Name: a method to manipulate surface tension of a liquid metal via surface oxidation and reduction
Uploaded: 2016-01-26T19:00:00
Description: Watch this Scientific Journal Video about A Method to Manipulate Surface Tension of a Liquid Metal via Surface Oxidation and Reduction at JoVE.com

The authors acknowledge support from Samsung, the NC State Chancellors Innovation Funds, NSF (CAREER CMMI-0954321 and Triangle MRSEC DMR-1121107), and Air Force Research Labs.

1. Manipulation der Grenzflächenspannung des flüssigen Metalls in Electrolyte <ol><li> Oxidation <ol><li> Gießen einer wässrigen Elektrolyten (sauer oder basisch) in eine Petrischale. Um sicherzustellen, daß das Oxid vollständig entfernt wird, verwenden eine Säure oder Base mit einer Konzentration von mehr als 0,1 M 24 (beispielsweise 1 M NaOH oder 1 M HCl). Verwenden Sie ein Volumen, das die Schale bis zu einer Tiefe von ca. 1-3 mm füllen wird. Vermeiden Sie Kontakt mit der Haut mit diesen...

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<html> Dieses Verfahren steuert die Oberflächenspannung von Gallium basierenden flüssigen Metallen mit kleinen Spannungen, um die Abscheidung und Entfernung einer Oberflächenoxidschicht zu fahren. Obwohl das Verfahren funktioniert nur in Elektrolytlösungen, es ist einfach und arbeitet in einer Vielzahl von verschiedenen Bedingungen, aber es gibt Feinheiten erwähnenswert. In Abwesenheit von elektrischem Potential, sowohl saure als auch basische Lösungen Wegätzen des Oxids 27. Die Anwendung eines Oxidatio...

This method provides a simple way to control the surface tension of liquid metals containing gallium. The method uses modest voltages (~1 V) applied directly to the liquid metal (relative to a counter electrode in the presence of electrolyte) to achieve enormous and reversible changes to the surface tension of the metal1. 
Surface tension is a dominant force for liquids at small length scales and is important for a number of capillary phenomena including wetting, spreading, and surface-tension driven flow. Consequently, the ability to control surface tension is a sensible way to manipulate the shape, position, and flow of liquids at sub-mm length scales. The most common way to alter surface tension between two fluids is to use a surfactant, which is a molecule that spans the interface between the fluids. Surfactants lower surface tension, but in a way that is not easy to reverse since it is difficult to remove surfactants from the interface. Surface tension can also be altered using a variety of techniques, including temperature gradients2,3, light4, surface chemistry5-7,and voltage8. But most of these methods result in modest changes to surface tension, particularly for liquid metals, which have notably large surface tensions.
The ability to control the surface tension of liquid metal could enable new opportunities for creating shape reconfigurable structures with metallic properties for electronic, thermal, and optical applications9-14. The most common liquid metal is Hg, which is noted for its toxicity. The methods described here are relevant for liquid metals based on gallium. These metals have low viscosity, large surface tension, low volatility (low vapor pressure), and low toxicity15. Importantly, these metals form surface oxides composed of gallium oxide that are a few nm thick in air16. This oxide layer creates a physical skin that historically has been a nuisance for electrochemical and fluid dynamic applications17. The method here utilizes the oxide in new ways to control surface tension.
The most common way to manipulate liquid metals in electrolyte is to apply a potential to the metal relative to a counter electrode18. Oppositely charged ions from the electrolyte match the charges on the metal, causing the interfacial tension to drop. This phenomenon-termed electrocapillarity-has been known since the 1870s as described by Lippman19and has been utilized for alloys of gallium20. Typically, electrocapillarity achieves modest changes to surface tension, since undesirable electrochemical reactions limit the range of voltages applied to the metal. In contrast, the method described here utilizes the surface oxidation of the metal (or conversely, the reduction of the surface oxide) as a way to achieve enormous changes in surface tension above and beyond changes resulting from electrocapillarity. The leading explanation for this phenomenon is that the oxide is asymmetric; that is, the outer surface of the oxide terminates with hydroxyl groups (making a low interfacial tension interface with the aqueous electrolyte), and the interior surface of the oxide terminates with gallium atoms (making a low interfacial tension interface with the metal). In contrast, the removal of the oxide via electrochemical reduction results in a bare metal-electrolyte interface, which returns the metal back to a state of high surface tension. We characterize the interfacial tension of the metal by analyzing the shape of sessile droplets as a function of voltage while assuming that gravity and surface tension are the dominant forces that define the curvature of its surface. 
The advantage of this technique relative to classic electrocapillarity is that it can reversibly tune the tension of low toxicity liquid metals over enormous ranges (from ~500 mN/m to near zero). This delta change in surface tension may be the largest ever reported in literature for any fluid and it can be accomplished in a tunable and reversible manner. These large changes in surface tension are useful for manipulating the capillary behavior of metals; for example, it can induce the metal to spread on a surface, withdraw the metal from microchannels, fill microchannels with metal, and overcome the Rayleigh instabilities to form liquid metal fibers1,21. 
A drawback of this technique is that it requires electrolyte. It works best in acidic or basic conditions, because these electrolytes remove excess surface oxide that would otherwise contaminate the surface of the metal and mechanically restrict the movement of the metal. The simultaneous removal and deposition of the oxide layer complicates the analysis of the interfacial phenomena and it is our hope the methods described in this paper empowers additional analysis. Another disadvantage is that the electrochemical reactions at the surface of the metal must be matched by complimentary half-reactions at the counter electrode22,23. This can lead to hydrogen bubbles forming at the counter electrode.

