Water film coated composite liquid metal marble and its fluidic impact dynamics phenomenon

Abstract

A composite liquid metal marble made of metal droplet coated with water film was proposed and its impact dynamics phenomenon was disclosed. After encapsulating the liquid metal into water droplets, the fabricated liquid marble successfully avoided being oxygenized by the metal fluid and thus significantly improved its many physical capabilities such as surface tension modification and shape control. The striking behaviors of the composite liquid metal marbles on a substrate at room temperature were experimentally investigated in a high speed imaging way. It was disclosed that such marbles could disintegrate, merge, and even rebound when impacting the substrate, unlike the existing dynamic fluidic behaviors of liquid marble or metal droplet. The mechanisms lying behind these features were preliminarily interpreted. This fundamental finding raised profound multiphase fluid mechanics for understanding the complex liquid composite which was also critical for a variety of practical applications such as liquid metal jet cooling, inkjet printed electronics, 3D printing or metal particle fabrication etc.

Keywords

liquid metal marble / metallic droplet / composite fluid / impact dynamics / multiphase fluid mechanics

Introduction

Liquid marbles, a kind of interesting material, are generally referred to as liquid droplets coated with hydrophobic particles [ 1]. Owing to their unique properties, such as absence of contact with substrate leading to quick motion, hydrophobic interactions with other fluids and the ability to be divided or merged together, such objects recently received intensive attentions [ 2]. Many exciting applications are being investigated in various fields including water pollution testing [ 3], gas detecting [ 4], manipulation and transport of small quantities of liquid [ 2], and preparation of micro-reactor [ 5] etc. A wide range of materials have been, therefore, employed to produce liquid marbles, including water, ionic liquid or organic reagents as the core and hydrophobic particles as the encapsulation [ 6]. The undesired evaporation of liquid usually influences the stability and lifetime of liquid marbles [ 7], though the ionic liquids offer a possible solution because of their low vapor pressure. These materials severely restrict further application of liquid marbles for their limitations.

Room temperature liquid metals are those metals that remain in liquid phase at room temperature. With high thermal and electrical conductivity and excellent deformability, such metals are regarded as materials that have rather wide application values. The most well-known liquid metal is mercury [ 8], whose toxicity, however, remained a tough issue for practical application. As an alternative, the gallium-based alloy is quickly emerging as an important functional liquid metal with its negligible toxicity, extremely low vapor pressure and sufficiently low melting point. Such advantages enable its many unconventional applications, such as chip cooling [ 9], printed electronics [ 10], and medical electronics [ 11].

Although the gallium-based alloy has many desirable properties, its surface usually immediately oxidizes in ambient air and forms a thin oxide layer [ 12] which would easily adhere to nearly any solid surface and thus may make certain application fail. To prevent the liquid metal from attaching to its surroundings and maintain the flexible re-configuration, Sivan et al. created liquid metal marbles by encapsulating the liquid metal droplets inside the coating of nanoscale powder [ 13]. Compared with the conventional liquid marbles, such marbles possess many extraordinary physical and electronic features with their high surface tension, native oxide layer, high density and electrical conductivity. However, due to the coating of nanoparticles, the liquid metal marble behaves like a soft solid, and thus somewhat loses deformability and liquidity of a fluid which is critical for some practical situations such as liquid metal jet cooling, printed electronics, 3D printing, metal droplet or particle fabrication, etc.

In this paper, an alternative way of making an entirely new liquid marble is proposed. The composite liquid metal marble is coated by liquid metal droplets with water film or more liquid candidates. Compared with the existing liquid marbles, the current composite liquid marbles have two kinds of fluid present distinctive characteristics, such as non-oxide layer, deformability and complete liquidity of fluid, easy to prepare and low cost.

