Ultrasound induced grain refinement of crystallization in evaporative saline droplets

Abstract

We investigate the effect of ultrasound on the evaporation and crystallization of sessile NaCl solution droplets which were positioned in traveling or standing wave ultrasound field. The experimental results indicated that the ultrasound field can significantly accelerate the evaporation rate of the sessile droplets and refine the crystal grains. By adjusting the distance between the sessile droplets and the ultrasound emitter, it is found that, in traveling wave ultrasound field, the sessile droplet evaporation time and the time for the appearance of NaCl grains exhibited a fluctuating increase as the droplet-emitter distance increased. While in the standing wave ultrasound, the sessile droplet evaporation rate increases with the increasing droplet-emitter distance. Overall, the traveling wave ultrasound field has a stronger effect on grain refinement of the sessile droplets than the standing wave ultrasound field. The grain refinement is attributed to the decrease of critical nucleation radius caused by ultrasound energy and the increase of the nucleation rate caused by the accelerated evaporation rate. In addition, the breakage of grains caused by ultrasonic cavitation would also lead to grain refinement.

1. Introduction

As a crucial physical process, crystallization plays an important role in many industries including food processing [1], [2], pharmaceutical production [3], [4], [5], chemical manufacturing [6], [7] and advanced material preparation [8], [9], [10], where the controlled crystallization is highly desired. Refined crystal grains have many outstanding advantages in practical applications. For example, the tiny grains in medications facilitates the absorption by human body [11]. In the preparation of crystalline metal films, refining grain size can increase the driving force of grain growth, thus facilitating the crystallization process [12]. In addition, metal materials composed of fine grains exhibit good performance of strength, plasticity and toughness [13], [14], [15]. Based on the above advantages, grain refinement has aroused extensive research interest in various fields.

In recent years, ultrasonic crystallization has been proven to be an effective method to facilitate the crystallization process, which could result in uniform size distribution of the crystal products due to the prevention of aggregation [16], [17], [18], and refine the crystal grains [19], [20]. Mao et al. [21] and Yu et al. [22] investigated the effect of ultrasound on the crystallization process of lysozyme and found that ultrasound can significantly improve the uniformity of crystal size and reduce the crystal size. Amara et al. [23] have proved that the average crystal size of potash alum crystals decreases with the increase of ultrasonic power and the morphology of potash alum crystals was modified by ultrasound waves. Zhong et al. [24] studied the anti-solvent crystallization of sucrose under ultrasonic action and reported that ultrasound significantly reduced the average size of crystals, improved the nucleation rate and accelerated the crystallization process but had no impact on the crystal morphology, which may be related to the frequency, power and action time of ultrasound. In the previous studies, the mechanism of ultrasound on grain refinement is still not clear. Since NaCl is easy to crystallize and its regular shape facilitate the crystal size statistics, in this paper, NaCl was selected to explore the mechanism of ultrasound on grain refinement.

The common experimental approach for investigating the ultrasonic effect on the crystallization process is usually to immerse the ultrasound emitter into the solution [21], [22], [24] and employ stirring as an auxiliary method to refine the crystal grains [17], [23]. However, due to the energy dissipation of the emitter in liquid and the thermal effect caused by the acoustic cavitation and acoustic streaming, the immersed emitter often results in a significant increased local temperature [25], [26], which is unexpected to obtain crystals with uniform size distribution. To eliminate the temperature variation in the solution caused by the ultrasound emitter and facilitate the collection of small size crystals, in this work, we focused on the crystallization in sessile droplets in the ultrasound field of 20.5 kHz, which is a typical frequency often used in power ultrasonic treatment. The sessile droplets in ultrasound field would evaporate in a short time to produce easily accessible refined grains. Through analyzing the droplet evaporation time, the time needed for the appearance of NaCl grains and the crystal size distribution, revealing the mechanism of ultrasound on the evaporative crystallization and grain refinement in the sessile droplets.

2. Materials and methods

2.1. Preparation of NaCl solution

The NaCl solution was prepared by adding sodium chloride salt (NaCl, 99.5 %, AR, Aladdin Industrial Corporation, China) into water and stirring with a magnetic stirrer (HJ-2B, SHUANGXU, China) at a speed of 600 ∼ 700 r/min for 15 ∼ 20 min. The water used in the experiments is ultra-pure water (18.25 MΩ·cm) which has been processed by an ultra-pure water system (UPTA-20L, China).

