Quantitative Estimation of the Parameters for Self-Motion Driven by Difference in Surface Tension

Abstract

Quantitative information on the parameters associated with self-propelled objects would enhance the potential of this research field; for example, finding a realistic way to develop a functional self-propelled object and quantitative understanding of the mechanism of self-motion. We therefore estimated five main parameters, including the driving force, of a camphor boat as a simple self-propelled object that spontaneously moves on water due to difference in surface tension. The experimental results and mathematical model indicated that the camphor boat generated a driving force of 4.2 μN, which corresponds to a difference in surface tension of 1.1 mN m(-1). The methods used in this study are not restricted to evaluate the parameters of self-motion of a camphor boat, but can be applied to other self-propelled objects driven by difference in surface tension. Thus, our investigation provides a novel method to quantitatively estimate the parameters for self-propelled objects driven by the interfacial tension difference.

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

“…The profile of camphor surface concentration on water results from the balance between the inflow of camphor molecules from the disks and camphor evaporation into the air and dissolution in the water [20]. It is known that water surface tension is a decreasing function of camphor surface concentration [6,20,30]. The averaged force acting on a camphor disk is directed towards the region with the lowest camphor surface concentration around the disk.…”

Unidirectional motion of a camphor disk on water forced by interactions between surface camphor concentration and dynamically changing boundaries

“…The experiments were performed at a temperature of 24 AE 1 1C. At such temperature the dissolution rate of camphor in water is negligible (less than 2 mg s À1 ) 5 and the dissolved camphor does not change the water density.…”

“…3 The surface tension is a decreasing function of the surface concentration of camphor molecules. 4,5 As a result, the camphor particle moves in the direction of a region with a lower surface camphor concentration. There are many advantages of using camphor in experiments with self-propelled particles.…”

Self-propelling rotator driven by soluto-capillary marangoni flows

“…0   ) rotating the tube is in a satisfactory agreement with the data extracted from the analysis of the motion of camphor boats. 40 The reported self-propelled rotator demonstrates a potential as a micro-mixer 41 and micro-electrical-generator. 42 …”