Research ArticleACTUATORS

Linear and rotational microhydraulic actuators driven by electrowetting

See allHide authors and affiliations

Science Robotics  19 Sep 2018:
Vol. 3, Issue 22, eaat5643
DOI: 10.1126/scirobotics.aat5643
  • Fig. 1 Actuator operation.

    Functional two-dimensional cross section of a linear microhydraulic actuator, drawn to scale, with the water droplets deformed by voltages on electrodes (P3,P4), relative to the water droplet voltage R. The polyimide droplet array is shown in orange, 8 M LiCl water drops are shown in blue, the dodecane oil between drops is shown in white, the electrode array dielectric is shown in light gray, and the electrodes are shown in gray and red for 0 and 40 V, respectively. (Top right) Droplet profile with the electrodes off. In this condition, the net forces on the array are zero. (Top left) When the droplets are deformed by the electrodes, a net horizontal force is produced, and the droplet array moves to align the drops to the energized electrodes. The energized electrodes are cycled with two on at any particular time, going through four states—(P1,P2), (P2,P3), (P3,P4), and (P4,P1)—at which point the cycle starts again at (P1,P2). The droplet array physically follows the energized states to result in actuator motion. For motion in the opposite direction, the order of the states in the cycle is reversed. The oil-water surface tension, γ, is 55 mJ/m2.

  • Fig. 2 Polyimide droplet array.

    Optical micrographs of polyimide droplet arrays wetted with 8 M LiCl. (Left) Rotational actuator array. (Right) Linear actuator array. Droplet arrays are suspended in air by using small custom vacuum wands made from blunt syringe needles. (Bottom) Magnified views show the details of the droplet shape. The narrow connectors that link each drop to the reference rail droplet equilibrate the droplet Laplace pressure, making the curvature and the height of each drop the same. Rail droplets align to hydrophilic regions in the electrode array and self-align the actuator orthogonal to the direction of motion. The reference rail droplet has a Pt wire that equilibrates the electronic potential of all the droplets with the reference rail, which, when assembled, is connected to the reference phase in the electrode array. The fluidic vias allow for the control of droplet pressure from the back.

  • Fig. 3 Linear actuator after assembly.

    (Left) The entire droplet array aligned to the electrode array; electrode array phases R, P1, P2, P3, and P4 are also indicated. Phase R is biased with a negative voltage Vt, and phases P1 to P4 are biased with 0 V or Vb as they are switched on and off. (Right) Magnified region of interest together with three cross-section diagrams of the actuator. All the droplets are electrically connected to the reference phase R. Cross-section 1 shows the Pt contact, which establishes this connection between the perforated phase electrode R and the reference rail droplet. Cross-section 2 is the main actuation cross section shown in Fig. 1. Cross-section 3 shows the rails and their self-alignment to the hydrophilic regions in the electrode array, shown as gaps in the green fluoropolymer. Other materials in the actuator cross sections are color-coded as indicated by the legend.

  • Fig. 4 Optical micrographs of maximum velocity unloaded actuation.

    (Left) Linear actuator moving six cycles at a cycle frequency of 4 kHz. For this actuation step, frequency is 16 kHz, velocity is 0.192 m/s, and acceleration on the first step is 3.07 km/s2. (Right) Rotational actuator moving 20 cycles at a cycle frequency of 2.5 kHz. The maximum cycle frequency of the unloaded rotational actuator is slightly lower than the linear one because the rotational droplet width is only 1 mm, and the rails, which produce drag, form a larger relative portion of the rotational actuator. Angular velocity and acceleration corresponding to a 2.5-kHz cycle frequency are 30 rad/s (or 286 rpm) and 0.3 Mrad/s2, respectively. Movies S1 to S7 show various unloaded actuations.

  • Fig. 5 Electrical characterization of microhydraulic actuation.

    (A) Voltage and current data for a three-cycle actuation with a cycle frequency of 250 Hz. Each graph segment shows data for a separate phase: electrode phases (P1, P2, P3, and P4) and the droplet reference phase (R). Also shown in a dotted line is the value of total electrode charge Q, normalized by the effective capacitance of the electrode. (B) Electrical power input data for each phase, calculated from the product of the voltage and the current. Integrated power averaged over the number of cycles for each electrode gives the phase energy per cycle; total cycle energy is the sum of all phase energies or 88 nJ. This can be further normalized by the active actuator area of 4 mm2 to Ecyc of 22 mJ/m2. (C) Contour plots of the maximum cycle energy Emax = C(Vb2/2 − VbVt); the hysteresis energy, Ehyst, of 10 mJ/m2 is highlighted in red. Open symbols show bias conditions that resulted in motion at a cycle frequency of 2.5 Hz; solid symbols show bias conditions that did not result in actuator motion. Also shown in orange are the bias conditions for (A), (B), and (E) and for Fig. 6 (B to D). Contact angle saturation energies are shown in white. (D) Measured energy per cycle Ecyc, as well as Emax and Ehyst as a function of Vb for Fcyc = 25 Hz actuation. (E) Measured cycle energy Ecyc as a function of actuation frequency for loaded and unloaded linear actuators and unloaded rotational actuators.

  • Fig. 6 Loaded actuation.

