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 of the Interfacial Tension of Liquid Metal in Electrolyte
<ol>
	<li>Oxidation
	<ol>
		<li>Pour an aqueous electrolyte (acidic or basic) into a Petri dish. To ensure that the oxide is completely removed, use an acid or base with a concentration greater than 0.1 M24 (e.g. 1 M NaOH or 1 M HCl). Use a volume that will fill the dish to a depth of approximately 1-3 mm. Avoid contacting the skin with these solutions.</li>
		<li>Use a syringe to place a drop (optimally between 10-500 &#181;l...

<ol>
	<li>Khan, M. R., Eaker, C. B., Bowden, E. F., Dickey, M. D. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Giant+and+switchable+surface+activity+of+liquid+metal+via+surface+oxidation.">Giant and switchable surface activity of liquid metal via surface oxidation.</a> Proc. Natl. Acad. Sci. 111 (39), 14047-14051 (2014).</li><li>Kataoka, D. E., Troian, S. M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Patterning+liquid+flow+on+the+microscopic+scale.">Patterning liquid flow on the microscopic scale.</a> Nature. 402 (6763), 794-797 (1999).</li><li>Daniel, S., Chaudhury, M. K., Chen, J. C. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Fast+Drop+Movements+Resulting+from+the+Phase+Change+on+a+Gradient+Surface.">Fast Drop Movements Resulting from the Phase Change on a Gradient Surface.</a> Science. 291 (5504), 633-636 (2001).</li><li>Ichimura, K., Oh, S. K., Nakagawa, M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Light-driven+motion+of+liquids+on+a+photoresponsive+surface.">Light-driven motion of liquids on a photoresponsive surface.</a> Science. 288 (5471), 1624-1626 (2000).</li><li>Gallardo, B. S., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Electrochemical+principles+for+active+control+of+liquids+on+submillimeter+scales.">Electrochemical principles for active control of liquids on submillimeter scales.</a> Science. 283 (5398), 57-60 (1999).</li><li>Zhao, B., Moore, J. S., Beebe, D. J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Surface-Directed+Liquid+Flow+Inside+Microchannels.">Surface-Directed Liquid Flow Inside Microchannels.</a> Science. 291 (5506), 1023-1026 (2001).</li><li>Chaudhury, M. K., Whitesides, G. M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=How+to+Make+Water+Run+Uphill.">How to Make Water Run Uphill.</a> Science. 256 (5063), 1539-1541 (1992).</li><li>Lahann, J., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=A+reversibly+switching+surface.">A reversibly switching surface.</a> Science. 299 (5605), 371-374 (2003).</li><li>Rogers, J. A., Someya, T., Huang, Y. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Materials+and+Mechanics+for+Stretchable+Electronics.">Materials and Mechanics for Stretchable Electronics.</a> Science. 327 (5973), 1603-1607 (2010).</li><li>Bauer, S., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=25th+Anniversary+Article:+A+Soft+Future:+From+Robots+and+Sensor+Skin+to+Energy+Harvesters.">25th Anniversary Article: A Soft Future: From Robots and Sensor Skin to Energy Harvesters.</a> Adv. Mater. 26 (1), 149-162 (2013).</li><li>Ozbay, E. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Plasmonics:+Merging+Photonics+and+Electronics+at+Nanoscale+Dimensions.">Plasmonics: Merging Photonics and Electronics at Nanoscale Dimensions.</a> Science. 311 (5758), 189-193 (2006).</li><li>Monat, C., Domachuk, P., Eggleton, B. J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Integrated+optofluidics:+A+new+river+of+light.">Integrated optofluidics: A new river of light.</a> Nat. Photonics. 1 (2), 106-114 (2007).</li><li>Schurig, D., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Metamaterial+Electromagnetic+Cloak+at+Microwave+Frequencies.">Metamaterial Electromagnetic Cloak at Microwave Frequencies.</a> Science. 314 (5801), 977-980 (2006).</li><li>Dickey, M. D. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Emerging+Applications+of+Liquid+Metals+Featuring+Surface+Oxides.">Emerging Applications of Liquid Metals Featuring Surface Oxides.</a> ACS Appl. Mater. Interfaces. 