TECHNICAL FIELD
The subject matter described herein relates to controlled reconfiguration of liquid metal structures.
BACKGROUND
Mercury is liquid at room temperature and atmospheric pressure. Because of the liquid nature of mercury, it is possible to change the configuration or shape of a liquid metal structure formed of mercury through application of mechanical force. However, mercury is highly toxic and therefore unsuitable for applications where the likelihood of exposure to humans is high.
Gallium alloys, like mercury, are liquid at room temperature and atmospheric pressure. Gallium metal is also liquid near room temperature and atmospheric pressure. Unlike mercury, gallium is considered to have low-toxicity. Because of its liquid phase at room temperature and atmospheric pressure and its relative low-toxicity, gallium may be useful for forming reconfigurable electrical, optical, thermal, or mechanical structures, such as filters in optical applications, antennas or wires in electronics applications, heat sinks in thermal applications, or microstructures in mechanical applications. However, one problem with using gallium as the medium for forming a reconfigurable structure is that an oxide skin forms on gallium. This skin causes the metal to stick to most surfaces and results in residue that remains even after a portion of the liquid gallium is moved, for example, from a fluid channel to a reservoir.
Accordingly, there exists a need for improved methods for voltage controlled reconfiguration of liquid metal structures.
SUMMARY
The subject matter described herein relates to voltage controlled reconfiguration of liquid metal structures. According to one exemplary method, a container is provided. An electrolyte is provided in the container. A liquid metal structure is provided in the container and at least partially in contact with the electrolyte. A voltage is applied between the liquid metal structure and the electrolyte to change the shape of the liquid metal structure such that the structure achieves a desired shape for an electrical, mechanical, optical, or thermal application.
A controller for voltage controlled spreading of liquid metal structures may be implemented in hardware, software, firmware, or any combination thereof. As such, the terms “function” or “module” as used herein refer to hardware, which may also include software and/or firmware components, for implementing the feature being described. In one exemplary implementation, the subject matter described herein may be implemented using a computer readable medium having stored thereon computer executable instructions that when executed by the processor of a computer control the computer to perform steps. Exemplary computer readable media suitable for implementing the subject matter described herein include non-transitory computer-readable media, such as disk memory devices, chip memory devices, programmable logic devices, and application specific integrated circuits. In addition, a computer readable medium that implements the subject matter described herein may be located on a single device or computing platform or may be distributed across multiple devices or computing platforms.
BRIEF DESCRIPTION OF THE DRAWINGS
The subject matter described herein will now be explained with reference to the accompanying drawings of which:
FIG. 1A is a schematic diagram illustrating the application of a reductive potential to a liquid metal structure in a microfluidic channel according to an embodiment of the subject matter described herein;
FIGS. 1B-1D illustrate withdrawal of a liquid metal structure from a microfluidic channel into a reservoir upon application of a reductive potential to the liquid metal structure according to an embodiment of the subject matter described herein;
FIGS. 2A-2C illustrate the selective withdrawal of a liquid metal structure from two segments of a multi-segment fluid channel and not from a third segment upon application of a reductive potential to the liquid metal structures in the first and second segments but not the third segment according to an embodiment of the subject matter described herein;
FIGS. 3A-3D illustrate the application of an oxidative potential to a liquid metal structure immersed in a pool of electrolyte to shape the liquid structure on a surface of a container according to an embodiment of the subject matter described herein;
FIGS. 4A-4C illustrate the application of an oxidative potential to a liquid metal structure to cause the liquid metal structure to move from a pipette and form a wire in a container according to an embodiment of the subject matter described herein;
FIGS. 5A and 5B illustrate the application of an oxidative potential to a liquid metal structure to cause the liquid metal structure to move from one chamber through a capillary into another chamber of a multi-chamber container according to an embodiment of the subject matter described herein;
FIGS. 6A and 6B illustrate the application of an oxidative potential to a liquid metal structure in a container where the liquid metal structure reconfigures itself in a direction that is opposite the direction of gravitational force according to an embodiment of the subject matter described herein;
FIGS. 7A-7C illustrate the application of an acid to a liquid metal structure to remove the oxide and cause a capillarity induced contraction of the liquid metal structure on a substrate according to an embodiment of the subject matter described herein;
FIGS. 8A and 8B illustrate the application of an oxidative potential to a liquid metal structure immersed in an electrolyte to cause spreading of the liquid metal structure within the electrolyte according to an embodiment of the subject matter described herein;
FIGS. 9A-9D illustrate a self-healing wire encapsulated in a self-healing material and the cutting of the self-healing wire according to an embodiment of the subject matter described herein;
FIGS. 10A-10D illustrate the reconnection and self-healing of a self-healing wire according to an embodiment of the subject matter described herein;
FIG. 11 is a block diagram of a system for voltage controlled reconfiguration of liquid metal structures according to an embodiment of the subject matter described herein; and
FIG. 12 is a flow chart illustrating an exemplary process for voltage controlled reconfiguration of liquid metal according to an embodiment of the subject matter described herein.