<table>
	<tbody>
		<tr>
			<td>Eutectic Gallium Indium</td>
			<td>Indium Corporation</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Sodium Hydroxide</td>
			<td>Fisher Scientific</td>
			<td>2318-3</td>
			<td></td>
		</tr>
		<tr>
			<td>Hydrochloric Acid</td>
			<td>Fisher Scientific</td>
			<td>A481-212</td>
			<td></td>
		</tr>
		<tr>
			<td>Sodium Fluoride</td>
			<td>Sigma-Aldrich</td>
			<td>201154</td>
			<td></td>
		</tr>
		<tr>
			<td>Optical Adhesive</td>
			<td>Norland</td>
			<td>NOA81</td>
			<td></td>
		</tr>
		<tr>
			<td>Polydimethylsiloxane (Sylgard-184)</td>
			<td>Dow Corning</td>
			<td></td>
			<td>Silicone Elastomer Kit</td>
		</tr>
		<tr>
			<td>Borosilicate Glass Capillaries</td>
			<td>Friedrich and Dimmoch</td>
			<td>B41972</td>
			<td></td>
		</tr>
		<tr>
			<td>Ag/AgCl Reference Electrode</td>
			<td>Microelectrodes Inc.</td>
			<td>MI-401F</td>
			<td></td>
		</tr>
		<tr>
			<td>Voltage Source</td>
			<td>Keithley</td>
			<td>3390</td>
			<td></td>
		</tr>
		<tr>
			<td>Potentiostat</td>
			<td>Gamry</td>
			<td>Ref 600</td>
			<td></td>
		</tr>
		<tr>
			<td>Laser Cutter</td>
			<td>Universal Laser Systems</td>
			<td>VLS 3.50</td>
			<td></td>
		</tr>
	</tbody>
</table>

a method to manipulate surface tension of a liquid metal via surface oxidation and reduction