6 (21), 18369-18379 (2014).</li><li>Dickey, M. D., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Eutectic+gallium-indium+(EGaIn):+A+liquid+metal+alloy+for+the+formation+of+stable+structures+in+microchannels+at+room+temperature.">Eutectic gallium-indium (EGaIn): A liquid metal alloy for the formation of stable structures in microchannels at room temperature.</a> Adv. Funct. Mater. 18 (7), 1097-1104 (2008).</li><li>Regan, M. J., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=X-ray+study+of+the+oxidation+of+liquid-gallium+surfaces.">X-ray study of the oxidation of liquid-gallium surfaces.</a> Phys. Rev. B. 55 (16), 10786-10790 (1997).</li><li>Gigu&#232;re, P. A., Lamontagne, D. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Polarography+with+a+Dropping+Gallium+Electrode.">Polarography with a Dropping Gallium Electrode.</a> Science. 120 (3114), 390-391 (1954).</li><li>Frumkin, A., Polianovskaya, N., Grigoryev, N., Bagotskaya, I. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Electrocapillary+phenomena+on+gallium.">Electrocapillary phenomena on gallium.</a> Electrochim. Acta. 10 (8), 793-802 (1965).</li><li>Lippmann, G. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=.">.</a> Relations entre les ph&#233;nom&#232;nes &#233;lectriques et capillaires. , (1875).</li><li>Tsai, J. T. H., Ho, C. M., Wang, F. C., Liang, C. T. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Ultrahigh+contrast+light+valve+driven+by+electrocapillarity+of+liquid+gallium.">Ultrahigh contrast light valve driven by electrocapillarity of liquid gallium.</a> Appl. Phys. Lett. 95 (25), 251110 (2009).</li><li>Khan, M. R., Trlica, C., Dickey, M. D. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Recapillarity:+Electrochemically+Controlled+Capillary+Withdrawal+of+a+Liquid+Metal+Alloy+from+Microchannels.">Recapillarity: Electrochemically Controlled Capillary Withdrawal of a Liquid Metal Alloy from Microchannels.</a> Adv. Funct. Mater. 25 (5), 671-678 (2015).</li><li>Saltman, W., Nachtrieb, N. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=The+Electrochemistry+of+Gallium.">The Electrochemistry of Gallium.</a> J. Electrochem. Soc. 100, 126-130 (1953).</li><li>Perkins, R. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Anodic-Oxidation+of+Gallium+in+Alkaline-Solution.">Anodic-Oxidation of Gallium in Alkaline-Solution.</a> J. Electroanal. Chem. 101, 47-57 (1979).</li><li>Xu, Q., Oudalov, N., Guo, Q., Jaeger, H. M., Brown, E. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Effect+of+oxidation+on+the+mechanical+properties+of+liquid+gallium+and+eutectic+gallium-indium.">Effect of oxidation on the mechanical properties of liquid gallium and eutectic gallium-indium.</a> Phys. Fluids. 24, 063101 (2012).</li><li>Rotenberg, Y., Boruvka, L., Neumann, A. W. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Determination+of+surface+tension+and+contact+angle+from+the+shapes+of+axisymmetric+fluid+interfaces.">Determination of surface tension and contact angle from the shapes of axisymmetric fluid interfaces.</a> J. Colloid Interface Sci. 93, 169-183 (1983).</li><li>Xia, Y., Whitesides, G. M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Soft+Lithography.">Soft Lithography.</a> Annu. Rev. Mater. Sci. 28 (1), 153-184 (1998).</li><li>Pourbaix, M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=.">.</a> Atlas of Electrochemical Equilibria in Aqueous Solutions. , (1974).</li><li>Gough, R. C., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Rapid+electrocapillary+deformation+of+liquid+metal+with+reversible+shape+retention.">Rapid electrocapillary deformation of liquid metal with reversible shape retention.</a> Micro Nano Syst. Lett. 3 (1), 1-9 (2015).</li><li>Wang, M., Trlica, C., Khan, M. R., Dickey, M. D., Adams, J. J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=A+reconfigurable+liquid+metal+antenna+driven+by+electrochemically+controlled+capillarity.">A reconfigurable liquid metal antenna driven by electrochemically controlled capillarity.</a> J. Appl. Phys. 117 (19), 194901 (2015).</li></ol>

