Noncontact rotation,levitation, and acceleration of flowing liquid metal wires

This paper reports the noncontact manipulation of free-falling cylindrical streams of liquid metals into unique shapes, such as levitated loops and squares. Such cylindrical streams form in aqueous media by electrochemically lowering the interfacial tension. The electrochemical reactions require an electrical current that flows through the streams, making them susceptible to the Lorentz force. Consequently, varying the position and shape of a magnetic field relative to the stream controls these forces. Moreover, the movement of the metal stream relative to the magnetic field induces significant forces arising from Lenz’s law that cause the manipulated streams to levitate in unique shapes. The ability to control streams of liquid metals in a noncontact manner will enable strategies for shaping electronically conductive fluids for advanced manufacturing and dynamic electronic structures.

Noncontact methods of manufacturing and manipulation can minimize disrupting objects of interest. Objects can be manipulated in a noncontact manner by magnetic methods (levitation and tweezers) (1, 2), acoustic manipulation (3, 4), optical tweezers (5), and other techniques (6, 7). However, to date, free-flowing liquid streams have been particularly difficult to manipulate in a noncontact manner. Realizing highly controlled changes in directionality or complex shaping of liquids, especially without disrupting the cross-sectional shape of the stream, is a challenge. Here, we explore the noncontact manipulation of free-flowing streams of liquid metals (LMs). Gallium-based LMs (Galinstan, the eutectic alloy of gallium indium and tin used in this work) have recently received significant attention due to their promises of soft and stretchable metallic conductors, low melting points, and simultaneous fluidity and metallic properties at room temperature as well as low toxicity (8–15).LM alloys are seemingly unlikely candidates to form stable fluid streams due to their enormous surface tension and water-like viscosity, which favor the formation of droplets (Fig. 1A). However, electrochemical oxidation of the surface of the LM in basic solution lowers the effective tension of the LM to extremely low values (16, 17). This electrochemical manipulation of interfacial tension enables various fascinating phenomena, such as reversible deformation (18), patterning (19), heartbeat effects (20), “superfluid-like” penetration through porous media (21), and other electrochemical effects (22–29). Most importantly, the presence of oxide species on the LM also enables long, stable wire-like streams of metal to form as it exits a nozzle into the solution (17, 30) (Fig. 1B). Because of their cylindrical cross-section and metallic conductivity, we call these fluidic streams liquid metal wires (LMWs), which form narrow diameters (∼100 to 200 µm). Although normally LM is not responsive to magnetic fields, the current passing through the wire to drive the electrochemical reactions makes it susceptible to magnetic forces via the Lorentz force (Fig. 1C). The Lorentz force arises by applying a magnetic field normal to the direction of electrical current. The Lorentz force is normal to both the current and magnetic field, as described by the so-called "left-hand rule."Open in a separate windowFig. 1.Shaping free-flowing liquid metal wires by the Lorentz force and Lenz''s law: (A) drops form at 0 V and (B) a liquid metal wire at 1.5 V. (C) Current-carrying LMW rotated by the Lorentz force within a magnetic field in which N and S refer to the north and south poles of the magnet. (D) Schematic illustration of the experimental setup; a blue piece of paper covered one wall of the vessel to facilitate imaging. (E) Photographs showing the LMW path resulting from different positions of the magnet with the N pole outward. The dotted lines indicate the location and the shape of the magnet. (F) False-colored images of LM (white) showing four sequences of frames with a force diagram and motion analysis. The yellow dotted line denotes the periphery of the magnet.In this work, we control the displacement of free-falling LMWs at room temperature using the Lorentz force. Because LM is soft, it provides almost no resistance to manipulation via the Lorentz force and therefore, accelerates radially. The displacement of the LMWs relative to the magnet also induces a secondary force according to Lenz’s law (i.e., a drag force that opposes the motion at the periphery of the magnet). Thus, the combination effects of the Lorentz force and Lenz’s law drive the metal into shapes that mirror the circumference of the magnet while levitating the metal. As shown here, the behavior depends on the location of the magnet relative to the LMW. We demonstrate and characterize the unique ability to manipulate LM streams in a noncontact manner using only a relatively low applied voltage and a common magnet.