Steuern der Grenzflächenspannung ist eine wirksame Methode zur Manipulation der Form, Position und den Fluss von Fluiden bei Submillimeterbereich Längenskalen, wobei die Grenzflächenspannung ist eine dominierende Kraft. Eine Vielzahl von Verfahren existieren zur Steuerung der Grenzflächenspannung von wässrigen und organischen Flüssigkeiten auf dieser Skala; Jedoch wurden diese Techniken Dienstprogramm für flüssige Metalle aufgrund ihrer großen Grenzflächenspannung begrenzt. Flüssige Metalle weich, elastisch und form rekonfigurierbare Komponenten in elektronischen und elektromagnetischer Geräte bilden. Obwohl es möglich ist, um diese Flüssigkeiten durch mechanische Verfahren (zB Pumpen) zu manipulieren, sind elektrische Verfahren einfacher zu miniaturisieren, zu steuern und zu realisieren. Allerdings haben die meisten elektrischen Techniken ihre eigenen Einschränkungen: Electrowetting-on-dielektrischen groß (kV) Potentiale für bescheidene Betätigung kann Elektrokapillarität relativ kleine Änderungen in der Oberflächenspannung beeinflussen und erfordert kontinuierliche electrowetting an Steckern des flüssigen Metalls in Kapillaren beschränkt. Hier stellen wir ein Verfahren zum Betätigen Gallium und Gallium basierenden flüssigen Metalllegierungen mittels einer elektrochemischen Oberflächenreaktion. Steuerung des elektrochemischen Potentials an der Oberfläche des flüssigen Metalls in Elektrolyten schnell und reversibel ändert die Grenzflächenspannung um mehr als zwei Grßenordnungen (̴500 mN / m, um in der Nähe von Null). Weiterhin erfordert dieses Verfahren nur eine sehr bescheidene Potential (&lt;1 V) relativ zu einer Gegenelektrode angelegt. Die resultierende Änderung in der Spannung ist im Wesentlichen auf die elektrochemische Abscheidung einer Oxidschicht auf der Oberfläche, die als ein oberflächenaktives Mittel wirkt; Entfernung des Oxids steigt die Grenzflächenspannung, und umgekehrt. Diese Technik kann in einer Vielzahl von Elektrolyten angewendet werden und ist unabhängig von dem Substrat, auf dem sie ruht.

Figur 1 A zeigt ein Beispiel eines einfachen Zwei-Elektroden-Verfahren für die Oxidation und Reduktion. In diesem Fall wird ein 70 ul Tropfen des flüssigen Metalls in einer 1 M NaOH-Lösung in Kontakt gebracht einen Kupferdraht eine elektrische Verbindung zu etablieren. Das 1 M NaOH beseitigt die Oberflächenoxidschicht von dem Metall und ermöglicht es dem Metall zu perlen aufgrund ihrer Grenzflächenspannung. Aufbringen einer 2,...

Watch this Scientific Journal Video about A Method to Manipulate Surface Tension of a Liquid Metal via Surface Oxidation and Reduction at JoVE.com

A Method to Manipulate Surface Tension of a Liquid Metal via Surface Oxidation and Reduction

Eine Methode, die Oberflächenspannung eines Liquid Metal manipulieren via Oberfläche Oxidation und Reduktion

We present a method to control the interfacial energy of a liquid metal in an electrolyte via electrochemical deposition (or removal) of a surface oxide layer. This simple method can control the capillary behavior of gallium-based liquid metals by tuning the interfacial energy rapidly, significantly, and reversibly using modest voltages.

Controlling interfacial tension is an effective method for manipulating the shape, position, and flow of fluids at sub-millimeter length scales, where ...

