Letter www.acsami.org
Self-Actuation of Liquid Metal via Redox Reaction Ryan C. Gough,* Jonathan H. Dang, Matthew R. Moorefield, George B. Zhang, Lloyd H. Hihara, Wayne A. Shiroma, and Aaron T. Ohta College of Engineering, University of Hawaii, 2540 Dole Street, Holmes Hall 483, Honolulu, Hawaii 96822, United States S Supporting Information *
ABSTRACT: Presented here is a method for actuating a gallium-based liquid-metal alloy without the need for an external power supply. Liquid metal is used as an anode to drive a complementary oxygen reduction reaction, resulting in the spontaneous growth of hydrophilic gallium oxide on the liquid-metal surface, which induces flow of the liquid metal into a channel. The extent and duration of the actuation are controllable throughout the process, and the induced flow is both reversible and repeatable. This self-actuation technique can also be used to trigger other electrokinetic or fluidic mechanisms. KEYWORDS: liquid metals, gallium alloys, galvanic cells, reconfigurable devices, autonomous motion
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results in a surface-tension gradient along the length of the liquid metal. This gradient drives motion of the droplet via continuous electrowetting,15 with the energy coming from the liquid metal itself and not from an external supply. This type of self-propelled motor has obvious applications in the field of autonomous robotics, but can be difficult to control once set in motion. The reaction between the Galinstan and the aluminum continues until the aluminum flake is broken down by the Galinstan,14 and the reaction rate cannot be externally controlled. A similar redox reaction can initiate a different type of actuation mechanism, one that is based on the electrical formation of a hydrophilic oxide layer16 instead of surfacetension gradients. This mechanism has a tremendous amount of control over the liquid-metal shape, beyond that achievable with electrocapillarity.17 In the past, this technique has been demonstrated by externally applying oxidative potentials across the interface between the liquid metal and a surrounding electrolyte, but here we show that this actuation can be obtained by direct conversion of stored electrochemical energy by the liquid metal. Figure 1 shows liquid metal (Galinstan) submerged in NaOH solution and trapped in an “anodic” reservoir with a glass floor and ceiling. The Galinstan is wetted to a copper probe that extends outside the test fixture. An identical “cathodic” reservoir is located on the opposite side of a fluidic channel, and contains an embedded copper electrode. The cathodic reservoir ceiling is a gas-permeable 130 μm thick silicone membrane, which allows atmospheric oxygen to diffuse
iquid metals have been demonstrated to be an excellent material choice for a wide variety of applications, from the rapid prototyping of wearable electronics,1 to reconfigurable RF devices,2−4 pumps,5 sensors,6 optical switches,7 and MEMS actuators.8 These metals combine traditional metallic properties (high electrical and thermal conductivity, high reflectivity) with the unique dynamics of a high-surface-tension fluid. The use of gallium alloys as a nontoxic replacement for mercury has further fueled interest in applications for this unique material. For many potential applications, the efficient, low-energy actuation of liquid metal is desirable. Pressure-driven liquid metal is typically used for proof-of-concept devices, but this actuation method relies on high-voltage, high-power pumps for commercial implementation. Electrowetting on dielectric (EWOD) achieves modest deformation of liquid metals without the need for pumps, although high actuation voltages are necessary.9 Low-voltage, low-power liquid metal actuation can be achieved with surface-tension based techniques like continuous electrowetting8,10 and electrocapillary actuation,11 which can effect significant changes in the liquid-metal shape and position. Recently, the tendency of gallium-based alloys to form a hydrophilic oxide layer under certain electric potentials has also been utilized to both introduce12 and remove13 these alloys from capillary channels with minimal voltage and power. All of the aforementioned liquid-metal actuation techniques require power from a dedicated source. There are many applications, such as remote sensing or autonomous robotics, for which this actuation would ideally be self-powered, drawing energy from the liquid metal itself. Recently, a self-propelled motor was demonstrated that derives its energy from a redox reaction between a droplet of gallium-based liquid metal (Galinstan) and an adhered flake of aluminum.14 These two dissimilar metals, in direct contact with each other and submerged in an alkaline electrolyte, form a galvanic cell that © 2015 American Chemical Society
Received: October 6, 2015 Accepted: December 22, 2015 Published: December 22, 2015 6
DOI: 10.1021/acsami.5b09466 ACS Appl. Mater. Interfaces 2016, 8, 6−10
Letter
ACS Applied Materials & Interfaces
