Methods of rapid 3D nano/microfabrication of multifunctional shell-stabilized liquid metal pipe networks and insulating/metal liquids electro-mechanical switch and capacitive strain sensor
1. An electric switch apparatus, comprising:
- a base comprising a first channel and a second channel, wherein the base comprises a flexible polymer and a strain-dependent gate coupling the first channel to the second channel;
an electrode disposed in the first channel, wherein the electrode comprises a cavity and a liquid metal disposed in the cavity; and
an electrically insulating liquid at least partially disposed in the second channel, wherein in a strained condition a portion of the electrically insulating liquid extends into the first channel dividing the electrode into at least two electrically isolated chambers within the first channel, and wherein in an unstrained condition the cavity of the electrode is undivided.
An electronic switch apparatus, a capacitive strain gauge apparatus and a nozzle apparatus, as well as methods for making electrodes such as liquid metal pipes are provided. A nozzle apparatus includes (a) a nozzle housing defining a cavity, where the nozzle housing defines an inlet at a first end and defines an outlet at a second end and (b) a first tube defining an inlet at a first end and defining an outlet at a second end, wherein the first tube is at least partially disposed in and is co-axially arranged with the nozzle housing, where the first tube defines a first flow channel, wherein a second flow channel is defined between an exterior surface of the first tube and an interior surface of the nozzle housing.
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Current AssigneeAgilent Technologies Incorporated
Sponsoring EntityAgilent Technologies Incorporated
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Sponsoring EntityAgilent Technologies Incorporated
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- 1. An electric switch apparatus, comprising:
a base comprising a first channel and a second channel, wherein the base comprises a flexible polymer and a strain-dependent gate coupling the first channel to the second channel; an electrode disposed in the first channel, wherein the electrode comprises a cavity and a liquid metal disposed in the cavity; and an electrically insulating liquid at least partially disposed in the second channel, wherein in a strained condition a portion of the electrically insulating liquid extends into the first channel dividing the electrode into at least two electrically isolated chambers within the first channel, and wherein in an unstrained condition the cavity of the electrode is undivided.
- View Dependent Claims (2, 3, 4, 5)
- 6. An electric switch apparatus, comprising:
a base comprising one or more channels, wherein the base comprises a flexible polymer; an electrode disposed in at least one of the one or more channels, wherein the electrode comprises a cavity and a liquid metal disposed in the cavity and the liquid metal comprises a gallium-based liquid metal comprising one or more of GaIn, GaInSn, and a gallium alloy; and a strain dependent gate defined by opposing walls of the channel, wherein in a strained condition the strain-dependent gate maintains an open position such that the cavity of the electrode is undivided, and wherein in an unstrained condition the strain-dependent gate maintains a closed position dividing the electrode into at least two electrically isolated chambers.
- View Dependent Claims (7)
- 8. An electric switch apparatus, comprising:
a base comprising a first channel and a second channel, wherein the base comprises a flexible polymer; an electrode disposed in the first channel, wherein the electrode comprises a cavity and a liquid metal disposed in the cavity, wherein the liquid metal comprises a gallium-based liquid metal comprising one or more of GaIn, GaInSn, and a gallium alloy; and
- View Dependent Claims (9, 10, 11)
- 12. An electric switch apparatus, comprising:
an electrode disposed in the first channel, wherein the electrode comprises; a cavity and a liquid metal disposed in the cavity; and a metal pipe having an oxidized external shell defining the cavity; and
- View Dependent Claims (13, 14)
- 15. An electric switch apparatus, comprising:
a base comprising one or more channels, wherein the base comprises a flexible polymer; an electrode disposed in at least one of the one or more channels, wherein the electrode comprises; a cavity and a liquid metal disposed in the cavity; and a metal pipe having an oxidized external shell defining the cavity; and
This application is a U.S. National Phase of International Application No. PCT/US2015/031174, filed May 15, 2015, which claims priority to U.S. Provisional Application No. 61/994,442, filed May 16, 2014, the disclosures of which are hereby incorporated by reference in their entireties.
