Liquid metal amoeba with spontaneous pseudopodia formation and motion capability

The unique motion of amoeba with a deformable body has long been an intriguing issue in scientific fields ranging from physics, bionics to mechanics. So far, most of the currently available artificial machines are still hard to achieve the complicated amoeba-like behaviors including stretching pseudopodia. Here through introducing a multi-materials system, we discovered a group of very unusual biomimetic amoeba-like behaviors of self-fueled liquid gallium alloy on the graphite surface immersed in alkaline solution. The underlying mechanisms were discovered to be the surface tension variations across the liquid metal droplet through its simultaneous electrochemical interactions with aluminum and graphite in the NaOH electrolyte. This finding would shed light on the packing and the structural design of future soft robots owning diverse deformation capability. Moreover, this study related the physical transformation of a non-living LM droplet to the life behavior of amoeba in nature, which is inspiring in human’s pursuit of advanced biomimetic machine.


The interaction between LM and graphite in alkaline solution
In our latest study, an intriguing and distinct phenomenon was discovered that when the spherical LM droplet was placed on the graphite substrate immersed in the electrolyte, it would easily change to a dull, flat puddle (Fig. S1). The mechanism underlying this flatten deformation is mainly related to the surface tension reduction induced by the formation of a thin oxide layer over the LM surface ( Fig. S2) 1 . The quick formation of this oxide layer should be caused by the electrochemical reaction induced by the graphite. It was predicted that the potential drop across the LM-NaOH interface was increased upon the LM-graphite contact. To testify this prediction, zeta potential of the graphite nanoparticle in NaOH was measured. At pH 11.6, the zeta potential of the graphite was -31.6 mV, which indicated that it was a positive potential drop across the graphite-NaOH interface. It is known that the LM droplet is negatively charged in NaOH 2 . Upon contact with the graphite, the negative potential drop across the LM-NaOH interface could be significantly pulled up in order to reach equal potential with the graphite (Fig. S3). Thus our prediction was proven that the LM was electrochemically oxidized, which as a result led to the electrochemical formation of the oxide layer over the LM and reduce the surface tension subsequently.
Further, the zeta potentials of other conductive substrate material including Cu and stainless steel were also measured (Table S1). Similar flattening behavior of the LM droplet was observed on the copper substrate which however should be mainly caused by the spreading of the gallium on copper as it is known that LM gallium alloy 3 amalgamates copper on the surface 3 . The zeta potential results also confirmed that this flattening is not related to the interface potential changes as the potential drop across the copper-NaOH was negative, similar to that of LM-NaOH interface. While on the stainless steel substrate, such flattening of LM was not observed. This is consistent with the zeta potential result. All these facts further confirmed our prediction that the oxidization of LM was due to its potential elevation pulled by the potential drop across the graphite-NaOH interface.    Table   Table S1. Zeta potential of several typical conductive substrate materials in alkaline electrolytes. Unit: mV. Data were extracted from our previous published reports. 4

Supplementary Movie
Movie S1 The transformation of the LM-Al droplet in Case 1.

Movie S2
The transformation of the LM-Al droplet in Case 2.

Movie S3
The transformation of the LM-Al droplet in Case 3. The first half showed a LM-Al droplet in Case 3 moving on the graphite. The second half showed the obvious vortices on the LM-Al droplet when the droplet turned a little leaned against the plastic wall of a petri dish.

Movie S4
The LM-Al droplet in NaOH stopped bubbling when contacted with the graphite.
When the droplet and graphite were separated (by a sucker), bubbles appeared against on the droplet. This experiment verified the galvanic interaction between Al and graphite substrate in NaOH solution.

Movie S5
When the Al continued to be consumed, the droplet may stop moving around with dark oxide membrane appeared on its surface.