    (A) Optical micrograph of a linear microhydraulic actuator operating with a load. Loading is accomplished by pushing some of the droplets off the electrode array. (B) Left axis shows the maximum output force on the load section as a function of cycle frequency and actuation velocity. Total maximum load force is calculated from the hysteresis energy, 10 mN/m, times the total load droplet width, and then normalized by the total width of the actuating droplets, to give the loaded maximum output force in millinewtons per meter. Right axis shows the output power density at maximum load as a function of actuation velocity and cycle frequency. Output power density is calculated as the product of velocity and output force normalized by the actuator weight. (C) Plot of the energy conversion efficiency and output power density as cycle frequency is increased. Conversion efficiency is the mechanical energy output divided by the electrical input energy. For points with energy recovery, the electrical input energy was calculated as the integral of the current-voltage product or the electrical energy applied minus the electrical energy returned. For points without energy recovery, the electrical input energy was calculated as the integral of only the positive values of the current-voltage product or just the electrical energy applied.

  • Fig. 7 Comparison of force and power density characteristics of microhydraulic actuators and biological muscle.

    (A) Output force and output power density characteristics of a 2-mm-long (42 droplets) microhydraulic actuator; output force was normalized by cross-section area of the actuator. (B) Characteristics of a 2-mm-long muscle fiber from (21), also normalized by fiber cross-section area. At this length, muscle fiber has a much lower actuation velocity but a higher cross-sectional force, resulting in a power density that is 10 times lower than microhydraulic actuators from Fig. 6. The shape of the characteristics is very similar.

  • Fig. 8 Comparison of characteristics of microhydraulic actuators to large-scale motors, micro-actuators, and biological muscle.

    Microhydraulic actuators presented in this paper are 10 times more powerful than human muscle and are projected to be more powerful than the best motors with further scaling. Piezoelectric bimorph actuators and all thermal actuators are below the 10% efficiency cutoff (29, 30). It is very rare for publications on electroactive polymers and dielectric elastomers to report both power density and efficiency; here, optimistically the best of each is plotted as the limit (7, 31, 32). Muscle efficiency is shown as 20%, which is the approximate conversion factor for metabolic cost to mechanical energy output in exercise machines. Data for USR30 ultrasonic motor were taken from datasheets of Shinsei Corporation, Japan. Prius and Tesla Model S motor specifications were taken from Toyota and Tesla publications for the appropriate electric vehicles.

Supplementary Materials

  • robotics.sciencemag.org/cgi/content/full/3/22/eaat5643/DC1

    Materials and Methods

    Fig. S1. Top-down micrograph of the electrode array.

    Fig. S2. The assembly sequence for a rotational actuator.

    Fig. S3. Droplet collapse.

    Fig. S4. Design of the custom output board for simultaneous current and voltage readout.

    Fig. S5. Detailed electrical data for a 2 mm–by–2 mm linear actuator at different cycle frequencies.

    Fig. S6. Detailed electrical data for a 2 mm–by–2 mm linear actuator at different voltages.

    Fig. S7. Extended voltage, current, and power profiles at a cycle frequency of 25 Hz.

    Fig. S8. Detailed electrical data for a 1-cm-diameter rotational actuator at different cycle frequencies.

    Movie S1. Highly magnified linear actuation at a cycle frequency of 1.25 Hz.

    Movie S2. Highly magnified linear actuation at a cycle frequency of 12.5 Hz.

    Movie S3. Magnified linear actuation at a cycle frequency of 1000 Hz.

    Movie S4. Magnified linear actuation at a cycle frequency of 4000 Hz.

    Movie S5. Magnified rotational actuation at a cycle frequency of 125 Hz.

    Movie S6. Magnified rotational actuation at a cycle frequency of 1000 Hz.

    Movie S7. Rotational actuation at a cycle frequency of 1000 Hz.

    Movie S8. Rotational actuation of a beam splitter cube.

    Movie S9. Droplet formation video.

  • Supplementary Materials

    The PDF file includes:

    • Materials and Methods
    • Fig. S1. Top-down micrograph of the electrode array.
    • Fig. S2. The assembly sequence for a rotational actuator.
    • Fig. S3. Droplet collapse.
    • Fig. S4. Design of the custom output board for simultaneous current and voltage readout.
    • Fig. S5. Detailed electrical data for a 2 mm–by–2 mm linear actuator at different cycle frequencies.
    • Fig. S6. Detailed electrical data for a 2 mm–by–2 mm linear actuator at different voltages.
    • Fig. S7. Extended voltage, current, and power profiles at a cycle frequency of 25 Hz.
    • Fig. S8. Detailed electrical data for a 1-cm-diameter rotational actuator at different cycle frequencies.
    • Legends for movies S1 to S9

    Download PDF

    Other Supplementary Material for this manuscript includes the following:

    • Movie S1 (.mp4 format). Highly magnified linear actuation at a cycle frequency of 1.25 Hz.
    • Movie S2 (.mp4 format). Highly magnified linear actuation at a cycle frequency of 12.5 Hz.
    • Movie S3 (.mp4 format). Magnified linear actuation at a cycle frequency of 1000 Hz.
    • Movie S4 (.mp4 format). Magnified linear actuation at a cycle frequency of 4000 Hz.
    • Movie S5 (.mp4 format). Magnified rotational actuation at a cycle frequency of 125 Hz.
    • Movie S6 (.mp4 format). Magnified rotational actuation at a cycle frequency of 1000 Hz.
    • Movie S7 (.mp4 format). Rotational actuation at a cycle frequency of 1000 Hz.
    • Movie S8 (.mp4 format). Rotational actuation of a beam splitter cube.
    • Movie S9 (.mp4 format). Droplet formation video.

    Files in this Data Supplement:

Stay Connected to Science Robotics

Navigate This Article