Stretchable electronic components have applications in flexible electronics, biomedical devices, and soft robotics. Room-temperature liquid metals may be attractive materials for fabrication of such devices because they retain their functionality even when stretched to several times their original length. One of the earliest examples of liquid-phase electronics is the Whitney strain gauge. This device measures strain of a mercury-filled rubber tube by measuring change in electric resistance of the metal. While in the past two decades mercury and rubber have been replaced by nontoxic liquid gallium alloys (i.e., GaIn and GaInSn) and more elastic polydiemethylsiloxane (“PDMS”), the resistive design of the liquid metal strain sensor remains popular. These resistive devices have a large footprint that may restrict the number of sensors that may be embedded into, for example, electronic fabric or skin. For example a ˜1Ω resistor made out of GaInSn with resistivity of 0.29 μΩm in a 200 μm diameter channel typically has a length of ˜10 cm. By winding the channel 10 times such a sensor may fit into an area of ˜1 cm2.
Improved understanding of the GaIn and GaInSn wetting characteristics and advances in their micro-fabrication may enable fabrication of smaller liquid metal filled microchannels with higher areal density; however, the serpentine geometry of these resistors remains quite complex. Several designs of capacitive strain sensors have been proposed as alternatives to the resistive devices. These capacitive devices consist of two microchannels filled with liquid metal separated by solid dielectric PDMS matrix. For in-plane sensing an order of magnitude estimate for the required sensor footprint can be obtained using the parallel plate capacitor model, C≈ε0εA/d (i.e., A≈Cd/ε0ε). To achieve a capacitance (C) of ˜1 pF, two liquid metal-filled microchannels with both height (h) and separation (d) of ˜400 μm within a PDMS matrix (ε˜2) typically have a length of 1˜10 cm (from the conductor-dielectric interfacial area, A˜1 h˜4×10−5 m2). By winding the parallel channels in a serpentine arrangement, such a sensor may fit into a base area of several square centimeters. Such a base area is typically needed for a variety of winding two-channel capacitive strain sensor designs to achieve C˜1-15 pF. With such a large footprint the sensor output is affected by stretching in multiple directions, not only in the desired principle direction. As a result, correlation of the physical strain with the sensor output is complex.
Example embodiments provide an electronic switch apparatus, a capacitive strain gauge apparatus and a nozzle apparatus and methods for making liquid metal pipes. Specifically, in example embodiments, the electronic switch apparatus and capacitive strain gauge apparatus may advantageously utilize smaller microchannels and/or an insulator material having a higher dielectric constant (ε) permitting a smaller footprint of these devices than sensors known in the alt. In other example embodiments, dielectric liquids may be used in place of solid elastomers, like PDMS, in the electronic switch apparatus and/or capacitive strain gauge apparatus and may beneficially decrease the required conductor-dielectric interfacial area of the electronic switch or capacitive strain gauge apparatus by 20 to 40 times compared to sensors known in the art.
In other example embodiments, the nozzle apparatus and methods of use may advantageously permit production of liquid metal pipes. Example embodiments of the nozzle apparatus and methods of use may permit formation of shells of the liquid metal pipes with different mechanical, chemical, electrical, and thermal properties and functions. Thus the choice of one or more fluids flowing through the nozzle apparatus may be tailored to the specific functionality desired for the shell material/surface, while at the same time enabling free liquid metal flow inside the liquid metal pipe. This may have the advantage of allowing rapid 3D nano- or micro-fabrication of liquid metal networks for a variety of applications. 3D printed liquid metal pipe networks may be functional on their own or may serve as the conductive skeleton for encapsulation in a flexible polymer matrix, for example, for use in the electronic switch apparatus and/or the capacitive strain gauge apparatus.
Thus, in one aspect, an electric switch apparatus is provided having (a) a base comprising a first channel and a second channel, (b) an electrode disposed in the first channel, wherein the electrode comprises a cavity and a liquid metal disposed in the cavity and (c) an electrically insulating liquid at least partially disposed in the second channel, where in a strained condition a portion of the electrically insulating liquid extends into the first channel dividing the electrode into at least two electrically isolated chambers within the first channel, and where in an unstrained condition the cavity of the electrode is undivided.