FIG. 13 is a schematic diagram illustrating an exemplary utility of oxidative spreading according to an embodiment of the subject matter described herein. (a) A schematic diagram depicting the electrochemical setup of injecting liquid metal into a capillary channel (˜0.9 mm ID). b) Four sequential top-down, optical micrographs of metal filling a glass capillary (˜0.9 mm ID) in response to an oxidative potential. c) Side view of a small droplet of EGaln pumped at a flow rate in am electrolyte, d) Formation of an oxide coated liquid metal fiber coming out of the tube.
DETAILED DESCRIPTION
The subject matter described herein includes methods, systems, and computer readable media for voltage controlled reconfiguration liquid metal structures. FIGS. 1A-1D illustrate the application of a reductive potential to a liquid metal structure to move the liquid metal structure from a microfluidic channel to a reservoir. As used herein, the term “reductive potential” will be used to refer to the application of a potential to a liquid metal structure that causes a reduction reaction on the surface of the liquid metal structure. In FIG. 1A, negative charges are pushed by the application of the potential from the electrolyte (lighter color) through the circuit to the gallium oxide skin of the liquid metal structure (darker color), which reduces the oxidized gallium in the skin. Known gallium alloys are believed to form oxide skins. Hence, the subject matter described herein is applicable to any gallium alloy that forms an oxide skin. The following reduction reaction is believed to occur on the gallium oxide skin:
Ga3++3e−→Ga
The result of the reduction of the oxide skin is that the high surface tension of the gallium metal causes the metal to spontaneously withdraw into the reservoir (oval shaped ball on left hand side of FIGS. 1A-1D) without leaving metal in the channel.
As will be described in more detail below, an oxidative potential may be applied to a liquid metal structure to form a liquid metal structure having a desired shape or to move liquid metal from a reservoir into a microfluidic channel. For example, if, after the liquid metal is in the reservoir as illustrated in FIG. 1D, the potential in FIG. 1A is reversed so that the positive electrode contacts the liquid metal in the reservoir and the negative counter electrode contacts the electrolyte, the liquid metal structure will move from the reservoir into the channel and will remain in the channel even after the oxidative potential is removed due to the oxide skin that forms around the metal. The following reaction is believed to occur when an oxidative potential is applied to the gallium metal:
Ga→Ga3++3e−
The gallium combines with oxygen or oxygen containing ions in the electrolyte to form the oxide skin. To maintain charge neutrality, an accompanying reduction reaction occurs at the counter electrode contacting the electrolyte.
In addition, after application of the reductive or oxidative potential is ceased, the liquid metal structure holds its shape. For example, FIG. 1C shows the liquid metal partially withdrawn. If application of the potential ceases, the oxide skin will reform, and the liquid metal structure will halt its movement into the reservoir and hold its shape. Such a property may be useful to define liquid metal structures of desired shape and to stop applying the potential once the desired shape is achieved.