the electrolyte and the liquid metal.17 This decrease in tension at the channel entrance initiates liquid metal flow from the anodic reservoir into the channel. Once the external electrical connection is severed, the halfcell reactions are halted, and with no oxidizing reaction to form new gallium oxide the NaOH rapidly dissolves the existing oxide layer on the liquid-metal surface. With its surface tension restored to preactuation levels, the liquid metal is expelled by the negative capillary force of the channel back into the anodic reservoir (see also Movie S1). To demonstrate that the cathodic reaction is driven by oxygen reduction and not the reduction of dissolved copper ions on the copper electrode itself, we fabricated two different test fixtures similar to the one depicted in Figure 1. Each of these fixtures had anodic and cathodic reservoirs with identical dimensions, 400 μm high with a 10 mm diameter. On one fixture, the cathodic reservoir was covered with the oxygenpermeable silicone rubber membrane described above, whereas the other fixture had a reservoir sealed with a glass ceiling. Both fixtures were filled with 0.25 M NaOH solution, and Galinstan was injected through a side inlet into the anodic reservoir. When an electrical connection was made between the liquid metal and the copper electrode in the glass-capped fixture, less than 5 μA of redox current was generated, and the liquid metal moved less than 3 mm into the channel in an hour. In contrast, the fixture with the silicone-membrane-covered cathodic reservoir achieved sustained currents of approximately 110 μA, and the liquid metal flowed through the channel at typical speeds of 150 μm/s. As oxygen is important for this reaction, an open cathodic reservoir with no ceiling or membrane may perform even better; however, an open reservoir suffers from rapid electrolyte evaporation, making repeatable measurements difficult. The redox current measured in fixtures with open reservoirs was generally consistent with the current levels from tests using the silicone-membrane-covered reservoirs. In addition, the manual injection of air onto the surface of the cathode via syringe during testing resulted in current spikes of 60 to 100 μA. Thus, it can be strongly inferred that oxygen is the reducing agent in the redox reaction, and that the silicone membrane is effective at allowing the oxygen access to the cathodic electrode, as well as maintaining a consistent volume of electrolyte in contact with the electrode. Figure 2 shows a sample profile of the liquid-metal motion over time, along with the generated redox current and actuation speed. Prior to actuation, the liquid metal is at rest in a 400 μm high anodic reservoir with a 10 mm diameter. A cathodic reservoir with identical dimensions is connected via a 2 mm wide by 10 mm long channel; this reservoir contains a 36 mm2 copper electrode, and its ceiling is comprised of 130 μm thick silicone rubber instead of glass. Both the reservoirs and the channel are filled with 1 M NaOH solution, and both reservoirs have side inlets to allow for the injection of NaOH solution and liquid metal into the fixture. The inlets are left unsealed to prevent pressure buildup during actuation that might inhibit liquid metal flow. Once an external electrical connection is made, there is a spike in current that is likely a result of the rapid reduction of oxygen near the cathode surface. This depletes the diffusion layer of oxygen, causing the current to decrease until a steady state is achieved where oxygen is replenished by diffusion through the silicone membrane. The generated redox current soon stabilizes at a level consistent with the steady-state rates of oxidation and reduction occurring at the two half-cells. The
Figure 1. (a) Galinstan in a cylindrical “anodic” reservoir wetted to a copper probe. An identical “cathodic” reservoir is on the opposite side of a fluidic channel and contains an embedded copper electrode. Both reservoirs and the intermediate channel are filled with 0.25 M NaOH solution. The ceiling above the liquid metal is glass, but the ceiling above the cathodic electrode is a silicone membrane that is permeable to oxygen. (b) An external electrical connection between the liquid metal and the copper electrode results in a spontaneous redox reaction, with the liquid metal acting as the anode and oxygen on the surface of the copper electrode acting as the cathode. The channel between the reservoirs serves as an electrolytic bridge between the two half-cells. The result of the redox reaction is the growth of Ga2O3 on the liquid-metal surface facing the channel. (c) This oxide growth lowers the localized interfacial tension δ, resulting in liquid-metal flow into the channel. (d) When the external electrical connection is broken, the redox reaction halts and the Ga2O3 is dissolved by the NaOH solution. This restores the ambient surface tension of the liquid metal, which is pushed back into its reservoir by capillary pressure Fcap from the channel.
