Breathing to harvest energy as a mechanism towards making a liquid metal beating heart

Abstract

Simulating nature to manufacture a self-powered device or motor has been an important goal in science and engineering. Conventional spontaneous motion has generally been achieved through the Marangoni flow of an organic liquid or water solution. Moreover, as a metallic material mercury has been developed as a beating heart, a kind of self-propulsion example. However, serious safety concerns about mercury restrict its extensive application. This study discovered an important mechanism to realize a GaIn alloy-based liquid metal beating heart by introducing a breathing mechanism in simulating living organisms. With the unique configuration of a semi-submerged liquid metal droplet partially immersed in alkaline solution, such a system produces a surface tension gradient perpendicular to the three-phase contact line which subsequently leads to the oscillation of the droplet and the surrounding solution. This finding suggests a feasible way to fabricate self-oscillating liquid metal motors without input of external electricity or fuels.

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Introduction

The spontaneous motion of droplets has attracted a great deal of attention over the past few decades. Since the famous phenomenon of ‘tears of wine’ was revealed, a typical example of Marangoni flow, it has been well known that surface tension gradients can cause liquids to flow against the force of gravity.^1,2 Based on such a mechanism, interesting stories have been developed rapidly. It was reported that aniline oil can show either circular or beeline motion in a circular Petri dish filled with solution, due to chemical non-equilibrium.³ Directional motion can be generated in many ways, such as differences in metal ion concentration,⁴ pH gradient⁵ and wettability gradient.⁶ Cira et al. demonstrated that two-component liquid droplets such as propylene glycol and water deposited on clean glass can cause motion in neighbouring droplets over a distance.⁷ For such two-component droplets, evaporation induces a gradient of surface tension and then a Marangoni flow sets in. These explorations of self-propulsion are beneficial for the fabrication of bionic robots, applications in microfluidic systems and physical analogues of biological organisms. Generally, the elements of the self-propelled system include water and organic solvent. In contrast, related works on Marangoni effects of metallic materials are exceedingly rare.

It is worth noting that a classical bionic model, the mercury beating heart, is perhaps the most popular self-propelled oscillator. It consists of a mercury droplet covered with aqueous acid or basic solution and an iron nail. The iron triggers chemical reactions on the surface of the Hg drop, inducing variations in the surface tension and further leading to oscillations. Such a phenomenon was first reported by Lippmann.⁸ A hundred years later, many researchers conducted comprehensive experiments on the phenomenon of the mercury beating heart under different conditions and elaborated the related mechanism.^9–11 Thereafter, variations of the autonomous beating heart system were developed widely. Distinctive phenomena, such as deformation of circles, triangles, squares, diverse multi-pointed stars^12–14 and rotational motion,¹⁵ can be achieved by adjusting the experimental conditions. Although these studies provide outstanding insights into bionic systems, the poisonous and pollutional characteristics of mercury restrict its extensive applications to a large extent. Clearly, finding a new strategy to obtain a spontaneous oscillator through a green metal material is of particular significance.

Over the years, we have made efforts to probe biomimetic behaviors via a safe liquid metal. It was found that a gallium-based liquid metal can transform into different morphologies, including large-scale reversible deformation, rapid merging of separate metal droplets, controlled self-rotation and planar locomotion.^16,17 In these cases, an electrical field is required as a major power resource. Such deformability of a liquid metal is similar to that of amoeba which can alter their own shape on demand. Apart from this, it was found that a magnetic field has a certain influence on the motion of a nonmagnetic liquid metal. When the liquid metal sphere is set between a pair of concentric ring electrodes in an NaOH solution on a permanent magnet, the sphere is capable of rotating centrifugally around the central electrode.¹⁸ To get rid of the limitation of an external energy supply and to further simplify the driving method, our group recently disclosed a synthetic self-fueled motor based on liquid metal at the micro scale.¹⁹ The liquid metal ‘eats’ Al flakes as fuel to power its swimming. It has been figured out that the mechanism is that the energy produced by the chemical reaction between gallium, Al and solution is converted into mechanical activity to drive the motion of the droplet. Such activities are comparable to those of an organism i.e. ingestion, digestion, conversion and locomotion. Further, such a self-fuelled liquid metal machine displays additional behaviors like autonomous convergence or divergence,^20,21 macroscopic Brownian motion²² and motion restriction resulting from a magnetic trap effect under the magnetic field.²³