As used herein, the term “oxidative potential” will be used to refer to the application of a potential to a liquid metal structure that causes oxidation of the liquid metal structure. Thus, the terms “oxidative” and “reductive” are defined with respect to the reactions they cause in the liquid metal structure.
In FIG. 1A, the liquid metal structure moves in the direction of the arrow when the reductive potential is applied to the liquid metal structure. In this example, the liquid metal structure is made of eutectic gallium indium (EGaln). However, other gallium based alloys may be used without departing from the scope to the subject matter described herein. The electrolyte can be any solution that includes ions capable of conducting current, such as aqueous solutions of NaCl, NaF, NaOH, HCl, polyelectrolytes, over a wide range of pH, such as 0-14. The electrolyte may be with or without dissolved oxygen, and the electrolyte concentration may vary, for example, between a concentration of 0 and 1M, depending on desired conductivity. In some experiments, reduction-induced withdrawal and oxidative spreading of a liquid metal structure occurred at voltages ranging from 1V to 5V DC. AC voltages and larger ranges of DC voltages can also be used to reconfigure liquid metal structures without departing from the scope of the subject matter described herein. Larger voltages may result in generation of gas bubbles, such as hydrogen gas bubbles, which should be avoided due to the disruptive effect of bubbles on the electrical pathway in the electrolyte. Also, reductive voltage ranges that are applied to a liquid metal structure in accordance with embodiments of the subject matter described herein may be the same in magnitude but opposite in polarity with respect to the oxidative voltage ranges.
The container that holds the liquid metal structure and the electrolyte may be made of any suitable material that is capable of containing gallium and the electrolyte without significant oxidation or reduction of the container. Exemplary container materials suitable for use with the subject matter described herein include glass, Teflon, polystyrene, and tungsten.
FIGS. 1B, 1C, and 1D illustrate stages of withdrawal of the liquid metal structure from the microfluidic channel into the reservoir on the left hand side of the channel in response to the application of the reductive potential.
In FIGS. 2A-2D, a reductive potential is applied between two segments of a multi-segment microfluidic channel but not a third segment. As a result, the liquid metal structure withdraws from the first and second segments (Segments 1 and 2) but the liquid metal in the third segment (Segment 3) remains stable. The embodiments illustrated in FIGS. 2A-2D demonstrate that applying potentials to liquid metal structures can be used to break wires and form complex patterns.
In FIGS. 3A-3D, a gallium sphere is immersed in an electrolyte, which in FIGS. 3A-3D is aqueous electrolyte. An oxidative potential is applied to the gallium sphere, where the positive electrode contacts the gallium sphere and the negative electrode contacts the electrolyte. Absent an oxidative potential, the metal does not spread due to surface tension and the presence of the oxide skin (see FIG. 3A). When the positive electrode is applied to the gallium metal structure and the negative electrode is immersed in the electrolyte, negative charge flows from the gallium metal structure, through the positive electrode, through the electrical circuit formed by the power supply outside of the electrolyte, and into the electrolyte through the negative electrode, causing the reduction of hydrogen ions to hydrogen atoms at the negative electrode. The bubbles in the electrolyte around the negative electrode in FIGS. 3B and 3C are believed to be caused by the reduction of hydrogen ions (protons) in the electrolyte and the resulting formation of hydrogen gas.
An oxidation reaction occurs on the surface of the gallium metal structure in contact with the electrolyte to form an oxide skin. It is believed that the spreading of the gallium metal occurs when the capacitive energy formed by a capacitor in which the gallium metal is one conductor, the oxide skin is the insulator, and the electrolyte as the other conductor exceeds the surface tension of the gallium structure. When the negative electrode is moved away from the original liquid metal structure, the oxide skin forms around the liquid gallium as it is drawn from its original position. As illustrated in FIG. 3D, when the electrodes are removed, a shaped liquid metal structure has been deposited on the surface of the container in which the liquid metal structure and the electrolyte reside. Such shaping can be used to form antennas, or wires, or other structures of desired configurations for an electronics application.
