Eutectic Gallium-Indium (EGaIn): A Liquid Metal Alloy for the Formation of Stable Structures in Microchannels at Room Temperature
Title: Eutectic Gallium-Indium (EGaIn): A Liquid Metal Allloy for the Formation of Stable Structures in Microchannels at Room Temperature
Authors: Michael D. Dickey, Ryan C. Chiechi, Ryan J. Larsen, Emily A. Weiss, David A. Weitz, and George M. Whitesides
Source: Advanced Functional Materials 2008, 18, 1097-1104 
Eutectic gallium-indium is an electrically conductive liquid metal at room temperature used in microfluidic devices to make microscale electronics and electromagnets. Interestingly, for small surface stresses, the fluid behaves as an elastic solid, but readily flows once enough stress is applied to the surface. Because of this property, eutectic gallium-indium can be forced through microfluidic channels by applying a pressure, like any other fluid, but once the pressure is relieved, the gallium-indium alloy remains structurally stable.
The rheological properties of the gallium-indium alloy are thought to be caused by the unusual surface of the alloy. In air, the alloy will spontaneously develop a “skin” composed mostly of gallium oxide, which confines the liquid gallium-indium alloy beneath it. The gallium oxide skin also acts as a surfactant, which serves to lower the surface tension of the material.
Gallium and indium are two metals which are soluble in each other in the liquid phase but not in the solid phase. Furthermore, adding small amounts of indium to liquid gallium will lower the freezing point of the liquid solution up to a certain point. This point is the eutectic temperature and composition. Based on the indium-gallium phase diagram, which depicts which phases are stable over a range of temperatures and compositions, this eutectic is around 15 atomic percent indium in gallium, and the eutectic composition melts at 15 degrees Celsius.
Gallium-indium eutectic surface
The surface of the gallium-indium eutectic liquid can be characterized by Auger electron spectroscopy, in which the material is bombarded by X-rays. Characteristic electrons are emitted from the surface, and their energies are based on the energy of the X-ray radiation and the atoms present at the material surface. The authors of this paper performed Auger electron spectroscopy on gallium-indium eutectic alloy which had been exposed to air, then removed the surface oxide while keeping the alloy under vacuum by sputtering with argon ions. The researchers found that the composition of the surface of the alloy which had been exposed to air was nearly pure gallium oxide, indicating that gallium is reacting with oxygen in the air, and that this gallium oxide segregates to the surface of the material, changing its surface properties.
Rheological properties of gallium-indium eutectic
The researchers found that gallium-indium eutectic behaves as an elastic solid for small applied stresses, because of the gallium oxide shell surrounding the fluid. At a certain critical stress, the oxide shell yields, and the material flows like a fluid. The researchers explored this property in two ways.
First, the researchers used a parallel-plate rheometer to quantify the fluid properties of the gallium-indium eutectic alloy. A parallel-plate rheometer measures the amount of force required to shear the fluid at a certain rate. In Newtonian fluids, the shear stress is linearly related to the rate of shear strain by the shear viscosity. However, in this case, a yield stress phenomenon is observed. The gallium oxide shell resists the deformation of the bulk material by the rheometer, and so the rheometer must apply a large stress. Once a critical stress is reached, the gallium oxide shell yields, and the fluid beneath the shell flows easily. The authors demonstrate this by plotting the elastic and viscous moduli of the material against the shear stress applied to the surface of the fluid. When the surface stress is increased beyond its critical value, the moduli drop several orders of magnitude, indicating the material can no longer resist deformation and flows easily.
Second, the authors tested the ability of gallium-indium eutectic alloy to flow through microfluidic channels. The authors built a testing device out of polydimethylsiloxane (PDMS), a silicone polymer commonly used in microfluidics. The testing device consisted of an inlet and outlet connected by a 20 micron wide channel. The inlet and outlet were tapered from a wide diameter to 20 microns. The authors then placed gallium-indium eutectic alloy in the inlet channel and pressurized the inlet. At different pressures, the gallium-indium alloy stops at different distances along the tapered inlet, which correspond to different curvatures of the alloy-air interface. Because the alloy is in mechanical equilibrium, the pressure applied must be equal to the Laplace pressure at the alloy-air interface. Therefore, the pressure applied must follow the Young-Laplace equation:
P = 2γ cos(θ)*(1/W + 1/H)
where W and H are the width and height of the channel, θ is the contact angle of gallium-indium eutectic alloy with PDMS, and γ is the surface tension of the alloy. The authors estimate the contact angle to be 150 degrees via a picture of the interface, and so they estimate the surface tension to be 630 mN/m, which agrees with values in previous literature.
The authors then compared the gallium-indium alloy to liquid mercury. They put gallium-indium eutectic alloy in one test channel and mercury in another, and pressurized both channels such that both liquids flowed through the narrow portion of the channel and out the outlet. Amazingly, upon releasing the pressure, the eutectic gallium-indium alloy remains in place, despite the added surface area, because it is held back by its oxide shell. Mercury, which has a lower surface tension (480 mN/m) and contact angle on PDMS (140 degrees), immediately withdraws from the channel upon release of pressure, due to its surface forces and the fact that it does not form an oxide shell under normal conditions.
The authors hope to use this unique property of gallium-indium eutectic alloys in several interesting applications. This alloy could be used to make micron scale electrodes for microfluidic or thin-film electrochemistry, for example. It could also be used to cast micron-scale metallic parts for use in a microfluidic device – the authors suggest miniature heating coils or connections for sensors or transistors.