In this paper, we have found a new mechanism to make a GaIn alloy-based liquid metal beating heart. Such a self-propelled system includes a liquid metal droplet and an alkaline solution. To trigger the oscillation of this system, a special configuration is designed where a part of the droplet is exposed to ambient air and the other part is immersed in the alkaline solution. The liquid film around the metallic droplet presents regular and periodic motion. To address the major issues, particular attention is paid to a comprehensive understanding of the mechanisms. It can be noted that such a unique semi-submerged design results in Marangoni gradients consisting of the gallium oxide diffusing along the droplet interface and gallium hydroxide spreading at the solution/air interface, which provides the impetus source for the oscillation. Thus, breathing air to harvest energy is similar to animals' physiological activity in nature.

Experimental

Materials

Liquid metal GaIn alloy (Ga 90%, In 10% by weight) was utilized to realize the spontaneous movement. The solution environments, i.e. NaOH solutions (0.5 mol L⁻¹), were prepared with deionized water and A.R. grade NaOH (99% purity). A liquid metal droplet was introduced into a glass dish containing NaOH solution by a disposable injector without a needle. The volume of the droplet was 0.4 mL with a diameter of 1.1 cm. To eliminate the interference of contact with the wall, the liquid metal was located in the center of the container. The solution level should be slightly lower than the top of the liquid metal droplet so that the liquid metal can be partly exposed to the air and the remainder can be submerged into the NaOH solution.

Characterization of the oscillation

The dynamic behaviour of the beating heart system was recorded by a digital camera (Canon 60D, Japan). The films were captured at 25 frames per second (fps), with 1920 × 1080 pixel resolution. Due to the high light reflection on the metal base, it is difficult to distinguish the boundary between liquid metal and the solution with the naked eye. To improve the contrast, the NaOH solution was dyed with red ink. In addition, the side view of the liquid metal droplet with the surrounding alkaline solution was also captured by the Canon camera.

Flow visualization

To observe the flow characteristics of the solution intuitively, nylon fabric fragments at the millimeter scale were randomly positioned as tracers in the liquid metal beating heart system. The fragments are light enough in weight that they can float on the NaOH solution and drift with the flow of fluid. Moreover, fluorescent polystyrene particles with a mean size of 1.84 μm (purchased from Spherotech) were utilized to display the flows of the solution. We employed a QIClick camera (QImaging) on an Olympus BX51 upright microscope to capture the motion of the fluorescent beads at a rate of 10 fps.

Results and discussion

Cyclic oscillation of liquid metal droplet

We found that the crown area of the liquid metal droplet exposed in the air was able to enlarge and shrink spontaneously and periodically in the NaOH solution without any external power supply (ESI movie 1†). It seems like the liquid metal droplet periodically takes a deep breath. The continuous oscillation lasted for 1 min 40 s; then the solution gradually calmed down. Before and after the regular movement, the interfacial solution rotated along the surface of liquid metal droplet in one direction. To investigate the relationship between time and the diameter of the exposed crown area quantitatively, we selected several stable cycles from the oscillation for analysis (Fig. 1(a) and (b)). If the moment when the exposed part had a minimum diameter was set to 0 s, the diameter expanded to the maximum within 5.2 s, and shrank rapidly to the minimum within only 1.2 s. Then, the five following cycles were extracted from the movie to clarify the motion pattern (ESI movie 1,† Fig. 1(c)). This shows that the dimension of the exposed part can recover to its original size during these six cycles. And the maximum diameter of the exposed part is almost one-and-a-half times as large as the minimum diameter. The corresponding oscillation period of these six cycles is 6.095 ± 0.335 s (T_expand = 4.930 ± 0.262 s, T_shrink = 1.165 ± 0.106 s), which proves that such variation is quite regular, as the heart is beating with a certain stable frequency.