down to the copper electrode. When an external electrical connection is made between the liquid metal (via its wetted probe) and the electrode, a galvanic cell is created. Spontaneous half-cell reactions occur at both ends of the channel as donated electrons from the oxidizing anode (the Galinstan) are consumed by a cathodic reaction at the copper electrode. This is analogous to a metal-air battery, in which the anode undergoes an oxidizing reaction, typically with hydroxyl ions, to release electrons that are used in an oxygen reduction reaction (ORR) at the cathode.18 The oxidizing reaction at the liquid metal results in the growth of hydrophilic gallium oxide (Ga2O3) on the surface of the Galinstan19 2Ga + 3H 2O → Ga 2O3 + 6H+ + 6e‐
(1)
In neutral solutions, the gallium oxide will form a thick film that inhibits the motion of the encased liquid metal. However, when immersed in a solution of NaOH the gallium oxide will be subsequently dissolved17 a reaction that has recently been characterized as20 Ga 2O3 + 2NaOH + 3H 2O → 2NaGa(OH)4
(2)
The combination of reactions 1 and 2 results in the growth of a thin layer of hydrophilic gallium oxide without the mechanical obstruction that would otherwise be imposed by a thick oxide layer, leading to a dramatically lower interfacial tension between 7
DOI: 10.1021/acsami.5b09466 ACS Appl. Mater. Interfaces 2016, 8, 6−10
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ACS Applied Materials & Interfaces
Figure 2. Liquid-metal velocity and redox current generation for a Galinstan droplet in a 1 M NaOH solution. The insets show the liquid metal at the indicated time. (a) Immediately before the electrical connection is made at t = 0 s, the liquid metal is at rest in a cylindrical anodic reservoir. (b) In the few seconds after an electrical connection is made between the liquid metal and the cathodic electrode, rapid oxide growth yields high levels of current as well as high actuation speeds. (c, d) Over the next 10−20 s, current and speed both gradually stabilize. (e, f) When the electrical connection between the copper electrode and the Galinstan is broken, the developed oxide layer is rapidly dissolved by the NaOH solution, and within 2−3 s the liquid metal has retracted back to the anodic reservoir.
term operation, as it exhibits immunity to corrosion at these same potentials.19 In addition to material compatibility, previous works have indicated that repeated gallium oxide formation and removal results in the eventual dissolution of gallium into the electrolyte.5 This dissolution forms an upper limit to the lifetime of any device utilizing this technique. The extent of this lifetime can be determined by calculating the relative rate of gallium loss versus the time-varying capacity of the electrolyte to dissolve fresh gallium oxide, resulting in a theoretical maximum number of device actuation cycles. For the test conditions described in Figure 2, these calculations estimate a device lifetime of over 9000 cycles before the Galinstan loses its eutectic properties (Appendix III in the Supporting Information). Tests were performed with a variety of electrolytes and concentrations, of which NaOH was by far the most effective. Tests using 1 M NaCl resulted in only 15 μA of sustained current and no appreciable liquid-metal actuation, confirming that the ability of the electrolyte to continuously dissolve the newly formed oxide layer is critical to sustained redox reaction. Higher concentrations of NaOH resulted in higher redox current and faster actuation, likely due to the increased solvency of the electrolyte, which allowed the oxide layer to be removed more quickly, increasing redox current (Appendix IV in the Supporting Information).