In a second aspect, an electric switch apparatus is provided having (a) a base comprising one or more channels, (b) an electrode disposed in at least one of the one or more channels, wherein the electrode comprises a cavity and a liquid metal disposed in the cavity and (c) a strain dependent gate defined by opposing walls of the channel, where in a strained condition the strain-dependent gate maintains an open position such that the cavity of the electrode is undivided, and where in an unstrained condition the strain-dependent gate maintains a closed position dividing the electrode into at least two electrically isolated chambers.
In a third aspect, a capacitive strain gauge apparatus is provided having (a) a base defining a channel having a first end and a second end, (b) a first electrode disposed in the first end of the channel, where the first electrode comprises a first cavity and a first liquid metal disposed in the first cavity, (e) a second electrode disposed in the second end of the channel, where the second electrode comprises a second cavity and a second liquid metal disposed in the second cavity and (d) an insulating liquid arranged between the first electrode and the second electrode.
In a fourth aspect, a nozzle apparatus is provided having (a) a nozzle housing defining a cavity, where the nozzle housing defines an inlet at a first end and defines an outlet at a second end and (b) a first tube defining an inlet at a first end and defining an outlet at a second end, wherein the first tube is at least partially disposed in and is co-axially arranged with the nozzle housing, where the first tube defines a first flow channel, wherein a second flow channel is defined between an exterior surface of the first tube and an interior surface of the nozzle housing.
In a fifth aspect, a method is provided including the steps of (a) flowing a liquid metal through a first flow channel to a first outlet, (b) flowing a first fluid through a second flow channel to a second outlet surrounding the first outlet and (c) upon exiting the respective first outlet or second outlet, reacting the first fluid with the flowing liquid metal, creating an exterior shell on the liquid metal.
These as well as other aspects, advantages, and alternatives, will become apparent to those of ordinary skill in the art by reading the following detailed description, with reference where appropriate to the accompanying drawings.
Example embodiments of an electronic switch apparatus, a capacitive strain gauge apparatus and a nozzle apparatus and methods for making liquid metal pipes are described herein. Any example embodiment or feature described herein is not necessarily to be construed as preferred or advantageous over other embodiments or features. The example embodiments described herein are not meant to be limiting. It will be readily understood that certain aspects of the disclosed apparatus and methods can be arranged and combined in a wide variety of different configurations, all of which are contemplated herein.
Furthermore, the particular arrangements shown in the Figures should not be viewed as limiting. It should be understood that other embodiments may include more or less of each element shown in a given Figure. Further, some of the illustrated elements may be combined or omitted. Yet further, an example embodiment may include elements that are not illustrated in the Figures.
As used herein, a “capacitive strain gauge” refers to an apparatus configured to measure strain in an object of interest. In operation, the capacitive strain gauge may be attached to the object of interest and receives the same strain as that applied to the object of interest. The applied strain may then be measured by the change in the electrical properties of capacitive strain gauge due to deformation of the strain gauge.
As used herein, an “electric switch” refers to an apparatus configured to complete and to open an electric circuit depending upon whether the switch is in a strained or unstrained position.
As used herein, an “electrode” refers to an apparatus having a shell or surface layer that may reduce adhesion and flow of a liquid metal therein. The electrode shell or surface may include grown oxide, textured elastomer, oil infused elastomer, an interfacial lubricating liquid or combinations thereof.
As used herein, a “liquid metal pipe” or “metal pipe” refers to a pipe having an oxidized external shell surrounding a liquid metal. Other materials like a liquid or solid dielectric may be disposed within or injected into the pipe.
As used herein, a flexible polymer may include any polymer capable of stretching (i.e., having a low Young'"'"'s modulus) and that acts as a dielectric, any suitable elastomer may be used, including but not limited to embodiments utilizing silicones, such as Polydimethylsiloxane (“PDMS”) and ecoflex.