In FIGS. 4A-4D, a pipette containing a liquid metal is placed within a container containing a basic electrolyte, which removes the oxide skin that forms around the liquid gallium. Absent the application of any electric potential, the liquid metal in the pipette or tube is mechanically pumped into the receiving container. A droplet of liquid metal forms at the outlet of the pipette (see FIG. 4A). When the droplet is large enough, it falls to the bottom of the receiving container because of gravitational forces. However, when an oxidative potential is applied to the liquid metal structure by placing the positive electrode in contact with the liquid metal structure in the pipette and the negative electrode in contact with the electrolyte, the protons in the electrolyte are reduced, and the gallium metal exiting the tubing is oxidized. The gallium begins to flow into the receiving container, and a gallium oxide skin forms around the liquid metal as it flows into the second container, creating a flowing wire from the pipette to the receiving container, as illustrated in FIG. 4B. In FIG. 4C, when the electrodes are removed, the formation of the wire ceases and the surface tension of the liquid metal in the receiving container causes it to form a sphere because the basic electrolyte removes the oxide skin if there is no oxidative potential. A liquid gallium alloy structure with an oxide skin is mechanically stable (i.e., it retains its shape after the cessation of application of the oxidative or reductive potential) unless there is a reductive potential applied or when the electrolyte used is basic (pH>10) or acidic (pH<3).
FIGS. 5A and 5B illustrate the application of an oxidative potential to a liquid metal structure in a multi-chamber container where a capillary filled with an electrolyte forms a conductive path between the chambers. In FIGS. 5A and 5B, the electrolyte fills the chamber on the left hand side of the container and extends through a horizontal passageway to the right hand side of the chamber and to a vertical passageway to rest in contact the lower surface of the liquid metal sphere in the right hand chamber. As illustrated in FIG. 5B, when an oxidizing potential is applied to the liquid metal structure by placing a positive electrode in contact with the liquid metal structure and the negative electrode in contact with the electrolyte, the liquid metal oxidizes and flows through the vertical passageway into the lower part of the right hand chamber.
FIGS. 6A and 6B illustrate the same response that occurs in FIGS. 5A and 5B except that in FIG. 6A and 6B, the liquid metal structure starts in the lower portion of the right hand chamber. In FIG. 6B, an oxidative potential is applied to the liquid metal structure by placing the positive electrode in contact with the liquid metal structure and the negative electrode in contact with the electrolyte. It can be seen in FIG. 6B that the oxidative potential applied to the liquid metal structure causes the liquid metal to flow upwards through the vertical passageway between the lower and upper sub-chambers on the right hand side of the container. Thus, FIGS. 6A and 6B illustrate that the movement of the liquid metal is caused by forces other than gravity because the metal flows upwards against the force of gravity.
FIGS. 7A-7C illustrate the application of one molar hydrochloric acid to a liquid metal structure on a slide. As illustrated in FIG. 8B and 8C, when the acid is applied to the liquid metal structure, the oxide skin of the liquid metal structure is reduced, and the surface tension of the liquid metal causes it to form a sphere. A liquid gallium alloy structure with an oxide skin is mechanically stable (FIG. 8A) unless there is a reductive potential applied or when the electrolyte used is basic (pH>10) or acidic (pH<3).
FIGS. 8A and 8B illustrate the application of an oxidative potential to a liquid metal structure immersed within an electrolyte solution. As illustrated in FIG. 8B, the result of application of the oxidative potential is the spreading of the liquid metal structure along a surface of a container that holds the liquid metal structure and the electrolyte.