speed of this actuation process is directly related to the rate of oxide formation, so that higher generated redox currents result in faster actuation (Appendix I in the Supporting Information). The relationship between actuation speed and redox current is highly linear, as expected by the actuation model (Appendix II in the Supporting Information). Disconnecting the electrical path halts the redox reactions, allowing the NaOH to dissolve the oxide layer on the liquid metal and resulting in the rapid withdrawal of the liquid metal from the channel. It is important to decouple the galvanic cell before the liquid metal is able to enter the cathodic reservoir, as the thin silicone ceiling of this reservoir is incapable of resisting the force imposed on it by the high-surface-tension liquid metal. This force will bow the ceiling upward, interfering with the ORR and potentially causing the leading edge of the liquid metal to split off. Similarly, allowing the liquid metal to reach the copper electrode will result in the formation of a new localized galvanic cell with an uncontrollable reaction rate (galvanic corrosion) that will also render the device unusable. Once the liquid metal has returned to the anodic reservoir the process can be repeated, and was limited in our tests by the eventual corrosion of the copper electrode by the NaOH. Copper was used here because of its low cost relative to more noble metals, but at high pH levels (measured in these tests at approximately 13.5), it is well within its corrosion regime at oxygen reduction potentials.19 Gold is a better option for long8
DOI: 10.1021/acsami.5b09466 ACS Appl. Mater. Interfaces 2016, 8, 6−10
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complex resonant circuit deployed by an autonomous robot, or an ultralow-energy sensor. The controllable transfer of electrochemical energy into kinetic actuation is a powerful tool in the manipulation of liquid metals, with applications in a variety of fields including ultralow-power electronics and autonomously tunable RF devices. We have demonstrated here how gallium-based alloys such as Galinstan can be used as an anode in a redox reaction with oxygen, resulting in gallium oxide growth and inducing flow into a channel. The redox reactions are more than sufficient to produce dramatic changes in the liquid-metal shape and position, with no external power source required. The techniques demonstrated here can be powerful tools for designers wishing to harness the unique electrochemical properties of this fascinating material.
Liquid-metal galvanic actuation is not limited to back-andforth motion within a capillary channel; it can also be used to trigger other electrokinetic actuation techniques (Movie S2 and Appendix V in the Supporting Information) or to rapidly form complex patterns and shapes. In Figure 3, a 300 μm high liquid-
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.5b09466. Further characterization of this self-actuation mechanism, comparisons to externally driven oxide growth, calculations of theoretical device lifetime, examples of galvanically triggered surface-tension-based actuation techniques, and descriptions of supporting videos (PDF) Movie S1, galvanic actuation (AVI) Movie S2, galvanic CEW transition (AVI) Movie S3, galvanic H (AVI)
Figure 3. (a) Liquid metal in a 300 μm high reservoir is connected to a 600 μm high target chamber in the shape of an H. The liquid metal is electrically connected via an external conductor to a copper cathode on the opposite side of the chamber, and the chamber itself is filled with 1 M NaOH solution. When a few drops of NaOH bridge the chamber to the cathode (i), galvanic actuation is initiated and the liquid metal begins flowing into the connecting channel. Once the liquid metal reaches the end of the channel (ii), its internal Laplace pressure drives it to rapidly fill the taller target chamber in order to lower its free surface energy (iii−iv). (b) The speed is a function of the actuation mechanism; in the channel, the liquid metal is driven by oxide growth induced by the galvanic cell, but given the chance to fill a taller chamber, it does so rapidly because of Laplace pressure. Thus, the liquid metal can quickly fill almost any arbitrary shape, using only its own stored electrochemical energy and without an external power source.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Author Contributions
The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.
metal reservoir is connected to a 600-μm-high target chamber of arbitrary shape (in this case forming an ‘H’, for Hawaii). The liquid metal is electrically connected to an external copper probe located immediately outside of the target chamber, which is filled with 0.25 M NaOH. When a few drops of NaOH solution are placed on the copper probe adjacent to the target chamber, the galvanic cell is completed, and liquid metal begins flowing from the reservoir into the connecting channel. The speed of the liquid metal before entering the target chamber is a relatively slow 90 μm/s, and is a function of both the channel dimensions as well as the internal electrical resistance of the galvanic cell. However, once the liquid metal reaches the ‘H’, it reduces its free surface energy by filling the 600-μm-high chamber and evacuating the 300 μm high reservoir. Thus, the actuation driver changes from oxide growth to the internal Laplace pressure of the liquid metal, which moves the liquid metal to rapidly fill the chamber at a linear velocity of approximately 2.5 cm/s, an increase of 2 orders of magnitude. Video of this actuation is available in Movie S3. The key to this rapid motion is the reduction in free surface energy of the liquid metal, which for low aspect ratio geometries (h/w ≪ 1) is largely a function of height. Because of this, the liquid metal can fill a target chamber of almost any shape, as long as the height increase is sufficient to provide a significant surface energy reduction. In Figure 3, the liquid metal forms an ‘H’, but this shape could just as easily be a more
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the National Science Foundation under Grant ECCS-1101936. In addition, Ryan C. Gough was partially supported by an IEEE Microwave Theory and Techniques Society Graduate Fellowship.
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