Liquid metals offer liquid-like fluidity and excellent conductivity at readily accessible temperatures [
1]. These futuristic metallic liquids have enabled kaleidoscopic applications, among which there are high performance electronic devices and thermal systems built with liquid metals [
2–
5]. The fluidity (or conformability) and conductivity are arguably the most widely recognized and utilized traits of liquid metals, in particular for energy-based applications. In comparison, their latent heat of solidification (referred to as latent heat hereafter), as an energy source, has attracted limited attention to date, despite the fast-increasing involvement of liquid metals in greatly diversified systems [
5–
10]. The possible reasons could be that either the solidification of the liquid metal in particular applications is intentionally avoided, or the latent heat is deemed insignificant [
9].
As a heat-releasing first-order phase transition, the solidification of a liquid metal is able to output a substantial amount of latent heat. Taking liquid gallium (Ga) as an example, it has a volumetric latent heat of 473 kJ/L, which is 40% higher than that of water. In addition, the low melting temperature (29.8°C) and the strong supercooling tendency of Ga ensure that its latent heat release can be regulated to take place at ambient temperatures. Leveraging these multi-facet advantages, a recent study has successfully achieved intelligent thermal-based stimulus perception and visualization by controlling the latent heat generation of liquid Ga droplets that are dispersed in an elastomer matrix [
11]. The work provides a liquid metal-based recipe for designing soft intelligent materials that are pivotal to future wearable electronics, thermal-based sensors, and human-machine interfaces [
12].
The design of the reported liquid metal-elastomer architecture (LMEA) starts with incorporating magnetic iron (Fe) particles in liquid Ga, after which the non-magnetic liquid metal becomes magnetically susceptible. The ‘magnetized’ liquid Ga is then thoroughly mixed with an elastomer base containing methyl vinyl polysiloxane and Pt-catalyzed polymethylhydrosiloxane till the liquid metal is broken into micro droplets. The elastomer is then let to cure to obtain the highly stretchable LMEA (Fig.1).
The Fe-containing Ga droplets, if undisturbed, will remain liquid as a result of the supercooling effect of Ga. Interesting behaviors emerge upon applying a suitable external stimulus to the LMEA. Under an infrared (IR) camera, the mechanical stress induced by pressing, stretching, or twisting, and the agitation generated by a changing magnetic field lead to temperature variations on the LMEA surface (Fig.2). It is found that the temperature field (or thermal map) is indeed a result of solidification and the accompanying latent heat release by the liquid metal droplets that are agitated by the applied stimuli. During the process, the liquid metal droplets act as tiny latent heat reservoirs which release their latent energy into the elastomer matrix. Since the droplets are discrete entities, locally configured stimuli or agitations trigger localized liquid metal droplet solidification, thereby resulting in a localized latent heat release and temperature increase. The recorded temperature depends on the room temperature (which can vary in different experiments), the size and compactness of the solidifying area, and the speed of solidification. The low thermal conductivity of the elastomer matrix further helps to effectively confine the heat released by the liquid metal within the LMEA.
It is noteworthy that the stimulus-induced temperature fields and patterns are visible to IR imaging but not to the bare eyes (Fig.1(b)), which makes the LMEA a potential material platform for information encryption. However, if it is desired, the temperature field can be visualized by further incorporating thermochromic particles (particles that change color when their temperature changes) into the LMEA (Fig.1(c)). Such a thermochromic LMEA(TC-LMEA) inherits the multi-stimulus-responsive functionalities from the LMEA, and meanwhile it further generates a visible color image together with a matching thermal map.
Through such a latent heat-enabled thermal or thermochromic imaging mechanism, a variety of information-coded stimuli have been perceived and visualized, and intelligent functionalities have been designed. For example, pressing-induced solidification and latent heat release enable continuous handwriting of thermal patterns on the LMEA (Fig.3(a)). It is also possible to display complex yet detailed thermal patterns with the material (such as in Fig.3(b) using a non-contact triggering magnetic field. In addition, the material platform has been used to probe the dynamic stress evolution in deformed elastic sheets. It is known that when biaxially stretched, an elastic sheet with a notch (crack) on its edge experiences stress concentration at the notch site, as have been reproduced by the finite element simulation in Fig.3(c) (top row). For a TC-LMEA sheet, the threshold stress for initiating solidification will thus first be reached at the notch tip, resulting in a latent heat release and temperature increase therein. Such a thermal (Fig.3(c)), middle row) and thermochromic (Fig.3(c), bottom row) change is indeed observed for a stretched notched TC-LMEA. Both thermal and optical imaging further reveal the gradual outward stress propagation from the notch tip to its surrounding area, indicating a successive solidification of the liquid metal droplets following a similar developing pattern as the material's stress evolution.
It is demonstrated in this work that the material system is long-term stable and that the solidified liquid metal droplets can be remelted by multiple methods such as Joule (electric) heating or IR heating, after which the material can be reused. Moreover, it has been validated that the liquid metal-elastomer combinations can be extended to different types of liquid metals and elastomers, so that the phase transition temperatures of the LMEA can be varied to meet different working temperature conditions. Apart from latent heat release, the mechanical and electrical properties of the LMEA are found to change considerably (in some cases drastically) after phase transition, which can be coupled with thermal (temperature) information to realize further integrations of material functionalities in the future.
Altogether, by implementing such an elaborate LMEA design, the latent heat stored in numerous discrete liquid metal droplets can become a powerful energy source for thermal-based detection and visualization. Such a thermal-based response mechanism of the LMEA does not require any electric circuitry to be involved in the material architecture, and in the meantime, it allows remote, non-contact detection of the output thermal signal. The multi-stimulus responsiveness, visualization strategies, and intelligent functionalities enabled by the LMEA have not yet been seen in other soft systems. In pursuing accurate on-demand control of latent heat release at the individual droplet level, efforts toward achieving uniform microscale droplet size and well-defined in-elastomer droplet arrangement can be challenging but rewarding. Nevertheless, the demonstrated highly diversified functionalities of the material platform forecast possibilities that have yet to be explored and defined. Liquid metals are making distinguishable contributions to the development of soft materials and systems. Unleashing and visualizing the latent heat of liquid metals is a previously underexplored path that can assist the realization of the overarching goal of soft intelligence.
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