Linear and rotational microhydraulic actuators driven by electrowetting

Abstract

Microhydraulic actuators offer a new way to convert electrical power to mechanical power on a microscale with an unmatched combination of power density and efficiency. Actuators work by combining surface tension force contributions from a large number of droplets distorted by electrowetting electrodes. This paper reports on the behavior of microgram-scale linear and rotational microhydraulic actuators with output force/weight ratios of 5500, cycle frequencies of 4 kilohertz, <1-micrometer movement precision, and accelerations of 3 kilometers/second2. The power density and the efficiency of the actuators were characterized by simultaneously measuring the mechanical work performed and the electrical power applied. Maximum output power density was 0.93 kilowatt/kilogram, comparable with the best electric motors. At maximum power, the actuator was 60% efficient, but efficiencies were as high as 83% at lower power. Rotational actuators demonstrated a torque density of 79 newton meters/kilogram, substantially more than electric motors of comparable diameter. Scaling the droplet pitch from 100 to 48 micrometers increased power density from 0.27 to 0.93 kilowatt/kilogram, validating the quadratic scaling of actuator power.

Nonspecular Reflection of Droplets

“…Examples include lipid droplets for intracellular lipid storage and lipid metabolism, [ 2 ] virion‐laden droplets for person‐to‐person transmission of respiratory diseases, [ 3 ] and falling raindrops that attract tiny aerosol particles for cleaning the atmosphere. [ 4 ] Navigating droplet transport holds great scientific significance [ 5,6 ] and has technological implications in diverse applications ranging from microfluidics, [ 7,8 ] biological/chemical assays, [ 9,10 ] water/energy harvesting [ 11,12 ] to liquid actuators/robots, [ 13,14 ] in which steering the transport direction plays a primary role. With different preferential directions, droplet transport resembles wave propagation in many ways, for instance, droplets passing through meshes for transmittance, [ 15,16 ] wicking on superhydrophilic membranes for absorption, [ 17 ] and bouncing on superhydrophobic surfaces for reflection.…”

“…droplet transport holds great scientific significance [5,6] and has technological implications in diverse applications ranging from microfluidics, [7,8] biological/chemical assays, [9,10] water/energy harvesting [11,12] to liquid actuators/robots, [13,14] in which steering the transport direction plays a primary role. With different preferential directions, droplet transport resembles wave propagation in many ways, for instance, droplets passing through meshes for transmittance, [15,16] wicking on superhydrophilic membranes for absorption, [17] and bouncing on superhydrophobic surfaces for reflection.…”

Micro Motion Amplification–A Review

“…Furthermore, electrothermal micro-walking devices have been considered, taking advantage of the fast temperature transients in the microscale [79]. Finally, surface tension based hydraulic micromotors have been demonstrated recently, showing significant potential for low-load microactuation [80]. Although micromotors arguably comprise a different field than motion amplification, a brief overview is presented in this paper, because they sometimes find similar applications, such as nano-positioning and xy stage systems.…”

Biologically inspired electrostatic artificial muscles for insect-sized robots

“…Building from prior designs, we focus on dry electrostatic actuators, because actuators depending on dielectric fluids suffer from evaporation in air and can freeze at low temperatures (although the output force can be greater owing to higher permittivity and dielectric strength of the dielectric fluid) (Kedzierski and Holihan, 2018; Niino et al, 1992, 1997a,b; Yamamoto et al, 1998, 2006). Most current electrostatic actuators at the microscale are based on comb structures, but their displacement is small (tens of micrometers, and less than 10% of the characteristic length of the actuator), as shown in Figure 2, which is ineffective for driving a millimeter-scale robot.…”