Fig. 1 Beating heart behaviors of a liquid GaIn metal droplet. (a) The semi-submerged liquid metal droplet results in the periodic oscillation of surrounding solution. The red arrows indicate the diameter of the exposed crown area. The blue dotted lines are marked according to the maximum diameter of exposed crown area for a comparison of dimensional changes at different times. (b) The variations in dimension on the part of the liquid metal droplet exposed to the air versus time in one cycle. (c) The periodic oscillation curves over several cycles.

From the top view, it is hard to distinguish whether the movement results from the metallic droplet or from the surrounding solution. Thus, side photos of this beating heart system were taken. The related videos of the oscillation from two different angles are displayed in ESI movie 2.† It was found that the solution near the interface obviously demonstrated back and forth movement around the metal droplet. In turn, the oscillation of the solution also leads to a slight vibration of the liquid metal droplet. Consequently, the oscillation of the beating system is mainly caused by the periodic fluctuation of the interfacial solution. This differs significantly from the mechanisms of a traditional mercury beating heart system where oscillation is caused by the motion of the mercury droplet itself. It is noteworthy that such a test can be simply utilized to clarify the source of the obvious oscillation.

Tracing the dynamic behaviors

In the tracing experiment, the movement of the nylon fabric fragments was recorded to describe the situation of the flow field (ESI movie 3†). Fig. 2(a) shows the overlaid time-lapse images of fragments at different times. The arrows represent the directions of motion of the fragments. These fragments can be divided into two groups according to their moving behaviors. One group of fragments near the droplet rapidly moved to the gas–solution–metal contact line, and this group was affected by the oscillation of the solution. Accordingly, an effective area can be determined within the blue circle, as shown in Fig. 2(a), region 1. The other group of fragments was far from the droplet and they fluctuated slightly depending on the natural state of the solution. Such an area outside the red circle is classified as region 2 in Fig. 2(a). The region between the red and blue circles does not involve any nylon samples, so this area is not considered or discussed in the current test. In region 1, a typical fragment termed sample 1 was selected and its position was labeled with the interval time. At first, this sample moved with the initial fluctuation of the solution, as the other fragments did in region 2. After 25 seconds, the solution surface became stable. After that, the fragments in region 2 were nearly static in situ. Meanwhile, sample 1 began to alter its direction of motion and went straight towards the center of the droplet along with each oscillation of the solution surface around the droplet. Fig. 2(b) shows the relative displacement of sample 1 between two adjacent time intervals in Fig. 2(a). From 50 s to 100 s, there was no large displacement of sample 1, because some fragments near the droplet contacted the interface and disturbed the periodic motion of the interfacial solution. After adjusting and adapting for about 50 s, the oscillation of the three-phase contact line recovered and the velocity of sample 1 significantly increased as it moved closer to the droplet. Evidently, the liquid metal beating heart system is capable of producing a certain level of dynamic power, which can be developed as a mini motor to deliver or collect objects in solution.

Fig. 2 The flow characteristics of the solution around the droplet. (a) Overlaid time-lapse images of nylon fragments moving with the flow of the solution for 125 s. (b) The relative displacement between labeled fragments in adjacent intervals. (c) Images of fluorescent beads (1.84 μm) tracing the flow direction of the solution.

It has been found that the nylon fabric experiment can intuitively embody the flow characteristics of a liquid metal beating heart system. Thus the effective scope of the system can be determined. However, nylon samples of a large size, as used here, cannot accurately reflect the actual flow field of the oscillation. Moreover, these light fragments floating on the liquid surface simply describe the surface status of the solution. Therefore, fluorescent polystyrene particles were utilized to further visualize the flow field around the droplet (Fig. 2(c)). Due to the limited field of vision of the microscope under high magnification, a liquid metal droplet of several hundred micrometres in size was adopted to demonstrate its surroundings. Such a small droplet formed into a sphere and was suspended in the solution. We found that such tiny droplets can realize the oscillation in the same way as those of large size (ESI movie 4†). However, it takes 100 ms for the exposed crown to change from its maximum area to the minimum, which is much shorter time than that of a droplet of bigger size, as shown in Fig. 1. This proves that the oscillation frequency is related to the dimension of the droplet. In addition, the tracer particles at the interface fluctuated with the solution and gathered from the edge to the center of the droplet. Such microscopic observation also shows that the particles near the top interface appear more mobile than those near the bottom (ESI movie 4†). This indicates that there are regional differences in the speed at which the surrounding solution moves, induced by oscillation of the beating heart system.