2.2. Experimental setup and procedure

The experimental setup is shown in Fig. 1. In this study, the ultrasound wave was produced by a uniaxial acoustic generator (SonoRh-1, Nanjing Sonodrive Technology Co., Ltd., China). The power ratio was set as 20 %, thus the power applied during the experiment was about 200 W. The ultrasound emitter emitted a sinusoidal ultrasonic wave with a frequency of 20.5 kHz and a wavelength (λ) of 16.43 mm. In the absence of the reflector, there formed a traveling wave ultrasound field. When a concave reflector is positioned coaxially under the emitter, the superposition of the incident wave and the reflected wave generated a standing wave ultrasound field between the emitter and the reflector. By using such an approach, the acoustic streaming can be introduced into the research system, and moreover, the droplet-emitter distance can be adjusted to tune the sound intensity. 5 μL NaCl solution droplets (10 wt%) were deposited on a glass substrate by microsyringes, followed by exposure to the traveling or the standing wave ultrasound field separately for evaporation and crystallization. The size of the glass substrate is 20 mm × 20 mm × 0.2 mm.

Fig. 1.

Schematic diagram of the experimental setup.

A high-definition CCD video microscope (GP-640S, GAOPIN, Shenzhen, China) was utilized to observe the entire evaporation process at real-time and record both the total evaporation time (t_E) of droplets and the time needed (t_G) for the appearance of NaCl grains. The acoustic streaming in the ultrasound field was observed by using a continuous sheet laser (MGL-N-532A-5 W, Changchun New Industries Optoelectronics Co., Ltd., China) to illuminate the water mist introduced in the sound field [27]. Under continuous laser illumination, the water mist was introduced into the sound field as tracer particle to visualize the acoustic streaming. When taking videos or photos, the CCD camera and the laser device were situated in the same horizontal plane, with the lens of the CCD camera oriented perpendicular to the direction of laser illumination. The use of sheet laser irradiation on droplet facilitates the precise recording of the time at which NaCl grains appear. The morphology and size of NaCl crystals were characterized by a field emission scanning electron microscope (FESEM, Zeiss Supra 55, Germany). The crystal size measurement and crystal size distribution statistics were achieved by Image-Pro Plus 6.0 and Origin 2022. The experimental temperature was maintained at 25 ± 2 °C and the relative humidity (RH) was controlled within a range of 35 ∼ 40 %.

2.3. Sound field simulation

COMSOL Multiphysics 6.0 was used to simulate and analyze the sound field. The uniaxial acoustic levitation used in this experiment met the requirements of spatial axisymmetric geometry and boundary conditions, so a two-dimensional axisymmetric model was used to calculate the sound field. The simulation domain was meshed by free triangular with extremely fine user-predefined size.

The emitter was set as the rigid boundary condition, and the side well was defined as a perfectly matched layer to simulated the absorption of sound waves as they travel away from the sound source. The bottom boundary of the simulation domain was defined in two ways. For the case with concave reflector, the bottom was set as the rigid boundary condition, and the standing wave sound field was formed in the simulation domain (Fig. 2(a)). The other was set the bottom as perfectly matched boundary condition, then simulated the traveling wave sound field without the reflector (Fig. 2(b)). The boundary acoustic pressure was restrained by the Helmholtz equation:

$$\mathrm{\nabla }\bullet \left(-\frac{1}{{\rho }_0}\mathrm{\nabla }p\right)-\frac{{\omega }^{2}p}{{\rho }_0{c}_0^2}=0$$ (1)

where p is the sound pressure, ${\rho }_0$ (=1.18 kg/m³) is the density of air, $\omega$ is the angular frequency, and $c_0$ (=346.12 m/s) is the speed of sound in air.

Fig. 2.

Simulation domain for (a) standing wave ultrasound field and (b) traveling wave ultrasound field.

The normal acceleration in the pressure acoustic frequency domain module was used as the initial condition of the emitter:

$$n\bullet \left[\frac{1}{{\rho }_0}\left(\mathrm{\nabla }p\right)\right]=a_0$$ (2)

where $a_0$ (=0.79 × 106 m/s²) is the normal acceleration. For simplicity, the material of the drop was assumed to be water (ρ = 998.2 kg/m³, sound velocity of water c_water = 1495.33 m/s) because the acoustic impedance mismatch between air and all of the liquids used in the experiments was similar.

