrium Laplace pressure, which contributes to the droplet rotation.…”