Various Motions of Micro Particles Under a DC Electric Field: An Approach to Make Electric Micro-Motor in Millimeter Scale
Authors: Zhang Wenyu , Kozaki Takahiro , Kakimoto Ichiro , Kosugi Kento , Yamamoto Daigo , Shioi Akihisa , Fujii Syuji , Kamata Kaori , Iyoda Tomokazu , Yoshikawa Kenichi 
Published: February 2020

1. Introduction
Micro-motor research has been undergoing intense study in recent years[1]. Due to the development of micro-electromechanical systems (MEMS), more and more attention has been payed to functional gears and devices in the micrometer scale. One of the obstacles in such system is how their power are generated. Traditional direct current (DC) motors are usually used in environments with high Reynolds number. They can work by the large inertia. In high Reynolds number flow, it is often modeled as an inviscid flow, an approximation in which viscosity is completely neglected. However, the viscosity effect becomes significance in a millimeter scale, especially in liquid phase. In such cases, an efficiency of traditional DC motor will be reduced significantly[2]. An electric motor that converts electrical energy into mechanical energy in millimeter scale, that can work in an environment with extremely low Reynolds number is needed.
2. Experimental method
Experimental setup[3] shown in Fig.a. The solution was made by adding di-(2-ethylhexyl) phosphoric acid (DEHPA) of 20μL in Silicone-oil of 100μL into a plastic tube. Then micro particle was added into the solution. The plastic tube containing the solution was put on vortex mixer for 1 minute. Various types of micro particle were used: polystyrene spherical particle (d=45μm); polystyrene rod-shaped particle; polyethene spherical particle (d=175μm); spirulina coil particle, a natural alga with helical shape; nickel coil particle, spirulina particle coated with nickel; carbonized spirulina particle; pediastrum particle, a natural alga with gear shape.
Mixture of 20 μL was put on the slide glass to form a drop. Two needle-shaped electrodes were inserted into the droplet from the opposite side. A DC voltage was applied between the electrodes. The motion of micro-particle was observed by an optical microscope. The applied voltage can be altered via the power amplifier.
3. Result and Discussion
Under a DC voltage, particles showed various types of motions. Fig.b shows these motions: (i) oscillating motions. Oscillating motion define as particle performing back-and-forth motion between two electrodes. Oscillating motions of polystyrene particle and spirulina coil particle are shown in Fig.b1, b2, respectively. (ii) rotational motion. Rotational motions of polystyrene trimer, spirulina coil particle and pediastrum particle are shown in Fig.b3[3], b4, b5, respectively; (iii) corkscrew rotational motion. Corkscrew rotational motion define as helical particle rotates around its long axis. It is a characteristic motion for helical particle. Motion of spirulina coil particle is shown in Fig.b6; (iv) revolution motion. Revolution motion of polystyrene spherical particle and polystyrene rod-shaped particle are shown in Fig.b7, b8[3], respectively.
Two theoretical models were applied to explain these motions. For oscillating motion, maybe the electron transportation was the driving force. Electron released from anode was attached to micro particle, and carried the particle moving towards cathode. After releasing the electron, micro particle moved back to anode. Then the cycle repeats again, hence the back-and-forth oscillating motion. For rotational, corkscrew rotational, and revolution motion, polarized particle itself and solution convection were the driving force. Micro particle polarized electrically while under an electric field. When the charge relaxation time of the solution is shorter than that of the particle, particle will begin to rotate[4]. Quincke rotation of micro particle has been reported when a parallel electrodes system was used[5]. Moreover, DEHPA is an anionic surfactant. When under electric field, DEHPA moved to cathode due to electrophoresis. Therefore, a solution convection was formed between the electrodes and carried micro particle to perform various motions.
4. Conclusion
Various motions of micro particles in an oil-phase solution under a DC electric field were found. They are: oscillating motion, which is particle performing back-and-forth motion between electrodes; rotational motion, which is particle rotates itself; corkscrew rotational motion, which is characteristic motion for helical motion; and revolution motion. Two theoretical models were used to explain the motions. Such motions show that electric energy converted into mechanical energy in millimeter scale. This result can contribute to future research of electric micro-motor development.
Reference:
[1] D. Yamamoto and A. Shioi, KONA Powder Particle J. 32, 2-22 (2015)
[2] T. Kurimura, M. Ichikawa, M. Takinoue, and K. Yoshikawa, Phys. Rev. E 88, 042918 (2013)
[3] D. Yamamoto, R. Yamamoto, T. Kozaki, A. Shioi, S. Fujii, and K. Yoshikawa Chem. Lett., 46 (10), 1470-1472 (2017)
[4] F. Peters, L. Lobry, and E. Lemaire. Chaos 15. 13102. (2005)
[5] M. Zrinyi and M. Nakano, PERIOD POLYTECH CHEM. Journal, Vol 61 No 1 (2017)

Figure 1