According to the above conditions, the sound pressure distribution of the sound field was obtained. By extracting the sound pressure value on the symmetry axis of the sound field, the position coordinates of each sound node and antinode could be obtained, which was convenient to determine the position of the droplet. After obtaining the sound pressure distribution of the sound field at different positions of the droplet, the distribution the sound radiation pressure P_A of the droplet surface was obtained according to King theory [28]:

$$P_A=\frac{1}{2{\rho }_0{c}_0^2}\langle p^2\rangle -\frac{1}{2}{\rho }_0\langle v^2\rangle$$ (3)

3. Results and discussion

As ultrasound waves, traveling waves and standing waves are distinctly different in energy storage and propagation. Traveling wave is accompanied by energy transfer during the propagation process, while standing wave has no energy transfer. Sessile droplets in different types of ultrasound fields would be affected by distinct forms of stimulus, and in turn various crystallization process. This inspired us to study the evaporation and crystallization of droplets in traveling and standing wave ultrasound fields respectively.

3.1. Evaporation and crystallization time in ultrasound field

Under natural conditions without an ultrasound field, the total evaporation time of a NaCl solution droplet (τ_E*) was recorded as 29.5 min and the time for the appearance of NaCl grains (τ_G*) was 20.5 min. In traveling wave ultrasound field, the sessile droplet evaporation time (τ_E) and the time needed for the appearance of NaCl grains (τ_G) exhibited a significant reduction compared to that of the natural conditions (Fig. 3). This indicated that traveling wave ultrasound field could increase the droplet evaporation rate and promote the formation of crystals. Furthermore, the promotion effect is stronger at the position $h=n\lambda /2$ (n = 1, 2, 3….…..) than that at its adjacent position $h=n\lambda /4$ (n = 1, 3, 5, 7......).

Fig. 3.

The total evaporation time (τ_E) of the sessile droplets (10 % NaCl, 5 μL) and the time for the appearance of the NaCl grains (τ_G) in traveling wave ultrasound field. Note: τ_E* and τ_G* respectively represents the total evaporation time and the time needed for the appearance of the NaCl grains under natural conditions without ultrasound field. h is distance between ultrasound emitter and droplet, λ is the wave length.

In standing wave ultrasound field, the total evaporation time of sessile droplets (τ_E) decreases with the increasing distance from sessile droplets to the ultrasound emitter (Fig. 4). It suggested that the evaporation rate of sessile droplets was increased with the increasing distance between droplet and the ultrasound emitter. The time (τ_G) needed for the appearance of NaCl grains in the standing wave ultrasound field was decreased by an average of 77.6 % which indicated that the standing wave ultrasound field promoted the formation of crystals. Fig. 4 showed that the facilitation of the standing wave ultrasound field on crystallization is insensitive to the droplet-emitter distance.

Fig. 4.

The total evaporation time (τ_E) of the sessile droplets (10 % NaCl, 5 μL) and the time needed for the appearance of the NaCl grains (τ_G) in standing wave ultrasound field.

The acceleration of droplet evaporation rate by ultrasound field is mainly caused by the acoustic streaming, because the boundary layer acoustic streaming around the droplet could significantly enhance the heat and mass transfer during evaporation [29], [30], [31]. The time for the appearance of NaCl crystals was advanced in ultrasound field mainly because the sessile droplets would obtain additional ultrasonic energy that enhance the energy fluctuation in the solution to enable the formation of crystal nuclei at smaller radius, which is conducive to nucleation [32], [33]. Meanwhile, the accelerated evaporation caused by the acoustic streaming would lead to increased concentration which is also favorable to the formation of nuclei.

3.2. Drying patterns in ultrasound field

As shown in Fig. 5, when the sessile droplets have evaporated in the ultrasound field, different kinds of drying patterns were formed. In traveling wave ultrasound field, the sessile droplet evaporated to form a ring-like drying pattern at the position $h=n\lambda /2$ (n = 1, 2, 3….…..) (Fig. 5(a)). While at the position $h=n\lambda /4$ (n = 1, 3, 5, 7......), NaCl crystals accumulated at the center of the drying pattern (Fig. 5(b)). In standing wave ultrasound field, the NaCl crystals were observed at the right edge of the drying pattern (Fig. 5(c)) at wave nodes and at the left edge of the drying pattern (Fig. 5(d)) at wave antinodes.