This method controls the surface tension of gallium-based liquid metals using small voltages to drive the deposition and removal of a surface oxide. Although the method only works in electrolyte solutions, it is simple, and works in a wide variety of different conditions, but there are subtleties worth noting. In the absence of electrical potential, both acidic and basic solutions etch away the oxide27. The application of an oxidative potential drives the formation of surface oxide in all aqueous electrolytes,...

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

Controlling interfacial tension is an effective method for manipulating the shape, position, and flow of fluids at sub-millimeter length scales, where interfacial tension is a dominant force. A variety of methods exist for controlling the interfacial tension of aqueous and organic liquids on this scale; however, these techniques have limited utility for liquid metals due to their large interfacial tension.
Liquid metals can form soft, stretchable, and shape-reconfigurable components in electronic and electromagnetic devices. Although it is possible to manipulate these fluids via mechanical methods (e.g., pumping), electrical methods are easier to miniaturize, control, and implement. However, most electrical techniques have their own constraints: electrowetting-on-dielectric requires large (kV) potentials for modest actuation, electrocapillarity can affect relatively small changes in the interfacial tension, and continuous electrowetting is limited to plugs of the liquid metal in capillaries.
Here, we present a method for actuating gallium and gallium-based liquid metal alloys via an electrochemical surface reaction. Controlling the electrochemical potential on the surface of the liquid metal in electrolyte rapidly and reversibly changes the interfacial tension by over two orders of magnitude ( &#820;500 mN/m to near zero). Furthermore, this method requires only a very modest potential (&#60; 1 V) applied relative to a counter electrode. The resulting change in tension is due primarily to the electrochemical deposition of a surface oxide layer, which acts as a surfactant; removal of the oxide increases the interfacial tension, and vice versa. This technique can be applied in a wide variety of electrolytes and is independent of the substrate on which it rests.

Figure 1A shows an example of the simple two-electrode technique for oxidation and reduction. In this instance, a 70 &#181;l drop of the liquid metal placed in a 1 M NaOH solution contacts a copper wire to establish an electrical connection. The 1 M NaOH removes the surface oxide from the metal and allows the metal to bead up due to its interfacial tension. Applying a 2.5 V potential between the drop and a platinum mesh counter-elect...

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

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.

a-method-to-manipulate-surface-tension-liquid-metal-via-surface

Research

Education

JoVE Journal

JoVE Core

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

While the first Electron Paramagnetic Resonance (EPR) studies regarding the effects of oxidation on the structure and stability of carbon radicals date back to the early 1980s the focus of these early papers primarily characterized the changes to the structures under extremely harsh conditions (pH or temperature)1-3. It is also known that paramagnetic molecular oxygen undergoes a Heisenberg spin exchange interaction with stable radicals that extremely broadens the EPR signal4-6. Recently, we reported interesting results where this interaction of molecular oxygen with a certain part of the existing stable radical structure can be reversibly affected simply by flowing a diamagnetic gas through the carbon samples at STP7. As flows of He, CO2, and N2 had a similar effect these interactions occur at the surface area of the macropore system.
This manuscript highlights the experimental techniques, work-up, and analysis towards affecting the existing stable radical nature in the carbon structures. It is hoped that it will help towards further development and understanding of these interactions in the community at large.

Stable radicals that are present in carbon substrates interact with paramagnetic oxygen through a Heisenberg spin exchange. This interaction can be significantly reduced under STP conditions by flowing a diamagnetic gas over the carbon system. This manuscript describes a simple method to characterize the nature of those radicals.