FIGS. 9A-9D illustrate a self-healing wire formed with a liquid metal structure according to an embodiment of the subject matter described herein. Referring to FIG. 9A, a liquid metal wire is encapsulated in a self-healing polymer, such as Reverlink available from Arkema, Inc. In FIG. 9B, the liquid metal wire is connected to a voltage source to light an LED. In FIGS. 9C and 9D, the self-healing polymer and the liquid metal within the polymer are cut.
In FIG. 10A, the self-healing wire is reconnected by mechanically aligning the two halves in physical contact with each other. In FIG. 10B, the two ends of the liquid metal wire reforms such that current flows to the LED and the LED is illuminated. In FIGS. 10C and 10D, the polymer that surrounded the liquid metal structure is healed.
FIG. 11 is a block diagram illustrating a system for voltage controlled reconfiguration of a liquid metal according to an embodiment of the subject matter described herein. Referring to FIG. 11, the system includes a container, an electrolyte, a liquid metal structure, and a controlled voltage source. The liquid metal structure may be a gallium material, including any known alloy of gallium that is liquid at the temperature and pressure of interest. The electrolyte may be any suitable aqueous or other protic solution or solution capable of conducting ions, including those described above. The container may be a microfluidic channel or any other structure suitable for containing or holding an electrolyte and a liquid metal. The container may be formed of any material suitable for containing the electrolyte and the liquid metal, including the exemplary container materials described above. The controlled voltage source may be any suitable source for applying voltages to the liquid metal structure and the electrolyte. The voltage source may be a controlled AC or DC voltage source capable of applying voltages over any suitable operational range, e.g., 1-12 volts. The controlled voltage source may be configured to apply oxidative or reductive potential to the liquid metal structure so that the liquid metal structure will move to a desired configuration, such as a desired conductor shape or antenna shape.
FIG. 12 is a flow chart illustrating exemplary overall steps for voltage controlled reconfiguration of a liquid metal structure. Referring to FIG. 14, in step 1200 a container is provided. For example, the container may be any container suitable for containing a liquid metal and an electrolyte. In step 1202, an electrolyte is provided in the container. For example, the container may be filled with an aqueous electrolyte solution suitable for conducting ions. In step 1204, a liquid metal structure is provided in the container at least partially in contact with the electrolyte. For example, a gallium alloy or other liquid metal that forms a surface oxide may be provided in the container. In step 1206, a voltage is applied to the liquid metal structure to change the shape of the liquid metal structure such that the structure achieves a desired shape for an electrical, mechanical, optical, or thermal application. For example, a reductive potential may be applied to the liquid metal structure and an electrolyte to withdraw the liquid metal structure from a microfluidic channel into a reservoir. In an alternate implementation, an oxidative potential may be applied to a liquid metal structure in a reservoir to move the liquid metal structure from the reservoir into a microfluidic channel to change its shape to a desired shape for an electrical, optical, thermal, or mechanical application. In one example, the liquid metal structure may be shaped to have a desired electrical property, such as a desired conductance, resistance, resonant frequency, inductance, directionality or other desired property. In another example, the structure may be shaped to have a desired optical, mechanical, or thermal property. The reconfigured electrical structure may be used for any suitable application, including electrodes, microfluidic structures, optical components, microfluidic cooling, etc.
According to another aspect of the subject matter described herein, an oxidative potential may be used to cause droplets of a liquid metal to form fibers, as illustrated in FIGS. 13A-13D. In the absence of applied potential, metal pumped out the end of the capillary forms beads that fall periodically due to the forces of gravity (FIG. 13C). Applying an oxidative potential to the metal relative to a counter electrode in solution causes the metal to form a stable fiber despite its large surface tension (FIG. 3D), ii.
It will be understood that various details of the presently disclosed subject matter may be changed without departing from the scope of the presently disclosed subject matter. Furthermore, the foregoing description is for the purpose of illustration only, and not for the purpose of limitation.
Patent Application
20160122888