Materials and methods

As is known, the conventional liquid marbles are generally obtained through rolling small liquid droplets in a highly hydrophobic powder. For aqueous liquid, the powder must be hydrophobic to prevent the particles from being immersed in the droplet [ 1]. Unlike such strategies, most liquids will not react nor fuse with liquid metal and hence are available to fabricate composite liquid metal marbles. This method significantly expands the range of fluid material that can be used as liquid marble coatings. Along this way, the composite liquid metal marbles have been prepared and their unique properties have been clarified. Particularly, the gallium-indium alloy (GaIn_24.5) is selected as the test material, which has a melting point of 15.5°C far below the room temperature. Its density, surface tension and dynamic viscosity are 6280 kg/m³, 0.624 N/m and 1.6956 × 10⁻³ Pa·s, respectively, at 25°C [ 14]. The details of the preparation of the material can be found in Ref. [ 15].

Figure 1 (a) and (b) presents schematic images of the liquid metal droplets at different conditions with or without a water coating layer, respectively. Gallium-based alloys tend to form oxide layer at the surface when exposed to the air, which prevents them from further oxidization. As has been reported, the surface tension of the GaIn25 pendant droplet at ambient condition is approximately 0.624 N/m [ 16]. Controlling of such physical property would allow the liquid metal to display a number of unique behaviors such as transformation or movement [ 17]. Figure 1 (c) to (e) depicts the fabrication process of the composite liquid metal marbles. A water droplet with a diameter of 4.77 mm was hung at the tip of a syringe needle by a stainless steel hypodermic needle. The liquid metal was then pumped into the water droplet slowly till the droplet began to fall down. As a result, the liquid metal droplet was entirely encapsulated in the water and hung at the needle point. In this way, a composite liquid metal marble can be created. After coated with water, the liquid metal droplet is absent of air so that the oxidization is avoided, which results in the unique properties of composite liquid metal marbles.

The basic behavior of the liquid marbles lies in their unusual physical and electronic properties, such as the capacity to remain the configuration under impact, to split into smaller marbles and to move quickly without any leakage due to large contact angle. To further reveal the physical properties of the composite liquid metal marbles, high speed visualization experiments on their dynamic impact behaviors on a substrate have been conducted.

Results and discussion

Many factors may influence the phenomena associated with droplet impact. In addition to the kinematic and fluid parameters of the droplets, such as droplet size, impact velocity and impact angle, surface tension, fluid viscosity and density, the impact surface such as roughness, wettability, and temperature should also be considered. For composite liquid metal marbles, the deformation, spreading and splashing of fluids depend not only on the above-mentioned factors, but also on the characteristics of the liquid coating, for example, the type of fluid and thickness of the coating layer.

In a typical experiment, composite liquid metal marbles, with diameters from 2.65 mm to 5.38 mm, were formed at the tip of a syringe needle and allowed to detach and fall under their own weight onto a stainless steel test surface. The needle was placed 459 mm above the test surface to obtain impact velocities from 1.27 m/s to 2.95 m/s. Photographs of impact droplets were taken via a high-speed camera (IDT, NR4.S3). The recorded high-resolution images (1016 × 1016 pixels, 3000 images per second) were transferred to a computer for later analysis. The image analysis software (Photron FASTCAM Viewer) was used to measure impact velocities and droplet dimensions as they were deformed.

To evaluate the dynamic characteristics of composite liquid metal marbles under impact after free fall, the behaviors of an uncoated liquid metal droplet were compared with a composite liquid metal marble. Figure 2 (a) and (b) illustrates the dynamic processes of a liquid metal droplet with native oxide layer and the impact of a composite liquid metal marble on the steel surface at the room temperature atmosphere, respectively. Figure 2 (a) indicates that there appears a tail at the end of the droplet until the impact occurs. This is because the oxide layer prevents the droplet from releasing the surface energy freely during the falling [ 15]. According to the variation of droplet diameter over the time, the dynamic process of the liquid metal droplet on the test surface can be divided into the initial, the spreading, the receding, and the oscillation stage, respectively. In the initial stage of the droplet impact, due to the effect of surface tension, the droplet could still maintain partly spherical. As the downward movement continued, the impact pressure made the droplet velocity turn sharply from the axial direction into the radial direction, thus a wall jet was generated. In the spreading stage, as a result of the Rayleigh-Taylor instability [ 18], fingers formed around the periphery of the droplet. Along with the increase of the diameter, fingers grew larger and the droplet gradually transformed into a lamella until it reached its maximum spreading diameter when t = 6.97 ms. After that, the surface tension and viscous forces overcame inertia and the fluid accumulated at the leading edge of the lamella when it was receding. Viscous dissipation finally led the droplet to achieve stability and obtain a rounded layer shape (t = 124.83 ms).

