Date Published: May 22, 2017
Publisher: John Wiley and Sons Inc.
Author(s): Vito Cacucciolo, Hiroki Shigemune, Matteo Cianchetti, Cecilia Laschi, Shingo Maeda.
Electrohydrodynamics (EHD) refers to the direct conversion of electrical energy into mechanical energy of a fluid. Through the use of mobile electrodes, this principle is exploited in a novel fashion for designing and testing a millimeter‐scale untethered robot, which is powered harvesting the energy from an external electric field. The robot is designed as an inverted sail‐boat, with the thrust generated on the sail submerged in the liquid. The diffusion constant of the robot is experimentally computed, proving that its movement is not driven by thermal fluctuations, and then its kinematic and dynamic responses are characterized for different applied voltages. The results show the feasibility of using EHD with mobile electrodes for powering untethered robots and provide new evidences for the further development of this actuation system for both mobile robots and compliant actuators in soft robotics.
The experiments were conducted in a cylindrical glass tank with a diameter of 135 mm and height of 74 mm, filled with a layer of dielectric liquid topped by a layer of conductive solution (Figure 1a). The liquids that were used were Novec 7000 (1‐methoxyheptafluoropropane (C3F7OCH3), density ρN7 = 1400 kg m−3, dynamic viscosity μN7 = 0.450 g m−1 s−1, volume resistivity 108 Ω cm) by 3 m as a dielectric and a solution of sodium chloride in water (density ρs = 1120 kg m−3, dynamic viscosity μs = 1.30 g m−1 s−1, resistivity 4.4 Ω cm) as conductive fluid. The Novec 7000 was chosen for the following reasons: (1) its density is higher than that of the NaCl solution and (2) the two fluids are not miscible (solubility ≤ 60 ppm by weight); (3) it is chemically stable; (4) its conductivity is very low, allowing to establish the high‐intensity electric field required by EHD and increasing the efficiency of the mechanism.15 On the bottom of the glass container, a copper sheet electrode with a thickness of 40 μm was glued, connected to the voltage source (+VH) through a conductive wire insulated for high voltages. As for the ground electrode, a tin wire with a diameter of 0.7 mm was inserted into the NaCl solution. In this set‐up, the positive electrode was in contact only with the dielectric fluid, while the NaCl solution only was connected to the ground (Figure 1a,c). The robot was composed by three elements (Figure 1b): (1) a hollow cylinder of polyurethane for the hull; (2) two fins made of poly(ethylene terephthalate) (PET) acting as stabilizers; (3) one submerged sail electrode, which generates the thrust, made by a square copper sheet. The extremes of the hollow cylinder were corked with hot‐melt adhesive to increase the buoyancy by sealing the internal chamber. The weight of this component was of 1.10 g, while the one of the fins 0.25 g. The electrode sail was conductive on one side only, while the other one was covered by a thin layer of dielectric glue. Its total weight was 0.20 g. All the experiments were performed using the same gap of 4.3 mm between the plane electrode on the bottom of the tank and the sail electrode on the mobile robot. This gap was chosen as a balance between obtaining enough thrust, which in EHD has been shown to depend on the gap squared and avoiding the breakdown.13 In all the experiments, the position of the robot was tracked through automatic tracking software (Kinovea 0.8.15) from movies acquired at 20 fps. The raw data were sampled at 5 ms with a spline interpolation.
The authors declare no conflict of interest.