Fig. 5.

Drying patterns of sessile droplets in ultrasound field. (a) and (b) correspond to the patterns at the position $h=3\lambda /2$ and the position $h=7\lambda /4$ in traveling wave ultrasound field; (c) and (d) correspond to the patterns at wave node and wave antinode in standing wave ultrasound field. Note: The white dashed lines represent the edges (contact lines) of the initial sessile droplets.

The difference in the drying patterns may be caused by the various internal flows of the droplets induced by the external acoustic streaming arising from the nonlinear effect of high-frequency (f > 4000 Hz) sound waves [34], [35]. In traveling wave ultrasound field, the acoustic streaming generated near the surface of the droplet, which showed an anticlockwise circulation on the left side at the position $h=n\lambda /2$ (n = 1, 2, 3….…..) and a clockwise circulation on the right side at the position $h=n\lambda /4$ (n = 1, 3, 5, 7......) (Fig. 6(a) and movie S1). At the position $h=n\lambda /2$ (n = 1, 2, 3......), the acoustic streaming induced the flows inside the droplet (Fig. 6(a1)) which would transport small NaCl grains to the contact line, eventually forming a ring-like drying pattern (Fig. 5(a)). At the position $h=n\lambda /4$ (n = 1, 3, 5, 7......), the direction of acoustic streaming (Fig. 6(b) and movie S1) was opposite to that at the position $h=n\lambda /2$ (n = 1, 2, 3......) (Fig. 6(a)), in turn, the internal flow of the droplet (Fig. 6(b1)) was also opposite to that of the droplet at the position $h=n\lambda /2$ (n = 1, 2, 3......) (Fig. 6(a1)). Although crystals first appeared at the contact lines owing to faster evaporation rate, the formed crystals may migrate towards the center (Fig. 5(b)) driven by the internal flow of droplet (Fig. 6(b1)).

Fig. 6.

The external acoustic streaming around the droplet surface and the corresponding internal flows. (a), (a1) and (b), (b1) correspond to the external acoustic streaming and internal flows at the position $h=n\lambda /2$ (n = 1, 2, 3….…..) and the position $h=n\lambda /4$ (n = 1, 3, 5, 7......) in traveling wave ultrasound field; (c), (c1) and (d), (d1) correspond to the external acoustic streaming and internal flows at wave node and wave antinode in standing wave ultrasound field.

In standing wave ultrasound field, the acoustic streaming around the sessile droplet surface was approximately a horizontal flow. The direction of the horizontal flow was leftward at wave nodes and rightward at wave antinodes, as shown in Fig. 6(c), movie S2 and Figure (d), movie S3. This acoustic streaming induced a unidirectional circulation in the sessile droplet (Fig. 6(c1) and (d1)), resulting in the crystals accumulating at the right edge of the drying pattern at wave node (Fig. 5(c)) and at the left edge of the drying pattern at wave antinode (Fig. 5(d)).

In ultrasound field, the internal flow of sessile droplet during evaporation includes not only the capillary compensation flow driven by evaporation, but also the flow induced by external acoustic streaming. For mass transfer from a droplet surface to a relatively strong forced convection, the effect of internal liquid circulation within the droplet on the acoustic streaming velocity and the mass transfer rate can be disregarded [36]. Therefore, it can be considered that the internal flow of the sessile droplet was mainly induced by acoustic streaming.

3.3. Grain refinement

The SEM images and crystal size distribution showed that, at the position h/λ = 3/2 in traveling wave ultrasound field, the sessile NaCl solution droplet evaporated to form a large number (∼400) of rectangular NaCl crystals with the size ranging from 5 to 35 μm (Fig. 7(a)). While, at the position h/λ = 7/4, there formed a smaller number (∼40) of NaCl crystals with a size ranging from 100 to 600 μm (Fig. 7(b)). Compared to NaCl crystals (1000 ∼ 1500 μm) formed by the natural evaporation of sessile droplets on the glass substrate (Fig. 8(a)), the traveling wave ultrasound field increased the crystal quantity and decreased the crystal size. As calculated from Fig. 8(b), the average crystal size is about 35.8 μm at the position $h=n\lambda /2$ (n = 1, 2, 3….…..) and 240.1 μm at the position $h=n\lambda /4$ (n = 1, 3, 5, 7......) in the traveling wave ultrasound field. The average crystal size at the position $h=n\lambda /4$ (n = 1, 3, 5, 7......) was approximately 7 times larger than that at the position $h=n\lambda /2$ (n = 1, 2, 3......), which indicated that the effect of grain refinement at the position $h=n\lambda /2$ (n = 1, 2, 3......) was much stronger than that at its adjacent position $h=n\lambda /4$ (n = 1, 3, 5, 7......) in the traveling wave ultrasound field.