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

Transport of Surface-modified Carbon Nanotubes through a Soil Column

Carbon nanotubes (CNTs) are widely manufactured nanoparticles, which are being utilized in a number of consumer products, such as sporting goods, electronics and biomedical applications. Due to their accelerating production and use, CNTs constitute a potential environmental risk if they are released to soil and groundwater systems. It is therefore essential to improve the current understanding of environmental fate and transport of CNTs. The transport and retention of CNTs in both natural and artificial media have been reported in literature, but the findings widely vary and are thus not conclusive. There are a number of physical and chemical parameters responsible for variation in retention and transport. In this study, a complete procedure of selected multiwalled carbon nanotubes (MWCNTs) is presented starting from their surface modification to a complete set of laboratory column experiments at critical physical and chemical scenarios. Results indicate that the stability of the commercially available MWCNTs are critical with their attached surface functional group which can also influence the transport and retention of MWCNT through the surrounding medium.

Surface properties of a nanoparticle are important for their interaction with the surrounding medium. Therefore the surface modification of carbon nanotubes can be critical for their transport and retention through porous media. Here, lab scale column experiments are used to understand the possible transport and retention of these nanoparticles.

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

Due to their abundance in natural clay and potential applications in advanced materials, discotic nanoparticles are of interest to scientists and engineers. Growth of such anisotropic nanocrystals through a simple chemical method is a challenging task. In this study, we fabricate discotic nanodisks of zirconium phosphate [Zr(HPO4)2&#183;H2O] as a model material using hydrothermal, reflux and microwave-assisted methods. Growth of crystals is controlled by duration time, temperature, and concentration of reacting species. The novelty of the adopted methods is that discotic crystals of size ranging from hundred nanometers to few micrometers can be obtained while keeping the polydispersity well within control. The layered discotic crystals are converted to monolayers by exfoliation with tetra-(n)-butyl ammonium hydroxide [(C4H9)4NOH, TBAOH]. Exfoliated disks show isotropic and nematic liquid crystal phases. Size and polydispersity of disk suspensions is highly important in deciding their phase behavior.

A two dimensional model material of discotic zirconium phosphate was developed. The inorganic crystal with lamellar structure was synthesized by hydrothermal, reflux, and microwave-assisted methods. On exfoliation with organic molecules, layered crystals can be converted to monolayers, and nematic liquid crystal phase was formed at sufficient concentration of monolayers.

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

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 series of factors were assessed, including drug product spike levels, spiking procedure, drug-excipient ratios, analyst-to-analyst variability, intraday variability, and cleaning procedure of the coupons. The lack of a well-defined procedure that consistently cleaned the coupon surface was identified as the major contributor to low and variable recoveries. Assessment of cleaning the surface of the coupons with clean-in-place solutions (CIP) gave high recovery (&#62;90%) and reproducible results (Srel&#8804;4%) regardless of the conditions that were assessed previously. The approach was successfully applied for cleaning verification of small molecules (MW &#60;1,000 Da) as well as large biomolecules (MW up to 50,000 Da).

The lack of a well-defined procedure that consistently cleaned coupon surfaces was identified as the major contributor to low and variable recoveries in cleaning verification. This manuscript describes the correct protocol of cleaning stainless steel coupons.

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

Advancing synthetic capabilities is important for the development of nanoscience and nanotechnology. The synthesis of nanowires has always been a challenge, as it requires asymmetric growth of symmetric crystals. Here, we report a distinctive synthesis of substrate-bound Au nanowires. This template-free synthesis employs thiolated ligands and substrate adsorption to achieve the continuous asymmetric deposition of Au in solution at ambient conditions. The thiolated ligand prevented the Au deposition on the exposed surface of the seeds, so the Au deposition only occurs at the interface between the Au seeds and the substrate. The side of the newly deposited Au nanowires is immediately covered with the thiolated ligand, while the bottom facing the substrate remains ligand-free and active for the next round of Au deposition. We further demonstrate that this Au nanowire growth can be induced on various substrates, and different thiolated ligands can be used to regulate the surface chemistry of the nanowires. The diameter of the nanowires can also be controlled with mixed ligands, in which another &#34;bad&#34; ligand could turn on the lateral growth. With the understanding of the mechanism, Au nanowire-based nanostructures can be designed and synthesized.