Figure 2 (b) depicts the deformation of a composite liquid metal marble with time from initial impact as indicated. Clearly, the presence of water coating significantly affected the observed properties. There was no tail at all and the composite liquid metal marble appears almost like a sphere during its free falling stages. This is because the oxidization was averted and the surface tension and attraction between metal and water molecules kept the marble spherical. At the beginning of the impact, the water coating touched the steel surface first and formed a water mat underneath the liquid metal, which considerably reduced the surface roughness and even insulated the liquid metal from the steel surface. The water coating also led to an increase in the number and size of fingers at the edge of the droplet. In the spreading stage, fingers were seen to form earlier, grow larger quickly, and their tips began to break off to generate small satellite droplets. The lamella reached its maximum spreading diameter earlier (t = 5.98 ms) and then began to recede, while the fingers continued to spread and finally split from the lamella. Under the effect of surface tension, the lamella was pulled back to the center, recoiled off the surface, and reached its maximum height when t = 28.55 ms. After 149.07 ms, the dissipation of energy brought the droplet into quiescence.

To observe more clearly the internal structure of the composite liquid metal marbles, a close-up of droplet impact was shot from level angle. Figure 2 (c) and (d) exhibit the side views of the collision in Fig. 2 (a) and (b), respectively. As shown in Fig. 2 (c), after the droplet landed on the surface, the fluid spurted from the bottom of the droplet and spread in all directions. When the initial energy was exhausted, the fluid formed a rounded lamella and adhered to the surface. As for the composite liquid metal marble (Fig. 2(d)), when it touched the surface, the water coating broke up first. Since the thickness of the water coating was relatively small, the collision between the liquid metal core and the steel surface soon occurred. In the process of spreading, the water covered the liquid metal all the time and isolated it from the air. Meanwhile, the water between the liquid metal and the steel surface diminished the friction and thus promoted the receding and recoiling of the droplet. In the end, the satellite droplets converged to a big droplet.

The effect of increasing droplet velocity on impact dynamics is evaluated in Fig. 3, which presents the impact of composite liquid metal marbles on the steel surface at a velocity of 2.95 m/s. It was observed that the spreading and splashing of the droplet were intensified and the spreading area increased. The number of fingers also dramatically increased so that more satellite droplets appeared.

In a composite liquid metal marble, the liquid metal preponderates over the water in volume and mass since the density of liquid metal is approximately six times that of the water. Therefore, the dynamic characteristics of composite liquid metal marbles are dominated by the liquid metal core. However, no recoil phenomenon occurred in the former experiments because the droplets were too big. To comprehensively investigate the dynamic characteristics of composite liquid metal marbles, further impact experiments of small marbles were also performed.