The present embodiments advantageously provide an electronic switch apparatus, a capacitive strain gauge apparatus and a nozzle apparatus and methods for making liquid metal pipes. Referring now to
In one embodiment, the base 105 may include a strain-dependent gate 130 coupling the first channel 110 to the second channel 115, facilitating the electrically insulating liquid'"'"'s movement between the unstrained 107 and strained 106 conditions. In one embodiment, the strain-dependent gate 130 may be biased closed in the unstrained position 107 and biased open in the strained condition 106.
Referring now to
Referring now to
In one embodiment, in an unstrained condition 307 the channel 310 may have a first length L1 and a first cross-sectional area A1, and in a strained condition 306 the channel 310 may have a second length L2 greater than first length L1 and a second cross-sectional area A2 less than first cross-sectional area A1. In other words, the base 305, and thereby the channel 310 and the first and second electrodes 315, 320, are elongated in the strained condition. In a further embodiment, the base 305 may have a surface area ranging from about 0.1 mm2 to about 10 mm2.
In another embodiment, in which the electrodes are metal pipes, the first metal pipe 315 and the second metal pipe 320 may each have one of a flat end (
In another embodiment, in which the electrodes are metal pipes, a tip-to-tip distance between the first metal pipe 315 and the second metal pipe 320 measured across the insulating liquid 325 may range from about 100 nm to about 10 mm (e.g.,
Referring now to
In one embodiment, as shown in
In another embodiment, the second end 419 of the first tube 415 may be coextensive with the second end 409 of the nozzle housing 405 such that the outlet 418 of the first tube 415 is arranged within the outlet 408 of the nozzle housing 405, as shown in
In a further embodiment, shown in
In still another embodiment, the nozzle apparatus 400 may also include a liquid reservoir arranged in fluid communication with the first flow channel 415 and a first fluid chamber arranged in fluid communication with the second flow channel 425 (not shown). This arrangement may permit the formation of a metal pipe 460 having an exterior oxide shell 465 and defining a cavity in which liquid metal 470 is disposed, as described in further detail below. In an alternative embodiment, shown in
As illustrated in
The foregoing method may induce complex surface dynamics on the liquid metal based upon the momentum ratio of the flow of the liquid metal and first fluid, the ratio of inertial to surface tension forces (Weber number), the ratio of inertial to viscous forces (Reynolds number), and the density and viscosity ratios of the liquid metal and the first fluid, for example. These factors may include Rayleigh-Plateau instabilities of the central liquid metal flow (i.e., small first fluid to liquid metal momentum ratio) and shear driven Kelvin-Helmholtz-type and Rayleigh-Taylor instabilities (i.e., large first fluid to liquid metal momentum ratio). Similar dynamics may occur in triple-orifice co-flow nozzles, discussed below and shown in
In one embodiment, the liquid metal 470 may be shaped via at least the first fluid 475, upon exiting the respective first outlet 418 or second outlet 408. In a further embodiment, the liquid metal may be shaped by focusing the flowing liquid metal 470 to a diameter smaller than a diameter of the first flow channel 420, which may be further achieved by adjusting the flow rate and/or pressure of the first fluid 475 or adjusting the position of the first outlet 418 relative to the second outlet 408, for example. In operation, in one embodiment, the thickness of the exterior shell 465 may increase with an increase in the amount of time that the exterior shell 465 is exposed to the first fluid. An example of this is shown in the detail view of
In yet another embodiment, the method may further include flowing a second fluid through a third flow channel to a third outlet (e.g.,
In yet another embodiment, the method may include depositing a portion of the liquid metal on a substrate, upon exiting the first outlet. In various embodiments, the substrate may be silicon, PTFE, PDMS, tungsten or glass.
The method may be performed using any of the embodiments of the nozzle apparatus 400 described above. The above detailed description describes various features and functions of the disclosed electric switch apparatus, capacitor strain gauge apparatus, nozzle apparatus and methods of use of the nozzle apparatus with reference to the accompanying figures. While various aspects and embodiments have been disclosed herein, other aspects and embodiments will be apparent to those skilled in the art. The various aspects and embodiments disclosed herein are for purposes of illustration and are not intended to be limiting, with the true scope and spirit being indicated by the following claims.