The overall goal of this procedure is to demonstrate the manipulation of liquid metal under the influence of electrochemical potentials. This method tunes the interfacial tension of liquid metal over an enormous range while elucidating the role of oxides on interfacial tension. The main appeal of this technique is that it's simple, it's reversible, and it only requires very modest potentials.This thing has been utilized to make electronic devices that are reconfigurable and composed almost entirely of To perform oxidation, pour an aqueous electrolyte into a petri dish. Use a volume that will fill the dish to a depth of approximately one to three millimeters. Use a syringe to place a drop of a gallium based alloy in the electrolyte.Here, eutectic gallium indium is used. Place a copper wire with a diameter less than that of the drop into the liquid metal to establish the working electrode. In acid or base, the liquid metal will wet the copper and thereby form an excellent electrical contact.Place a conducting counter electrode in the solution, but not in contact with the liquid metal. If the counter electrode has a resistance of less than one ohm, its dimensions are irrelevant. Next, connect the wires to a voltage source and apply a positive potential to the liquid metal.For small shape deformation, apply positive voltages less than one volt. For larger shape deformation, apply greater than one volt. To perform reduction, dispense a drop of the liquid metal from a syringe into an empty petri dish.Then, poor a neutral aqueous electrolyte into the petri dish to a level that submerges the metal. Place a copper wire into the liquid metal to act as a working electrode, and place a conducting wire into the electrolyte to act as the counter electrode. Connect the wires to a voltage source and apply a negative potential to the liquid metal.Apply approximately minus one volt to remove the surface oxide and cause the metal to de-wet from the substrate. The metal should de-wet first on the side closest to the counter electrode. Apply more negative potentials to remove the oxide layer completely.Avoid applying excessively large negative voltages to prevent hydrogen bubbles from appearing on the liquid metal due to reduction of the electrolyte. Using a laser cutter, or milling tool, cut a direct path from the center to the edge of a piece of polymethylmethacrylate, or PMMA. Do not cut the path all the way through the thickness of the PMMA.This piece will serve as a substrate for the liquid metal. With the same tool, cut a one square millimeter hole through the center of the PMMA. Using the path as a guide, run an insulated copper wire with only the tip exposed, to the center of the PMMA.Position the wire so that it is protruding over the PMMA surface. Seal the wire in place with a leak proof adhesive. Cut the wire just above the surface of the PMMA, but do not let it extend too far or it will disturb the shape of the drop.Tape the PMMA piece down into a transparent container through which a clear image can be obtained. Place the container in a contact angle goniometer so that the surface profile of the drop is clearly visible. Fill the container with one molar sodium hydroxide.Then, place a 25 to 50 microliter drop of liquid metal on the protruding copper wire. This wire will serve as the working electrode and will wet the droplet. Connect all of the electrodes to a potentiostat.Next, place a platinum mesh counter electrode, and a saturated silver silver chloride reference electrode in the solution. Use the potentiostat to control the voltage with respect to the reference electrode, and use the goniometer to measure the shape and thereby the interfacial tension of the drop. Ensure that the goniometer is capable of measuring sessile drop interfacial tension.Fill a glass capillary with a solution of one molar sodium hydroxide. The capillary diameter should be approximately one millimeter. Place one end of the capillary flush against a drop of liquid metal.Align the capillary so that it is parallel with the surface of the table. Avoid air gaps between the liquid metal drop and the electrolyte filled capillary. Using a wipe, dab off any excess electrolyte that may have leaked during assembly.Place a copper wire in the liquid metal and a conductive counter electrode in the open end of the capillary so that it contacts the solution. Connect the wires to a voltage source and apply a positive potential to the liquid metal. The liquid metal should begin filling the capillary.Utilize soft lithographic and replica molding techniques to fabricate microfluidic channels composed of polydimethylsiloxane, or PDMS. Fabricate channels that are approximately 100 to 1, 000 microns wide, 100 microns tall, and 25 to 65 millimeters long. Inject liquid metal either manually, or using a syringe pump, to fill the channel completely.Using a cotton swab that has been dipped in one molar sodium hydroxide or one molar hydrogen chloride, remove excess amounts of liquid metal from the inlet of the channel so that the metal remains flush with the top surface of the PDMS. Submerge one end of the channel in electrolyte, and place the anode such that it touches the electrolyte but not the metal. At the other end of the channel, contact a separate electrode to the metal surface so that liquid metal itself acts as a cathode.Connect these wires to a voltage source, and complete the electric circuit. For a three-electrode system, place the reference electrode such that it barely submerges into the drop of electrolyte. Before applying a reducing voltage, mount a video camera on a tripod or in a microscope to record the experiments.Use the auto focus mode to get everything in focus. Utilize the manual focus to have better control over depth of field, white balance, and ISO. As necessary, use a higher f stop, a shutter speed of one one hundredth of a second, auto white balance, and auto ISO.Start recording the experiment. Apply approximately minus one volt to withdraw the liquid metal from the micro channels. Turn the voltage off to cause the metal to stop moving in neutral electrolyte.Shown here is a goniometer video of the liquid metal spreading under the influence of the electrochemical potential. A decrease in height signifies a more oxidative potential. The interfacial tension can be determined from the curvature of the droplet.The region to the left of the dotted line where the oxide layer is absent shows typical electro-capillary behavior. However, when the oxide layer is present, the interfacial tension shows a precipitous decline, further decreasing as larger voltages are applied. Applying a reducing potential to the metal removes the oxide and causes it to withdraw from microchannels.This graph shows the velocity profile of the capillary withdrawal at a constant voltage. As the liquid metal is replaced with electrolyte, the electrical resistance in the microchannel increases causing a drop in the velocity over time. Once mastered, this technique can be done in a matter of minutes and should allow scientists and engineers to further study this unique phenomenon.