Mechanisms of the self-propulsion

To clarify the principle of this self-propelled system, we first employed deionized water and NaCl solution (0.5 mol L⁻¹) in comparison with NaOH solution. Fig. 3(a) demonstrates the status of droplets in three different solutions. There were no obvious changes in the morphology of the droplets in water or NaCl solution. The surface of these droplets was rough, lacking in brightness compared to that of droplets in NaOH solution. In the two situations, oscillation did not occur even if the droplets were semi-submerged.

Fig. 3 The characteristics of interfacial interaction of air, NaOH solution and liquid metal droplet. (a) The comparison of performances among liquid metal in water, NaCl solution and NaOH solution. (b) The bottom and side views of the liquid metal droplet in air and NaOH solution, respectively. (c) Mechanical analysis of triple-line (left) and schematic illustration of the flow field around the droplet (right).

It is well known that Ga-based alloy surface is susceptible to oxidation when exposed to air, causing a thin, solid film to cover the surface of the alloy.²⁴ For the GaIn alloy here, the content of gallium is dominant. Thus the main constituent of its oxide film is Ga₂O₃. This oxide film can be dissolved effectively in NaOH solution but not in water or NaCl solution. Moreover, the performance of the liquid metal in air and NaOH solution was studied due to the semi-submersion of the liquid metal droplet. Fig. 3(b) shows the morphology of liquid metal droplets in air and NaOH, respectively. Contour features of the droplets were extracted both from the bottom and the side views. This shows that the contact angle of the droplet in NaOH solution (θ₁) is larger than that in air (θ₂), indicating the higher surface tension in NaOH solution. This result is consistent with the conclusion previously reported that oxidation lowers the surface tension of the liquid metal significantly.²⁵

For oscillation, we define the wetting parameter M according to the force balance: M = γ_DS − γ_DA + γ_SA cos θ_a, where γ_DS, γ_DA and γ_SA represent the surface energy of the droplet/solution, the droplet/air and the solution/air, respectively; θ_a represents the angle between γ_SA and γ_DS. The direction of M is along the X axis, as shown in Fig. 3(c). The X axis represents the direction of the interfacial tangent. Initially, M is larger than 0 due to γ_DS > γ_DA, when the contact line reaches position 1. In Fig. 3(c), position 1 indicates the peak of the dynamic solution surface around the droplet; positions 2 and 3 are the different moments of the surface proceeding downwards; the lowest height of the interfacial surface is shown in position 4. The resulting stresses are large enough to shift the interfacial solution several millimeters downwards along the droplet surface; hence the exposed diameter subsequently increases. Moreover, there is a phenomenon that effectively supports this statement. During the course of the droplet being exposed from the solution at the beginning, the liquid surface first went down until the exposed diameter of droplet reached a maximum (ESI movie 1†). Such a maximum diameter is larger than that under the condition of a stable and balanced liquid surface. This proves that the interfacial solution has been subjected to the downward drag force since the very beginning.

In the fluctuation area, the lower the height of the solution, the longer the reaction time between the solution and the oxide of gallium will be. Therefore, the difference in the interfacial tension of the two patches beside the boundary becomes smaller since the interface solution goes down from position 1 to 4 (Fig. 3(c)). Meanwhile, the value of θ_a gets larger, which decreases cos θ_a markedly. Therefore, the downward motion of the solution is decelerated due to M becoming smaller. These statements are in accordance with the result shown in Fig. 1(b). Once the liquid surface descends to its lowest degree, i.e. position 4, the direction of γ_SA will turn to its opposite position. Such a sudden comprehensive alteration leads to M < 0, inducing an inverse direction of motion for M. As long as M < 0, the interfacial solution goes upwards along the outer face of the droplet. Moreover, the distant hydrostatic pressure tends to push the solution with a lower surface adjacent to the droplet back to the horizontal. The whole procedure is similar to the vibration of a spring.