This example describes use of a co-flow nozzle for rapid 3D nano/microfabrication of multifunctional shell-stabilized liquid metal pipe networks. The method described herein may provide a significantly faster extrusion rate than current methods. It may also provide the opportunity for tailoring the mechanically stabilizing liquid metal oxide shell to varied functionalities through adjustment of the secondary reactive fluid. Having observed liquid metal flow within the flexible oxide shell, also introduced is an electric switch and sensitive capacitive strain sensor based on flow of an insulator/metal liquid within a flexible channel.
Besides polymer encapsulated strain sensors discussed above, liquid metals have also been proposed for a number of other applications including microelectronic heat sinks, thermal interface materials, electrical interconnections and contacts, droplet-based micro-switches, microsyringes for cells, radio-frequency switches, magneto-hydrodynamic pumps, stretchable antennas, resonators, and tunable-frequency selective surfaces. Because of this wide application space, several new routes of manufacturing liquid metal structures aiming to replace the current labor intensive, non-scalable, and geometry-limited, syringe injection-molding-like fabrication process have been developed. In particular, routes to making liquid metal features using vacuum-induced patterning, contact printing, roller-ball pen, masked deposition, micro-fluidic flow focusing, co-electro-spinning, freeze-casting, airbrushing, and 3D printing have been proposed. However, manufacturing of gallium-based liquid metals may be complicated by rapid surface oxidation. Despite EGaIn and GaInSn having surface tension about an order of magnitude higher than that of water, formation of the thin oxide-skin leads to strong adhesion of these materials to a variety of surfaces. Recently, the fundamentals of complex wetting dynamics caused by the surface oxidation and several ways of mitigating its negative effects of the oxide skin have been developed.
To overcome high adhesion of surface-oxidized EGaIn and EGaInSn to different surfaces embedding of metal droplets in hydrophobic nanoparticles (i.e. liquid marbles), and textured metal-phobic surfaces, hydrochloric acid liquid or vapor treatment, and acid-impregnated surfaces have been proposed. In contrast, the oxide-skin formation has been used to mechanically stabilize GaInSn extruded from a moving syringe. Using this approach, a variety of simple and complex free-standing 3D liquid metal structures were able to be printed. While opening-up a new avenue for variety of applications, 3D printing of GaInSn using these methods may be limited to extrusion rates of 2 to 200 μm/s. Past this rate, the forming liquid metal structure breaks-off the nozzle tip. This mechanical failure may be due to exceeding the critical surface stress of ˜0.5 N/m of the oxide formed through brief air exposure. This extrusion rate is an order of magnitude slower than typical extrusion rates of 2 to 3 cm/s achieved in commercial polymer 3D printers.
The inventors'"'"' research has shown that oxide-layers with different thicknesses may co-exist and that the mechanical properties of the oxide-shell may be dramatically altered through chemical treatment. In particular, using Focused Ion Beam-Scanning Electron Microscope (FIB-SEM) equipped with EDS and nanomanipulator, the nano-to-microscale wetting and adhesion properties of GaInSn before and after HCl vapor treatment were investigated. Images in
The sequence of images in
Rapid 3D Printing of Functional Oxide Liquid Metals Pipes with Co-Flow Nozzles
Instead of relying on relatively slow air driven oxidation to form the fragile oxide shell, this example describes the use of a co-flow nozzle with liquid metal and various other gases or liquid for optimizing the surface of liquid metals for a variety of applications. The secondary fluid may have multiple purposes including increased rate of shell formation, tailoring of various properties of the shell, and/or shaping the liquid metal flow (e.g. focusing to a smaller diameter, even down to nanoscale).
A variety of different reactive fluids may be used to induce formation of shells with different mechanical, chemical, electrical, and thermal properties and functions. Thus the choice of the outer fluid may be tailored to the specific functionality desired for the shell material/surface, while at the same time enabling free liquid metal flow inside. This may lead to rapid 3D nano/microfabrication of liquid metal networks for a variety of applications. 3D printed liquid metal pipe networks may be functional on their own or serve as the conductive skeleton for encapsulation in a flexible polymer matrix.