We report a solution-based method to synthesize substrate-bound Au nanowires. By tuning the molecular ligands used during the synthesis, the Au nanowires can be grown from various substrates with different surface properties. Au nanowire-based nanostructures can also be synthesized by adjusting the reaction parameters.

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

Here, a method to synthesize gold, palladium, and platinum aerogels via a rapid, direct solution-based reduction is presented. The combination of various precursor noble metal ions with reducing agents in a 1:1 (v/v) ratio results in the formation of metal gels within seconds to minutes compared to much longer synthesis times for other techniques such as sol-gel. Conducting the reduction step in a microcentrifuge tube or small volume conical tube facilitates a proposed nucleation, growth, densification, fusion, equilibration model for gel formation, with final gel geometry smaller than the initial reaction volume. This method takes advantage of the vigorous hydrogen gas evolution as a by-product of the reduction step, and as a consequence of reagent concentrations. The solvent accessible specific surface area is determined with both electrochemical impedance spectroscopy and cyclic voltammetry. After rinsing and freeze drying, the resulting aerogel structure is examined with scanning electron microscopy, X-ray diffractometry, and nitrogen gas adsorption. The synthesis method and characterization techniques result in a close correspondence of aerogel ligament sizes. This synthesis method for noble metal aerogels demonstrates that high specific surface area monoliths may be achieved with a rapid and direct reduction approach.

A rapid, direct solution-based reduction synthesis method to obtain Au, Pd, and Pt aerogels is presented.

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

Metal-organic frameworks (MOFs) are a class of porous inorganic materials with promising properties in gas storage and separation, catalysis and sensing. However, the main issue limiting their applicability is their poor stability in humid conditions. The common methods to overcome this problem involve the formation of strong metal-linker bonds by using highly charged metals, which is limited to a number of structures, the introduction of alkylic groups to the framework by post-synthetic modification (PSM) or chemical vapour deposition (CVD) to enhance overall hydrophobicity of the framework. These last two usually provoke a drastic reduction of the porosity of the material. These strategies do not permit to exploit the properties of the MOF already available and it is imperative to find new methods to enhance the stability of MOFs in water while keeping their properties intact. Herein, we report a novel method to enhance the water stability of MOF crystals featuring Cu2(O2C)4&#160;paddle-wheel units, such as HKUST (where HKUST stands for Hong Kong University of Science &#38; Technology), with the catechols functionalized with alkyl and fluoro-alkyl chains. By taking advantage of the unsaturated metal sites and the catalytic catecholase-like activity of CuII ions, we are able to create robust hydrophobic coatings through the oxidation and subsequent polymerization of the catechol units on the surface of the crystals under anaerobic and water-free conditions without disrupting the underlying structure of the framework. This approach not only affords the material with improved water stability but also provides control over the function of the protective coating, which enables the development of functional coatings for the adsorption and separations of volatile organic compounds. We are confident that this approach could also be extended to other unstable MOFs featuring open metal sites.

Robust functional catechol coatings were produced in one step by direct reaction of the material known as HKUST with synthetic catechols under anaerobic conditions. The formation of homogeneous coatings surrounding the entire crystal is ascribed to the biomimetic catalytic activity of Cu(II) dimers on the external surface of the crystals.

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

In this work, we report the synthesis and characterization of ordered nanoporous carbon material (also called ordered mesoporous carbon material [OMC]) with a 4.6 nm pore size, and ordered silica porous matrix, SBA-15, with a 5.3 nm pore size. This work describes the surface properties of nanoporous molecular sieves, their wettability, and the melting behavior of D2O confined in the differently ordered porous materials with similar pore sizes. For this purpose, OMC and SBA-15 with highly ordered nanoporous structures are synthesized via impregnation of the silica matrix by applying a carbon precursor and by the sol-gel method, respectively. The porous structure of investigated systems is characterized by an N2 adsorption-desorption analysis at 77 K. To determine the electrochemical character of the surface of synthesized materials, potentiometric titration measurements are conducted; the obtained results for OMC shows a significant pHpzc shift toward the higher values of pH, relative to SBA-15. This suggests that investigated OMC has surface properties related to oxygen-based functional groups. To describe the surface properties of the materials, the contact angles of liquids penetrating the studied porous beds are also determined. The capillary rise method has confirmed the increased wettability of the silica walls relative to the carbon walls and an influence of the pore roughness on the fluid/wall interactions, which is much more pronounced for silica than for carbon mesopores. We have also studied the melting behavior of D2O confined in OMC and SBA-15 by applying the dielectric method. The results show that the depression of the melting temperature of D2O in the pores of OMC is about 15 K higher relative to the depression of the melting temperature in SBA-15 pores with a comparable 5 nm size. This is caused by the influence of adsorbate/adsorbent interactions of the studied matrices.

