Self-activated elastocapillary wave promotes boiling heat transfer on soft liquid metal surface
Graphical abstract
Introduction
For a long history, mankind observed that solid can be deformed when compression is exerted on a soft material. Fundamental studies on the deformation of soft materials have only been received attention in the past two decades [[1], [2], [3]]. When the three phases of solid, liquid and gas meet at the contact line, the well-known Young equation gives cosθ = (γsv − γsl)/γlv, where θ is the contact angle, γ is the surface tension, the subscripts of s, v and l represent solid, vapor and liquid, respectively. Even though the Young equation has been widely used, one notes that the equation emphasizes the force balance along the surface plane, but the vertical component of the surface tension is neglected. The latter plays key role in deforming a soft solid near the three-phase contact line [[3], [4], [5]]. The elastocapillary length is expressed as lec = γlv/μ, characterizing the relative importance of surface tension of liquid and vapor relative to the elasticity of soft solid μ. The elastocapillary effect is important when the characteristic length of soft solid is smaller than lec. Crease and wrinkle are two types of deformation [[6], [7], [8]]. The elastocapillary induced deformation and instability are related to the wrapping of a liquid drop by a thin elastic film [9], coalescence of thin wet fibers [10,11], cavitation in soft hydrogel [12], formation of dynamic patterns on curved surface [13] to name a few.
The elastocapillary effect for droplet interacting with soft solid have been widely investigated [1,3,9,14,15]. Bubbles are more active than droplet, thus the elastocapillary effect with bubbles in the system is less reported. Boiling is phase-change heat transfer process governed by the bubble dynamics. The elastocapillary effect is also seldom considered in boiling heat transfer. Currently, boiling on solid surface use stiff micro/nano structures to promote the heat transfer performance [[16], [17], [18]], including micro-pin-fin, nanoparticles and porous materials, etc. There is a superheating liquid boundary layer on solid surface. Transient conduction, microconvection and microlayer evaporation in the superheating boundary layer are the mechanisms for boiling [19].
As a soft material, liquid metal is introduced in boiling system in this paper. In contrast to the three-phase system of solid, liquid and vapor, boiling on a liquid metal surface involves two-phases of liquid and vapor, but liquid contains two components including a deformable liquid metal and a boiling liquid such as ethanol. We show that, compared to hard copper surface, liquid metal obviously reduces wall superheating at the onset of nucleate boiling (ONB). Most importantly, bubble nucleation, growth and departure create dynamic wrinkles and elastocapillary wave of liquid metal. In turn, this interface instability reduces the superheating boundary layer thickness of boiling liquid, thus boiling heat transfer is augmented.
Section snippets
Experiment
Comparative experiments were performed on hard copper surface (CS) and soft Galinstan liquid metal surface (SS) at atmospheric pressure. The test surface was a circle with a diameter of 10.0 mm, pool liquid temperatures were Tb = 78 °C, 60 °C and 40 °C, and ethanol was the boiling liquid. The details of test section, experiment setup, surface characterization and data reduction are as follows.
Boiling curves and heat transfer performance
Fig. 3 shows the comparison of boiling heat transfer performance on CS and SS. Following conclusions can be drawn: (i) At ONB, wall superheating ΔΤ = Τw-Τs is ~5 K for SS and ~ 13 K for CS, indicating easier triggering of boiling on SS, where Tw is the wall temperature and Ts is the saturation temperature of ethanol, noting that Tw is defined at the copper surface for boiling on both copper surface and soft surface. (ii) SS significantly enhances boiling heat transfer, increasing heat transfer
Conclusions
In summary, we spread a thin soft layer of liquid metal on a hard substrate to alter the behavior of superheating boundary layer of boiling liquid. We show that, wall superheating at ONB decreases from ~13 K on hard surface to ~5 K on soft surface. The soft surface increases bubble departure frequency by ~7 times and reduces bubble departure size by half, corresponding to a maximum 60.5% heat transfer increasement for saturated boiling of ethanol. The elastocapillary wave is self-activated by
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgement
We appreciate the funding support from the National Natural Science Foundation of China (No. 51821004 and 51806065) and Fundamental Research Funds for Central Universities (No. 2020DF002).
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