Novel Insulator/Metal Liquid Electro-Mechanical Switch and Capacitance-Based Strain Sensor
The inventors'"'"' research has also shown that, with application of minimal pressure, the liquid metal can flow freely within its own shell (i.e.,
Where ε is the permittivity of the dielectric liquid, A is the area of the plate, and L is the plate thickness. Assuming that the dielectric liquid is incompressible, change in length from L1 to L2, will cause a cross-sectional area change to A2=A1L1/L2.
This shows that the capacitance change is proportional to the square of inverse of the stretch ratio, which may allow for highly sensitive sensors
This example explores a capacitor composed of two liquid metal electrodes separated by a liquid dielectric material within a single straight cylindrical microchannel (see schematic in
2.1 Effects of Geometry and Dielectric Liquid on Capacitance of the Two-Liquid System
In order to systematically study the effects of geometry on capacitance of the two-liquid system without the hassle of fabrication of multiple devices we have developed a simple testing setup shown in
2.1.1 Effects of Separation Distance and Dielectric Liquid
The capacitance was measured for two flat-ended cylindrical electrodes with diameters of 1 mm and 2 mm separated by 0.1 mm to 13.5 mm that were submerged in glycerol, water, and as reference, silicone oil. The latter insulator has a dielectric constant (ε) between 2.2 and 2.8 and served as a control to mimic the PDMS elastomer. The plots in
where r is the electrode radius. The plot in
2.1.2 Effect of the Liquid-Liquid Interface Shape
Next, the effect of the curvature of the liquid metal-liquid dielectric interfaces on the system'"'"'s capacitance was investigated. Quantification of this effect is particularly relevant to the two-liquid capacitor because a meniscus is likely to form at the liquids'"'"' interface due to the large difference between their surface tensions. The capacitance of electrodes with two types of menisci shown in
In eqn (2) the two spheres are positioned along the z-coordinate and are defined by a and ηi in the bi-spherical system with ri=a|csch ηi| and zi=a coth ηi. The function Cl (cosh η1) is defined as:
In eqn (3) N1 is the normalization factor of the Legendre polynomials (N0=1, N1=1, N2=0.5, N3=0.5, N4=0.125, N5=0.125 etc.) and 2F1 is the hypergeometric function. Unfortunately, as shown in
2.1.3 Effect of Oxide-Skin Growth at the Liquid-Liquid Interface
The effect of the capacitance of the two-liquid capacitor by formation of the gallium oxide skin was investigated. This few nanometers-thin film rapidly forms at the interface between the liquid metal and all studied dielectrics due to high solubility of oxygen in the dielectric liquids as well as its high permeability through PDMS. The effect of oxide skin growth on the device capacitance was quantified using simple modification of the setup shown in
2.2 Single Channel Two-Liquid Capacitive Strain Sensor
2.2.1 Device Fabrication Procedure
The results presented in section 2.1 show that a two-liquid capacitor can have C ˜5-8 pF with a footprint of only 1-2 mm2 (i.e. ˜50-fold reduction compared to two channel capacitors). Motivated by these results, this section explores the viability of a single-channel two-liquid capacitive strain sensor. This device differs from the experimental setup used in section 2.1 in that the liquid metal electrodes and the liquid dielectric are encapsulated within a single channel as opposed to having two liquid metal filled tubes separated by a liquid dielectric bath. As shown in the schematic in
2.2.2 Device Performance
The described procedure could be used to repeatedly fabricate single channel two-liquid capacitors with glycerol and water as dielectric liquids. However, fabrication of a complete two-liquid device that was stable under “static” conditions did not guarantee its stability during stretching. For example, the image in
The capacitance of several glycerol-liquid metal capacitor channels was measured during stretching using the setup illustrated in
A capacitor was introduced consisting of a liquid dielectric material sandwiched in between two liquid metal electrodes within a single straight cylindrical channel. As discussed above, simulations and a simple setup consisting of two liquid metal filled tubes submerged in a dielectric liquid bath were used to quantify the effects of the electrode geometry including the diameter, separation distance, and meniscus shape as well as the dielectric constant of the insulator liquid on the system'"'"'s capacitance. It was demonstrated that by replacing silicone oil with glycerol and water a three- to five-fold increase in the system'"'"'s capacitance can be achieved. This increase is substantial but not as large as expected based on the ratio of the dielectric constants of the insulator liquids. Using simulations it was shown that this effect is due to the presence of low dielectric constant tubing (PVC and PDMS). Using simulations it was also demonstrated that electric fringe effects outside the separation gap and along the cylinder are responsible for the capacitance scaling with the radius of the electrodes and not their end areas. It was found that for all geometries the measured capacitance cannot be predicted by classical analytical models for parallel plate or two-sphere capacitors and that full system numerical simulation is required to adequately capture the electrical field distribution. With the optimal geometry composed of hemispherical menisci and minimal separation distance, it was found that glycerol and water systems with a capacitance of ˜5 pF to ˜8 pF and a footprint of only ˜1-2 mm2 are feasible.