Figure 4 shows sequential images of the impact of liquid metal droplets and composite liquid metal marbles at smaller diameters and at a speed of 1.27 m/s (Fig. 4 (b)), 1.90 m/s (Fig. 4 (a), (c)) and 2.95 m/s (Fig. 4 (d)), respectively. Figure 4 (a) illustrates the deformation process of liquid metal droplet at a velocity of 1.90 m/s. Previous studies [ 19] have used the dimensionless time t* = tV₀/D₀ to replace the real time (t). Compared with Fig. 4 (c), one can find that the differences between the experimental results of liquid metal droplets and composite liquid metal marbles are much more significant. Without the increase in surface tension resulting from oxidization, the liquid metal marble recoiled off the surface and leaped into the air after being pulled back together by surface tension. Undergoing a vertically upward projectile motion, the droplet eventually rested on the surface when t* = 96.34. Figure 4 (b) to (d) shows the evolution of the configurations of composite liquid metal marbles with the increase of impact velocity. At a low speed of V₀ = 1.27 m/s (Fig. 4 (b)), the droplet reached its maximum wetting area after t* = 1.69. Then, the lamella was pulled back by the surface tension and produced a jet, which achieved its maximum height at t* = 14.72. With the jet rising, the neck became narrow due to the inward pull of the surface tension. Eventually, a small droplet was ejected from the tip of the jet. It is worth noting that this ejected droplet is spherical but not fusiform [ 15], which is believed to be related to oxidization. This proves that the water layer on the surface of the liquid metal efficiently prevents it from oxidization. When t* = 51.52, the droplet finally subsided and formed a rounded lamella. Increasing the impact velocity led to the increase in spreading area and the decrease in lamella thickness. The recoil of the droplet was also distinctly enhanced so that the entire droplet was lifted completely off the surface. At the highest speed of 2.95 m/s in the experiments (Fig. 4(d)), the droplets spread much quickly and the fingers could be seen earlier. More fingers detached as satellite droplets because of higher kinetic energy (t* = 6.41). At the same time, the work done by the viscous force was increased notably. Consequently, there was relatively less kinetic energy left for the liquid metal marble to recoil. The water coating was dragged by the liquid metal when it was pulled back toward the center and ejected under the effect of the great inertial of the liquid metal. A comparison of Fig. 4 (d) with Fig. 3 demonstrates the influence of the marble size on the number of satellite droplets. It was shown that bigger marbles would generate more small droplets after splashing. The population of the satellite droplets was approximately determined by the fingers formed around the lamella at its maximum extension. However, not every finger could produce a satellite droplet, and some fingers even released more than one droplet because the velocity distribution and surface roughness were neither uniform nor symmetric.

The extension of droplet can be quantified by the dimensionless diameter (also called spreading factor ξ=D/D₀), which is defined as the ratio of the measured diameter (D) to the initial diameter (D₀). Figure 5 presents the variation of the diameter of droplets with dimensionless time in the process of impact on the steel surface at 26°C at impact velocities from 1.27 to 2.95 m/s. Figure 5(a) depicts the diameter variation of a liquid droplet during the impact at a speed of 1.90 m/s. After the droplet reached the maximum diameter, the spreading factor basically remained constant because the droplets were adhered to the surface. When it comes to the impact of composite liquid metal marbles, with the increase of impact velocity (Fig. 5(b) to (d)), the maximum spread factor as well as the dimensionless time required for a marble to reach quiescence increased. Because of the rebound of the liquid metal core, the spreading factor was rapidly reduced after it came to a climax. Hence there is a peak in the curve of the spreading factor.

Conclusions

Composite liquid marbles produced with droplets of liquid metal coated with water film have been presented. Such metal droplet significantly improves its many physical capabilities such as surface tension modification and shape control. The unique differences between composite liquid marbles and the conventional liquid marbles were disclosed by impact dynamics experiments and high speed photography. The spreading factors were quantitatively measured from the photographs. The oxidation of the liquid metal surface was efficiently prevented by the water coating, thus generating spherical marbles through free falling. The water film also worked as lubricant which reduced the friction between the marbles and the substrate and accelerated the spreading. Increasing the impact velocity enhanced the spreading of marbles and increased the time required for a marble to reach the maximum spread factor. The marbles with higher velocity were more likely to splash. The number of the fingers and the satellite droplets formed at the edge of the broken marble increased with droplet velocity and size. The interaction between the water and the liquid metal, high surface tension and high density of liquid metal were the dominating factors that molded their extraordinary physical features. This finding is important for a variety of practical applications such as liquid metal jet cooling, inkjet printed electronics, 3D printing or metal particle fabrication, etc. It especially opens possibilities for the convenient production of liquid marbles. Such kind of matter also poses profound multiphase fluid mechanics with its complexity of free surface flow and interactions between different multiple phases. Further works should focus on the kinematic and fluid parameters of marbles, such as marble diameter, impact velocity and thickness of water coating, so that a comprehensive understanding of composite liquid metal marbles could be obtained. Besides, the high-speed images provide obvious evidence that there are some governing non-dimensional parameters which indicate the relative influence of the coating fluid and the dynamic parameters of the liquid system. In addition, the type of encapsulation material is also worth pursuing. For example, many small liquid metal droplets can easily be made by encapsulating the liquid metal droplet with surfactant which can prevent the satellite droplets from merging together. These features illustrate the significance of composite liquid metal marbles for future scientific research and technological applications.

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