Research

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Chemistry

Liquids, Solids, and Intermolecular Forces

SCIENTISTS IN ACTION

Exploring the Radical Nature of a Carbon Surface by Electron Paramagnetic Resonance and a Calibrated Gas Flow

Exploring the Radical Nature einer Kohlenstoffoberfläche durch Elektronenspinresonanz und einem kalibrierten Gasdurchfluss

While the first Electron Paramagnetic Resonance (EPR) studies regarding the effects of oxidation on the structure and stability of carbon radicals date ...

Forschung

Chemie

Transport of Surface-modified Carbon Nanotubes through a Soil Column

Transport von oberflächenmodifizierte Kohlenstoff-Nanoröhren durch eine Bodensäule

Carbon nanotubes (CNTs) are widely manufactured nanoparticles, which are being utilized in a number of consumer products, such as sporting goods, ...

Synthesis and Exfoliation of Discotic Zirconium Phosphates to Obtain Colloidal Liquid Crystals

Synthese und Exfoliation von diskotischen Zirkoniumphosphaten Erhalten Kolloidales Liquid Crystals

Due to their abundance in natural clay and potential applications in advanced materials, discotic nanoparticles are of interest to scientists and ...

Failure of Cleaning Verification in Pharmaceutical Industry Due to Uncleanliness of Stainless Steel Surface

Mangelnde Reinigung Überprüfung in der pharmazeutischen Industrie wegen Unreinheit des Edelstahl-Oberfläche

The aim of this work is to identify the parameters that affect the recovery of pharmaceutical residues from the surface of stainless steel coupons. A ...

Synthesis of Substrate-Bound Au Nanowires Via an Active Surface Growth Mechanism

Synthese von Substrat-gebundenen Au Nanodrähten über eine aktive Oberfläche Wachstumsmechanismus

Advancing synthetic capabilities is important for the development of nanoscience and nanotechnology. The synthesis of nanowires has always been a ...

A Rapid Synthesis Method for Au, Pd, and Pt Aerogels Via Direct Solution-Based Reduction

Eine schnelle Synthese-Methode für Au, Pd und Pt-Aerogelen über lösungsorientierte Direktreduktion

Here, a method to synthesize gold, palladium, and platinum aerogels via a rapid, direct solution-based reduction is presented. The combination of various ...

Surface Functionalization of Metal-Organic Frameworks for Improved Moisture Resistance

Oberfläche Funktionalisierung der Metall-organischen Frameworks für verbesserte Feuchtigkeitsbeständigkeit

Metal-organic frameworks (MOFs) are a class of porous inorganic materials with promising properties in gas storage and separation, catalysis and sensing. ...

Surface Properties of Synthesized Nanoporous Carbon and Silica Matrices

Oberflächeneigenschaften von synthetisierten nanoporöse Carbon und Silica-Matrizen

In this work, we report the synthesis and characterization of ordered nanoporous carbon material (also called ordered mesoporous carbon material [OMC]) ...

Bio-inspired Polydopamine Surface Modification of Nanodiamonds and Its Reduction of Silver Nanoparticles

Bio-inspirierte Polydopamine Oberflächenmodifizierung von Nano-Diamanten und der Reduktion von Silber-Nanopartikeln

Surface functionalization of nanodiamonds (NDs) is still challenging due to the diversity of functional groups on the ND surfaces. Here, we demonstrate a ...

U2O5 Film Preparation via UO2 Deposition by Direct Current Sputtering and Successive Oxidation and Reduction with Atomic Oxygen and Atomic Hydrogen

U2O5 Film Preparation via UO2 Deposition by Direct Current Sputtering and Successive Oxidation and Reduction with Atomic Oxygen and Atomic Hydrogen

U2O5 Film Vorbereitung über UO2 Ablagerung von Gleichstrom-Sputtern und aufeinanderfolgenden Oxidation und Reduktion mit atomarem Sauerstoff und atomarer Wasserstoff

We describe a method to produce U2O5 films in situ using the Labstation, a modular machine developed at JRC Karlsruhe. The Labstation, an essential part ...