Here we report the synthesis and characterization of ordered nanoporous carbon (with a 4.6 nm pore size) and SBA-15 (with a 5.3 nm pore size). The work describes the surface and textural properties of nanoporous molecular sieves, their wettability, and the melting behavior of D2O confined in the materials.

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

Surface functionalization of nanodiamonds (NDs) is still challenging due to the diversity of functional groups on the ND surfaces. Here, we demonstrate a simple protocol for the multifunctional surface modification of NDs by using mussel-inspired polydopamine (PDA) coating. In addition, the functional layer of PDA on NDs could serve as a reducing agent to synthesize and stabilize metal nanoparticles. Dopamine (DA) can self-polymerize and spontaneously form PDA layers on ND surfaces if the NDs and dopamine are simply mixed together. The thickness of a PDA layer is controlled by varying the concentration of DA. A typical result shows that a thickness of ~5 to ~15 nm of the PDA layer can be reached by adding 50 to 100 &#181;g/mL of DA to 100 nm ND suspensions. Furthermore, the PDA-NDs are used as a substrate to reduce metal ions, such as Ag[(NH3)2]+, to silver nanoparticles (AgNPs). The sizes of the AgNPs rely on the initial concentrations of Ag[(NH3)2]+. Along with an increase in the concentration of Ag[(NH3)2]+, the number of NPs increases, as well as the diameters of the NPs. In summary, this study not only presents a facile method for modifying the surfaces of NDs with PDA, but also demonstrates the enhanced functionality of NDs by anchoring various species of interest (such as AgNPs) for advanced applications.

A facile protocol is presented to functionalize the surfaces of nanodiamonds with polydopamine.

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

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 of the Properties of Actinides under Extreme Conditions laboratory (PAMEC), allows the preparation of films and studies of sample surfaces using surface analytical techniques such as X-ray and ultra-violet photoemission spectroscopy (XPS and UPS, respectively). All studies are made in situ, and the films, transferred under ultra-high vacuum from their preparation to an analyses chamber, are never in contact with the atmosphere. Initially, a film of UO2 is prepared by direct current (DC) sputter deposition on a gold (Au) foil then oxidized by atomic oxygen to produce a UO3 film. This latter is then reduced with atomic hydrogen to U2O5. Analyses are performed after each step involving oxidation and reduction, using high-resolution photoelectron spectroscopy to examine the oxidation state of uranium. Indeed, the oxidation and reduction times and corresponding temperature of the substrate during this process have severe effects on the resulting oxidation state of the uranium. Stopping the reduction of UO3 to U2O5 with single U(V) is quite challenging; first, uranium-oxygen systems exist in numerous intermediate phases. Second, differentiation of the oxidation states of uranium is mainly based on satellite peaks, whose intensity peaks are weak. Also, experimenters should be aware that X-ray spectroscopy (XPS) is a technique with an atomic sensitivity of 1% to 5%. Thus, it is important to obtain a complete picture of the uranium oxidation state with the entire spectra obtained on U4f, O1s, and the valence band (VB). Programs used in the Labstation include a linear transfer program developed by an outside company (see Table of Materials) as well as data acquisition and sputter source programs, both developed in-house.

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

This protocol presents the preparation of U2O5 thin films obtained in situ under ultra-high vacuum. The process involves oxidation and reduction of UO2 films with atomic oxygen and atomic hydrogen, respectively.

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 ...