1 Introduction
In the 1920s and 1930s, microchemical research based on microreactions began to attract wide attention
[1]. In 1923, Fritz Pregl from Austria won the Nobel Prize in Chemistry for his research on microanalysis in inorganic chemistry. In 1935, the American Microchemical Association was formally established. Today, microreaction systems are not only used for chemical manipulation and analysis, but also widely used in materials, biology, medicine and other fields. Compared with the reaction system based on large containers such as beakers and flasks, the micro-reaction system has many advantages, such as less reagents and energy, less toxic and harmful substances, larger specific surface area, faster heat and mass transfer, and more accurate control of the reaction
[2⇓⇓~5]. Generally speaking, the core component of the microreaction system is a solid micropipeline system containing microfluids, that is, a microreactor. With the development of the study of non-wetting phenomena, it has been found that droplet systems with spherical-like shapes can also be used for microreactions. Since conventional solid wall confinement is not required, these systems can be called self-supporting liquid microreactors. Among them, spherical droplet microreactors on superhydrophobic surfaces have been successfully applied in chemical reactions, cell culture, and other fields
[6⇓~8]. Compared with the spherical droplet in the wetted state on the general surface, the spherical droplet in the non-wetted state can well avoid the mass loss of liquid reactants and products during transportation, and can also largely avoid the siltation of particles in the liquid on the substrate. However, spherical droplets directly exposed to air (bare droplets) will face many problems in practical applications, such as too fast evaporation leading to short life, long solid-liquid contact leading to the destruction of non-wetting state, and so on.
In 2001, a spherical droplet with hydrophobic particles attached to its surface, liquid marble (LM), was reported in Nature
[9]. As the "armor" of the internal droplet, the surface particle shell can act as a barrier, so that the droplet can not be in direct contact with the substrate, so that the liquid marble can remain in a non-wetting state on any solid surface, and can even float stably on the liquid surface
[10⇓~12]. Compared with bare droplets, liquid marbles have many advantages as microreactors. In terms of preparation, the formation of spherical bare droplets needs to obtain superhydrophobic surface first, and the preparation of superhydrophobic surface is more complex. Liquid marbles, by contrast, can be obtained by spreading a layer of hydrophobic powder on any substrate and rolling droplets back and forth on it. In terms of stability, liquid marbles will not soak the substrate even if they are placed for a long time. In addition, the particles attached to the surface may play a role in slowing down the evaporation of droplets
[13]. More importantly, surface particles can also endow the droplet system with more functionality and operability, so people have prepared stress liquid marbles such as magnetic response marbles, light response marbles and acid-base indicator marbles, which greatly broaden the application range of the droplet system
[14⇓~16][17⇓~19][20⇓⇓~23]. In the past two decades, liquid marbles have been used in many research fields, including cell culture, sensors, material preparation, viscous substance storage, and so on
[24][25⇓~27][28⇓⇓⇓~32][33][34]. In addition, the use of liquid marbles in the field of fundamental research related to interfaces and droplets has also increased awareness of issues such as effective surface tension and droplet bouncing dynamics
[11,35⇓⇓~38][39,40].
However, although liquid marbles have many applications, their limitations can not be ignored. For example, a single sphere-like shape tends to limit experimental means and functional design. In fact, as far as shape details are concerned, even without considering the surface roughness caused by particle attachment, the contour of liquid marbles often deviates from the symmetrical and smooth spherical shape.Locally, it shows sharp corners and straight lines (Fig. 1-a), but this kind of shape deviation has little effect on specific applications, so this phenomenon and related mechanisms have not attracted people's attention for a long time
[38]. In fact, the phenomenon that particles attach to the fluid interface and cause interface deformation does not only occur in liquid marbles, which are droplets with gas-liquid interface. As early as 2005, Subramaniam et al. Found that when bubbles or oil droplets in water are wrapped by particles, the bubbles or oil droplets can take on stable and complex shapes by increasing the packing density of particles to produce interface blockage (Fig. 1-b)
[41]. Interface blockage can cause interface deformation, which is also the reason for the shape deviation of liquid marbles. In the process of preparation and evaporation of liquid marbles, random interface blockage is easy to occur. In the blocked state, the particles can not move, and the surface of the marbles loses its fluidity and rigidity, so it will produce and maintain solid morphological characteristics such as tips and straight lines. As mentioned above, such local morphological features are not important at the application level, however, the understanding that solid features arise from interface clogging has led to the conscious use of interface clogging to achieve holistic complex shape design.