In the second part of this paper, we explored the feasibility of using a two-liquid capacitor within a single PDMS channel for hyperelastic strain sensing. Residual GaInSn adhesion to the channel walls was prevented by lubricating the PDMS channel with water and glycerol prior to liquid metal injection. This enabled fabrication of single-channel liquid metal capacitors separated by glycerol and water. Unfortunately, oxide regrowth at the GaInSn-PDMS interface in the presence of water rendered strain sensors with water as the dielectric material impractical. In particular, when stretching, the liquid metal electrodes did not deform gradually but suddenly “snapped” leaving behind residual GaInSn on the PDMS walls. This behavior was not observed in glycerol devices, indicating persistent PDMS wall lubrication by this dielectric liquid. For glycerol devices fit within a 1.6 mm diameter channel, the minimal meniscus tip-to-tip distances achieved with and without compressing of the device'"'"'s outer edges were ˜0.5 mm and ˜2.3 mm, corresponding to capacitance values of ˜2 pF and ˜1.1 pF with footprints of ˜0.8 mm2 and ˜3 mm2, respectively. Thus, it was demonstrated that a single PDMS channel two-liquid capacitor can have about a ˜25-fold higher capacitance per sensor'"'"'s base area as compared to the current winding two-channel capacitors (2 pF/0.8 mm2 vs. ˜10 pF/100 mm2). However, further experiments revealed that the liquid metal electrode geometry was altered by stretching. In particular, pressure induced by the stretching caused the outflow of glycerol from the region separating the electrodes to the annual region between the electrode and the PDMS channel wall. As a result, necking and breaking of the liquid metal electrodes were observed. Thus, while enabling fabrication and facile movement of the liquid metal electrodes, the presence of the “lubricating” glycerol also causes failure of the two-liquid capacitor. This work illustrates that single channel two-liquid capacitors could provide a ˜25 times more compact alternative to the current capacitive liquid metal strain sensors. Alternative designs may include modification of the channel wall surface to reduce GaInSn adhesion, an alternative channel filling procedure or a hybrid approach between the single channel design and multichannel design. Another option may be to include an “overflow” region for the glycerol squeezed out of the center region.
GaInSn with a composition of 68.5% Ga, 21.5% In, and 10% Sn was purchased from Rotometals. Water was purified to a resistivity of 18 MΩ cm using a Thermo Scientific™ Barnstead™ NanoPure™ system. Glycerol (≥99.5%, Sigma-Aldrich G9012) and silicone oil (viscosity, 100 cSt, Sigma-Aldrich, 378364) were used as the other two dielectric liquids. Elastomer substrates for capacitive strain sensors were fabricated by mixing elastomer base weighing 10 parts (around 15 g) and curing agent weighing 1 part (around 1.5 g) (Dow Corning, Sylguard 182, silicone elastomer kit).