Fig. 7.

Morphology and crystal size distribution at (a) the position $h=n\lambda /2$ (n = 1, 2, 3….…..) and (b) the position $h=n\lambda /4$ (n = 1, 3, 5, 7......) in the traveling wave ultrasound field.

Fig. 8.

(a) Individual NaCl crystal formed after natural evaporation of a sessile droplet; (b) The distribution of the average crystal size at the position $h=n\lambda /2$ (n = 1, 2, 3….…..) and the position $h=n\lambda /4$ (n = 1, 3, 5, 7......) in traveling wave ultrasound field. d₀ represents the size range of crystals formed after the natural evaporation of the sessile droplets (10 wt% NaCl, 5 μL).

In the standing wave ultrasound field, the crystal size also varies at wave node and wave antinode (Fig. 9(a) and (b)). The average crystal size distribution (Fig. 9(c)) showed that the crystal size decreased with the increasing distance between the sessile droplets and the ultrasound emitter (the minimum was about 95 ∼ 100 μm). Overall, compared with the crystals formed by natural evaporation of droplets, ultrasound field reduced the grain size and played a role in grain refinement.

Fig. 9.

Morphology and crystal size distribution for evaporation in the standing wave ultrasound field at (a) wave node and (b) wave antinode; (c) The distribution of the average crystal size as function of h/λ in the standing wave ultrasound field.

The ultrasonic energy may be the main factor for grain refinement. In ultrasound field, the sessile droplets would obtain the ultrasonic energy when it was exposed to ultrasound field, which reduced the r_c during crystallization. The small r_c facilitates the formation of a large number of tiny NaCl nuclei during crystallization. Crystal growth involves diffusion from the bulk of the solution to the crystal surface and the reaction on the crystal surface [37]. Due to the limited amount of solute in the sessile droplet, these large number of small nucleation points restricted the rapid growth of crystal grains [19], thus forming lots of small-sized crystals after evaporation.

To understand the different effects of traveling and standing wave ultrasound field on droplet evaporation and crystallization, the acoustic pressure in the ultrasound field and the acoustic radiation pressure on the surface of the sessile droplets were calculated. In the traveling wave ultrasound field, the distribution of acoustic pressure were shown in Fig. 10(a) and (b) when the sessile droplets were at the position $h=3\lambda /2$ and $h=7\lambda /4$. The acoustic radiation pressure P_A on the entire droplet surface were shown in Fig. 10(c) and (d). The simulated calculation (Fig. 10(e)) indicated that the acoustic radiation pressure on the droplet surface at the position $h=3\lambda /2$ was much stronger than that at the position $h=7\lambda /4$. At positions with higher acoustic radiation pressure, a stronger acoustic streaming may be generated around the droplet. This could be supported by the shorter evaporation time of droplets in positions with higher acoustic radiation pressure.

Fig. 10.

Acoustic radiation pressure in traveling wave ultrasound field. Distribution of acoustic radiation pressure in the ultrasound field when the sessile droplet was positioned at the position (a) $h=3\lambda /2$ and (b) $h=7\lambda /4$; Distribution of acoustic radiation pressure in the sessile droplet at the position (c) $h=3\lambda /2$ and (d) $h=7\lambda /4$; (e) Acoustic radiation pressure on the surface of the sessile droplets. Notes: D is the droplet-emitter distance; R is the distance in the direction of the sessile droplet radius.

In the standing wave ultrasound field, the distribution of acoustic radiation pressure was different when the sessile droplet was deposited at the wave node and wave antinode (Fig. 11(a) and (b)). The acoustic radiation pressure in the entire droplet was shown in the inset images of Fig. 11(c) and (d)). The acoustic radiation pressure on the droplet surface at wave node and wave antinode was almost the same (see the red curve in Fig. 11(c) and (d)). This indicated that the energy density near the droplet surface at wave nodes and wave antinodes was almost the same. Therefore, the time needed for the appearance of the NaCl grains was almost unaffected by the droplet-emitter distance (see the red curve in Fig. 4).

