Eutectic Gallium-Indium

A Newtonian fluid is one in which the stress versus sheer strain is linear and passes through the origin.  The practical implication of this is that the fluid continues to flow regardless of the rate of sheer; water is a Newtonian fluid.  Wikipedia has a good explanation of this, but what we're more interested in is non-Newtonian fluids.  Figure 1 is a movie (lifted from YouTube) of an example of a non-Newtonian fluid.  Notice how the fluid exhibits both elastic and viscous properties--it behaves both like a liquid and a solid depending on the rate and magnitude of the forces acting on it. 

Figure 1.
The non-newtonian fluid in this case acts like an elastomer while experience the sheer stresses of a foot colliding with it. Once the person stands the sheer stress decreases and the fluid behaves like a viscous fluid again.

Liquid metals are interesting for a variety of reasons, but most metals melt at temperatures that are incompatible with organic materials.  Mercury is the most well-known example of a metal that is a liquid at room temperature, but there are also eutectic mixtures of metals that melt at or near room temperature.  Almost all liquid metals, however, are not only Newtonian fluids, but have very high surface free energies.  The practical implication of this is that they will spontaneously rearrange themselves in the bulk to minimize their surface areas.  There is a notable exception--Eutectic Gallium-Indium, or EGaIn (which is pronounced "ee-gain").  The unusual rheological properties of EGaIn are shown in Figure 2.

[videojs:videos/original/egain_movie.webm|height=320|width=240]

Figure 2.
Further characterization of the rheological properties of EGaIn1 revealed that the behavior can be classified as shear-yielding.  This means that, as a function of shear strain, there is a clear transition where the material converts form an elastomer to a viscous material.  This is different from a phase-change--the EGaIn remains a liquid, but in the absence of shear stress it behaves like an elastomer.  This means that it can used in microsolidics2 applications where a molten solder is injected into microchannels and then allowed to solidify, but since there is no phase change the contents of the channels remain a liquid and are therefore self-repairing.  This is not possible with liquids metals that are Newtonian fluids.  The high surface free energy forces the metal out of the channels spontaneously. Figure 3 (taken from reference 1) is a comparison between EGaIn and Hg in microchannels where the differences in behavior are obvious; EGaIn remains in the channels when the shear stress (i.e., pressure drop) is removed whereas Hg does not.

A practical use for this unusual behavior is the formation of small "tips" that can be used as conformal top-contacts for electrical measurements on delicate systems such as self-assembled monolayers (SAMs).[bib]EGaIn[bib]  Figure 3 is a picture of the formation of one of these tips.
 

Figure 3.

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1) Dickey, M.D.; Chiechi, R.C.; Larsen, R.J.; Weiss, E.A.; Weitz, D.A.; Whitesides, G.M. Adv. Func. Mater. 2008, 18, 1.
2) Siegel, A.C.;  Bruzewicz, D.A.; Weibel, D.B.; Whitesides, G.M.  Adv. Mater. 2007 19 727.
3) Chiechi, R.C.; Weiss, E.A.;  Dickey, M.D.; Whitesides, G.M. Angew. Chem. Int. Ed. 2008, 47, 142.

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