Two Liquid Metal Filled Tubes within a Dielectric Bath Setup
The ABS system housing was 3D printed using Makerbot Replicator 2× and filled with dielectric liquids. The housing had two ports for passing liquid metal channels made of 1 mm or 2 mm internal diameter (ID) Masterkleer PVC tubing (Mcmaster-Carr), which was connected to 1 mL plastic syringes (Mcmaster-Carr) with corresponding 0.08 cm or 0.2 mm ID and 1.26 cm length blunt tip dispensing needles (Mcmaster-Carr). The spacing between the ends of the liquid metal tubes was adjusted using two micro-positioning stages (Deltron, 1201-XYZ) attached to the 3D printed syringe holders using adhesive tape. For all of the tests, a 889B Bench LCR/ESR Meter was used to measure capacitance and dissipation factors at a frequency of 200 kHz and a voltage of 1 V in parallel mode. A home-made faraday cage was used to shield the devices from electromagnetic interference during measurements. To make electrical contacts, copper wires were soldered onto the conductive syringe needles. The stray capacitance of the system was quantified with just air at different levels of relative humidity as well as long electrode separation distances. The results were that the stray capacitance was negligible. To study the effect of the liquid metal meniscus shape on device capacitance, the bottom of the 3D printed housing was replaced by a pre-cleaned glass slide (Thermo Scientific). This modification enabled detailed visualization of the meniscus shape using transmitted light in a Zeiss Axio-Zoom V 16 microscope fitted with a Z 1.5×/0.37 FWD 30 mm lens. Menisci with two different spherical-cap shapes characterized by the metal-tube contact angles of ˜60° and ˜90° were fabricated by manually adjusting the syringes. The capacitance was measured off-site within the faraday cage after the geometry of the device was adjusted under the microscope.
Single Channel Two-Liquid Capacitor Fabrication Procedure
The channel was fabricated within a single step by casting the elastomer solution over a 3D printing rectangular casing pierced by a 1.6 mm diameter and 7.6 cm length stainless steel shaft (McMaster-Carr). The PDMS solution was mixed in Petri dishes and poured into the mold. Before curing the PDMS mixture was degassed for around 30 minutes until all air bubbles escaped. Subsequently, the casted elastomer was heated for 1.5 h at 85° C. on a hot plate. After curing, the cast was removed from the mold, with the steel shaft simply pulled out to create the two-open-ended channel with circular cross-section. To fabricate the three-sandwiched liquid layout while preventing the liquid metal from adhering to the wall of the channel during stretching, the dielectric liquids were injected first in order to lubricate the channel. Next, one side of the channel was blocked with a syringe and GaInSn injected by piercing a hole through the top of the PDMS into the channel using a 1 mL syringe with 1.26 cm long 0.03 cm ID blunt tip dispensing needles (McMaster-Carr, 75165A686). As a result the insulating liquid was ejected from the free side of the channel. To prevent leakage during testing, the vertical hole was sealed with a few drops of the uncured elastomer to the hole on the top and let it cure naturally without additional heating (to prevent hardening of the PDMS). After the first hole was sealed, another point located at a specified distance away from the first one was made and liquid metal was injected again to push out majority of the dielectric liquid from the channels. The result was a controlled amount of dielectric liquid separating the GaInSn. To finalize the device fabrication, wires were inserted into the ends of the channels.
PDMS Sensor Stretching Experiments
The ends of the PDMS devices were mounted onto two micro-positioning stages (Deltron, 1201-XYZ) by sandwiching them between flat acrylic “clamps”. For experiments involving the devices with “compressed” non-facing ends, the channel ends were fitted with a thin copper wire and sealed using silicone prior to mounting on the stage (thus clamping compressed the ends of the device). For testing of the non-squeezed devices, the channel ends were sealed after clamping onto the stage. During each stretching step the device was photographed from a top-down view using a Nikon 3200 camera. The capacitance of the system without the PDMS channel being filled with liquid metal electrodes (so wires, stages etc) in air below 50% relative humidity was below 0.15 pF for PDMS with a length of ˜2.7 cm.