Fig. 11.

Acoustic radiation pressure in standing wave ultrasound field. Distribution of acoustic radiation pressure in the ultrasound field when the sessile droplet was positioned at the position (a) $h=3\lambda /2$ and (b) $h=7\lambda /4$; The acoustic radiation pressure in the sessile droplet and on the surface of the sessile droplet at the (c) wave node and (d) wave antinode. Notes: D is the droplet-emitter distance; R is the distance in the direction of the sessile droplet radius.

Acoustic streaming promoted the droplet evaporation and increased the nucleation rate, while the bulk acoustic wave produced ultrasonic cavitation effect inside the droplet. On the one hand, the ultrasonic cavitation bubbles can serve as nucleation sites, further facilitating the formation of crystal nuclei. On the other hand, when the cavitation bubbles accumulate sufficient acoustic energy to explode, there would generate a localized high temperature and high pressure in their vicinity [38], which can fragment the crystal grains and produce more small grains, thereby effectively refining grain size [22], [33]. For clarity, the schematic in Fig. 12 list the possible causes behind grain refinement.

Fig. 12.

Schematic for possible causes behind grain refinement.

While refining the NaCl crystal grains, ultrasound wave also destroyed the original morphologies of the crystals. In the traveling wave ultrasound field, cavities were generated in the crystals, with the dimensions of 6 ∼ 10 μm at the position $h=n\lambda /2$ (n = 1, 2, 3….…..) (Fig. 13(a)) and 80 ∼ 100 μm at the position $h=n\lambda /4$ (n = 1, 3, 5, 7......) (Fig. 13(b)). In the standing wave ultrasound field, irregular craters were formed on surface of crystals, with the dimensions of 100 ∼ 300 μm (Fig. 13(c) and (d)). These cavities and craters in crystals may be caused by ultrasonic cavitation effect. In this experiment, there exist many tiny bubbles (cavitation nuclei) in the solution due to the stirring during the solution preparation. These microbubbles in sessile droplet were activated by ultrasound and experienced a dynamic process including oscillation, growth, contraction and explosion, that is, ultrasonic cavitation effect [39], [40], [41].

Fig. 13.

The morphologies of the NaCl crystals formed after the evaporation of sessile droplets in ultrasound fields. (a) At the position $h=3\lambda /2$ and (b) and the position $h=7\lambda /4$ in traveling wave ultrasound field; (c) At wave node and (d) at wave antinode in standing wave ultrasound field.

4. Conclusions

This work investigated the effect of ultrasound field on the evaporation and crystallization of sessile NaCl solution droplets, i.e., in traveling and standing wave ultrasound field respectively. The results indicated that in traveling wave ultrasound field, the sessile droplet evaporation rate at the position $h=n\lambda /2$ (n = 1, 2, 3….…..) was faster than that at the position $h=n\lambda /4$ (n = 1, 3, 5, 7......). While in the standing wave ultrasound field, the sessile droplet evaporation rate increased with the increase of droplet-emitter distance. In traveling wave ultrasound field, the NaCl crystal size has been reduced from 1000 ∼ 1500 μm (under natural condition) to 5 ∼ 35 μm, while in standing wave ultrasound field, the crystal size was reduced to about 100 μm. This demonstrated that the traveling wave ultrasound field exhibited a more significant effect on grain refinement than standing wave ultrasound field. The grain refinement is mainly due to the additional ultrasonic energy obtained by the sessile droplets from the ultrasound field during crystallization, which reduced the critical nucleation radius r_c and increased the nucleation rate. Meanwhile, the external acoustic streaming accelerated the droplet evaporation rate, resulting in a shortened crystal growth time. These allowed the sessile droplets to form smaller crystals after drying in the ultrasound field. This study provides a reference for grain refinement and evaporation rate control for saline droplets.

CRediT authorship contribution statement

Xiaoqiang Zhang: Writing – original draft, Investigation, Formal analysis, Data curation, Conceptualization. Hongyue Chen: Writing – review & editing, Formal analysis. Yuhan Wang: Formal analysis. Xin Gao: Formal analysis. Zhijun Wang: Formal analysis. Nan Wang: Formal analysis. Duyang Zang: Writing – review & editing, Supervision, Conceptualization.

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.

Appendix A. Supplementary data

The following are the Supplementary data to this article: