Self-Running Liquid Metal Drops that Delaminate Metal Films at Record Velocities

This paper describes a new method to spontaneously accelerate droplets of liquid metal (eutectic gallium indium, EGaIn) to extremely fast velocities through a liquid medium and along predefined metallic paths. The droplet wets a thin metal trace (a film ∼100 nm thick, ∼ 1 mm wide) and generates a force that simultaneously delaminates the trace from the substrate (enhanced by spontaneous electrochemical reactions) while accelerating the droplet along the trace. The formation of a surface oxide on EGaIn prevents it from moving, but the use of an acidic medium or application of a reducing bias to the trace continuously removes the oxide skin to enable motion. The trace ultimately provides a sacrificial pathway for the metal and provides a mm-scale mimic to the templates used to guide molecular motors found in biology (e.g., actin filaments). The liquid metal can accelerate along linear, curved and U-shaped traces as well as uphill on surfaces inclined by 30 degrees. The droplets can accelerate through a viscous medium up to 180 mm/sec which is almost double the highest reported speed for self-running liquid metal droplets. The actuation of microscale objects found in nature (e.g., cells, microorganisms) inspires new mechanisms, such as these, to manipulate small objects. Droplets that are metallic may find additional applications in reconfigurable circuits, optics, heat transfer elements, and transient electronic circuits; the paper demonstrates the latter

Self-Running Liquid Metal Drops that Delaminate Metal Films at Record Velocities

This paper describes a new method to spontaneously accelerate droplets of liquid metal (eutectic gallium indium, EGaIn) to extremely fast velocities through a liquid medium and along predefined metallic paths. The droplet wets a thin metal trace (a film ∼100 nm thick, ∼ 1 mm wide) and generates a force that simultaneously delaminates the trace from the substrate (enhanced by spontaneous electrochemical reactions) while accelerating the droplet along the trace. The formation of a surface oxide on EGaIn prevents it from moving, but the use of an acidic medium or application of a reducing bias to the trace continuously removes the oxide skin to enable motion. The trace ultimately provides a sacrificial pathway for the metal and provides a mm-scale mimic to the templates used to guide molecular motors found in biology (e.g., actin filaments). The liquid metal can accelerate along linear, curved and U-shaped traces as well as uphill on surfaces inclined by 30 degrees. The droplets can accelerate through a viscous medium up to 180 mm/sec which is almost double the highest reported speed for self-running liquid metal droplets. The actuation of microscale objects found in nature (e.g., cells, microorganisms) inspires new mechanisms, such as these, to manipulate small objects. Droplets that are metallic may find additional applications in reconfigurable circuits, optics, heat transfer elements, and transient electronic circuits; the paper demonstrates the latter

Illustration of a cyber-physically organized under-rubble biobotic swarm based mobile sensor network.

The illustration uses real images of insect biobots with neurostimulation backpacks and implanted stainless steel wire electrodes. Each of the ZigBee enabled system-on-chip based backpacks can be configured as a sensor node in the network.</p

Charge injection capacity for all electrodes types in 2-electrode cells.

The charge injection capacity of all electrode types are determined from CV plots and were found to be correlated to the corresponding EIS impedance. AuP and EGaIn electrodes have the highest charge capacity, with AuP having a prominent redox reaction.</p

Equivalent circuit model of an electrolyte-electrode interface.

(a, b) Model of an interface between working and reference electrodes labeled as W.E. and R.E. respectively, shown in a 3-cell circuit (a) with a counter electrode C.E. and in a 2-cell circuit (b). The charge conducting interface is conceived of a parallel RC network with parameters Rct (the charge transfer resistance) and Cdl (the double layer capacitance). Rs is the electrolytic resistance. (c, d) Modified version of the equivalent circuit model in 4(a, b) for SS-EGaIn electrodes. An additional RC parallel network is considered for the intermediate layer between SS and EGaIn, where network parameters Rint and Cint function corresponding to Rct and Cdl respectively.</p

Paired sample <i>t</i>-test of 2-cell and 3-cell measurement of electrodes.

Paired sample t-test of 2-cell and 3-cell measurement of electrodes.</p

Interface circuit model parameters–non-ideal interface.

Interface circuit model parameters–non-ideal interface.</p

Charge injection capacity of electrodes calculated from accelerated aging data.

The AuP electrodes were found to have the highest charge injection capacity which appeared consistent throughout the two weeks duration. It was followed by EGaIn electrodes which had the second highest charge capacity, and SS-EGaIn and SS electrodes which had relatively low capacity with SS electrodes having the lowest capacity. SS-EGaIn had increased charge injection capacity compared to SS, but it dropped and became more like that of SS with time as the EGaIn dislodged from the SS surface.</p

Mean change in 2-cell circuit model parameters with time.

(a) Rct−charge transfer resistance, (b) Cdl−double layer capacitance, (c) Rs−electrolytic resistance, and (d) Rint and Cint−charge transfer parameters at the intermediate layer between SS and EGaIn in the SS-EGaIn electrodes. Changes in the parameter values reflect the trends observed in the EIS plots. Rct changed for all electrodes except for EGaIn whose Cdl decreased by about five times over the period of two weeks. The Cdl of SS stayed fairly constant, while that of AuP changed, but remained considerably high. Parameters of the SS-EGaIn electrodes were found to gradually become similar to the initial parameters of the SS electrodes.</p

Self-Running Liquid Metal Drops that Delaminate Metal Films at Record Velocities

This paper describes a new method to spontaneously accelerate droplets of liquid metal (eutectic gallium indium, EGaIn) to extremely fast velocities through a liquid medium and along predefined metallic paths. The droplet wets a thin metal trace (a film ∼100 nm thick, ∼ 1 mm wide) and generates a force that simultaneously delaminates the trace from the substrate (enhanced by spontaneous electrochemical reactions) while accelerating the droplet along the trace. The formation of a surface oxide on EGaIn prevents it from moving, but the use of an acidic medium or application of a reducing bias to the trace continuously removes the oxide skin to enable motion. The trace ultimately provides a sacrificial pathway for the metal and provides a mm-scale mimic to the templates used to guide molecular motors found in biology (e.g., actin filaments). The liquid metal can accelerate along linear, curved and U-shaped traces as well as uphill on surfaces inclined by 30 degrees. The droplets can accelerate through a viscous medium up to 180 mm/sec which is almost double the highest reported speed for self-running liquid metal droplets. The actuation of microscale objects found in nature (e.g., cells, microorganisms) inspires new mechanisms, such as these, to manipulate small objects. Droplets that are metallic may find additional applications in reconfigurable circuits, optics, heat transfer elements, and transient electronic circuits; the paper demonstrates the latter