It is well known that changes in interfacial tension under non-equilibrium conditions play an important role in directional sensing.³ For a semi-submerged droplet, different interfacial tensions are triggered between the interfaces of droplet/solution and droplet/air, respectively. Remarkably, the metal ions localized around the droplet can result in an inhomogeneous interfacial tension.⁴ Therefore, there are two sources of Marangoni gradients in this system: the diffusion of gallium oxide along the droplet interface and gradients at the solution/air interface due to the spreading of gallium hydroxide. Notably, the Marangoni flow moves towards the region with the higher interfacial tension;²⁶ thus an asymmetric flow field is created around the droplets (Fig. 3(c)). The flow generates a local vortex in the surrounding solution, which is also the reason why the nylon fabric in certain regions can move towards the droplet.

Additionally, we conducted repeated experiments under an oxygen-free, i.e. argon gas, atmosphere. Before being used the GaIn alloy was adequately washed in NaOH solution to guarantee that its oxide was completely removed. It was found that the oscillation can still be observed in this situation (ESI movie 5†). There were six apparent oscillation periods, lasting for 25 s. However, the oscillation time of liquid metal in the oxygen-free situation is shorter compared to the one in the air environment. It can be concluded that oxygen is beneficial to increasing the difference in surface tension between the two sides of the interface.

Over a rather long period of time, the oscillation amplitude remained almost constant under the air condition. However, at the beginning and end of the oscillation, the interface experienced inducing and adjusting periods, respectively. During these stages, the interfacial solution rotated around the droplet (Fig. 4(a)–(c)). The rotation occurs due to the inhomogeneous oxidation of the surface of the liquid metal droplet. Such inhomogeneity results in different amplitudes of motion of the surrounding solution at different interfacial positions. The solution which goes higher towards the center of the droplet will flow to the lower nearby position. Thus, the rotation is triggered and the rotational direction is random. With the reaction proceeding, the difference in Marangoni gradients was reduced; thus the oscillation amplitude got smaller. Additionally, the concentration of NaOH solution in the microenvironment around the droplet gradually decreased due to consumption, especially that of the liquid film near the upper solution surface. This can be confirmed by the fact that if we shake the liquid metal droplet with the dish, the oscillation will restore the activity. This is attributed to the complementary action of fresh NaOH solution and the redistribution of oxide film on the surface of droplet. Another reason for the solution ceasing to oscillate is the accumulation of gallium hydroxide on the interfacial surface of the droplet (see Fig. 4(d)). The solid gallium hydroxide produced from the reaction between gallium and the alkali solution results in the barrier boundary restricting the motion of solution. Consequently, the balance of the system will be established.

Fig. 4 The disturbed activities of the liquid surface around the droplet at the start and end of the oscillation. (a)–(c) are photos of the liquid surface rotating in a clockwise direction. (d) Local image of the liquid droplet at the end of an oscillation.

Conclusions

In summary, we have found and investigated a self-propelled liquid metal beating heart that can harvest energy via a breathing mechanism without an external power supply. Such a system has the unique design that the droplet is semi-submerged in NaOH solution and partially exposed to the air. The different characteristics of the liquid metal droplet performing in these two environments lead to a surface tension gradient across the interface in three phases on the droplet, which induces the Marangoni flow. Studies of the Marangoni effect are common, but it is rare to see one applied to a sphere realizing periodic oscillation. Interestingly, such a system imitates not only the motion of a living creature but also its energy acquisition strategy. Without using external electricity or fuel, the ambient air provides one energy source for the spontaneous motion of liquid metal in alkaline solution, which is in close analogy to the breathing behavior of a living organism. A conventional beating heart metal is generally composed of mercury and sulphuric acid, which are harmful and dangerous. Conversely, non-toxic GaIn alloy-based liquid metals offer an alternative approach to realize beating heart behaviors. In particular, such a beating heart has the advantages of low cost, easy accessibility, and being quick to make. This finding would shed light on a variety of future applications, such as a self-propelled motor in microfluidics or a biomimicry system for partially duplicating biological activity in nature.

Acknowledgements

This work is partially supported by Dean's Research Funding and Frontier Project of Chinese Academy of Sciences.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra17486h

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