Page 11 - Science Focus (Issue 017)
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PROGRAMMABLE LIQUID MATTER:
A Metal That “Bends” to Your Will
可編程液態物質：
能被隨心所欲地操縱的變形金屬
By Henry Lau 劉以軒
To your knowledge, how many elemental composed of gallium (Ga) and indium (In), which
metals are liquid at room temperature? The answer, looked like mercury in its appearance. In order to
of course, is one: mercury as it possesses a melting allow the liquid metal to twist and move to assume
point of –38.8°C. Besides mercury, scientists have different shapes, it was pre-treated with sodium
been busy at work to create alloys, by the fusion hydroxide (NaOH), a highly alkaline solution.
of two or more metals, which would also assume The hydroxide ions served to generate negative
the liquid state at room temperature. What may charges on the liquid metal surface by reacting
astound you more is that in recent years, scientists with gallium to form some tetrahydroxogallate(III)
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have also created liquid metals that can assume anions (Ga(OH)4 ). Meanwhile, Na cations in the
+
predetermined shapes on command. This exciting solution would be electrostatically attracted to the
avenue of research, albeit in its infancy, has been negatively charged blob, coating it with a uniform
dubbed by many as “programmable liquid matter.” positive charge (Figure 1). Under this equilibrium
T h e b a s i c i d e a t h at l e d to t h e r i s e of state, the blob would assume its most stable shape
programmable liquid matter is a concept first — a perfectly symmetrical sphere.
suggested in 1991: “programmable matter.” [1] This charged form of the liquid metal can be
From the name itself, you can already infer that the deformed when charges are applied to it via
material is meant to be able to change its physical electrodes. When the anode (positive terminal)
properties based on the user’s instructions. The comes into contact with the liquid metal sphere,
change is triggered by an input of some sort, be it the surface charge of the blob would switch from
environmental or user-induced. Hence, the term negative to positive. This disrupts the previously
“programmable” is used as it is similar to giving the established charge equilibrium, causing the
matter a “program” to run on when changing its spherical blob to collapse, flatten, and gradually
physical properties. In the context of programmable spread towards the cathode (negative terminal)
liquid matter, it means that the liquid metal in (Figure 2). As a result, we could make the liquid
question will be morphed into various fixed shapes. metal move in any desired direction by controlling
Sometimes, even its physical properties such as the position and voltage of the electrodes. To
electrical conductivity and surface tensions can enable an all-rounded control of the liquid metal’s
be modified. A particular research team from the movement, researchers suspended it on a 7 x 7
Universities of Sussex and Swansea created an electrode array. By programming a sequential
alloy that can morph from one shape to another change of the voltage and connectivity for the
according to the direction of an electric field [2]. 49 individual electrodes in the array, they have
Before actually making the liquid metal move, managed to control the liquid metal’s form to take
researchers had to do a lot of work. To start with, the shape of the letter “S” or that of a “heart.”
they created a low melting point eutectic alloy (Figure 3(a) & 3(b))
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