In 2013, Zhao yuanjin et al. Reported that two liquid marbles could be blocked by collision and coagulation, thus forming a dumbbell-shaped complex liquid (Fig. 1-c), which was probably the beginning of the research on liquid shaping from conventional powder liquid marbles
[42]. However, in the next three or four years, little research was reported in this direction. In today's view, this is mainly due to the difficulty of liquid shaping with common large-sized powder particles, and the lack of arbitrary and precise shaping means lack of practical value. However, powder particles are not the only option for liquid shaping. Also in 2013, Li Xiaoguang et al. reported a phenomenon of liquid shaping by squeezing and rubbing bare droplets with a superhydrophobic film as a tool (Fig. 1-d), and in subsequent studies, they clarified that the cause of this phenomenon was that the outermost particles of the film were transferred to the surface of the droplet and the interface was blocked
[43][44]. Because the particle shell transferred to the droplet surface is composed of a single layer of particles with a size of only ~ 20 nm, it lays the foundation for precise shaping, and also makes the transparency and appearance of the deformed droplet very close to that of pure liquid. In addition, such a small size severely weakens the shielding effect of the particles, which makes the two deformed droplets prone to coalescence even when they are slowly approaching each other (Fig. 1-e). Taking advantage of this characteristic, the research group established a Liquid shaping technology based on the "splicing" strategy to realize the arbitrary design of liquid shapes (Fig. 1-f), and proposed the concept of "Liquid Plasticine" (LP) to name the resulting complex-shaped liquids
[44]. After that, a series of studies were carried out on the preparation, characteristics and functional applications of liquid plasticine, a liquid precise shaping method based on the "partition" strategy was established, a liquid plasticine sensor capable of reflecting the gas diffusion rate was designed, and a miniaturized and multi-functional liquid plasticines protein analysis platform was constructed
[45][46][47]. At the same time, more and more researchers have begun to pay attention to this direction. On the one hand, they have continued the preparation route of single-layer nanoparticle structure liquid plasticine with superhydrophobic film as the particle source, on the other hand, they have devoted themselves to the preparation of liquid plasticine with more readily available particle sources such as powder
[48]. At present, the main particle sources reported are nano-silica powder, micro-stearic acid powder, micro-elemental sulfur powder and millimeter-sized polymer flake stack
[49][50][51][52,53].
It should be pointed out that there is a problem of inconsistent use of concepts in the literature on liquid molding. Because both liquid plasticine and liquid marbles are complex liquids with particles attached to their surfaces, and liquid marbles have been studied earlier and more extensively, some researchers regard liquid plasticine as a kind of liquid marbles and call it liquid marbles directly, or add some modifiers about the shape before "liquid marble". However, a close examination of the concept of liquid marbles shows that it is contradictory to refer to liquid plasticine as liquid marbles or liquid marbles in a specific form. People are familiar with the core feature of liquid marbles-droplets surrounded by particles in air, but seldom consider the origin of the name. The concept of Liquid Marbles was first proposed in a paper published under the title of "Liquid Marbles" in 2001. When the concept was proposed, it was clearly stated that the droplet volume used was 1 ~ 10 μL
[9]. In this volume range, the shape of the non-wetting droplet is almost not affected by gravity and appears spherical. Judging from this, the author's use of the word "marble" is to consider the spherical factor. As the volume increases, the shape of the "liquid marble" becomes flat, but it still resembles a sphere, so it is reasonable to continue to call it a liquid marble based on the principle of simplification. However, some people have already realized this shape problem and used "liquid puddle" instead of "liquid marble" to describe the particle-droplet system when the volume is so large that the curvature of the top of the droplet is much smaller than that of the two sides
[54,55]. On the contrary, when the size of the liquid marbles is reduced to the micron scale and piled up in large quantities to form a powder with extremely high water content and good fluidity, people used to name the micro liquid marble cluster as "dry water". As for dry water, its technology sprouted in the 1960s and was officially named in the 1970s (much earlier than liquid marbles). At present, it has been applied in cosmetics, fire extinguishing agents and other fields. For its history and development status, please refer to the recent review
[56,57].
The above facts show that even if the structure of things is essentially the same, when their characteristics are obviously different, it is a common phenomenon driven by the pursuit of rigorous scientific concepts. Compared with the volume-dominated oblate spherical system, the liquid system with complex shape is more contradictory to the spherical connotation of the term "liquid marble", while "liquid plasticine" can well reflect the complexity and plasticity of the shape. Therefore, we believe that it is necessary to distinguish the two concepts of "liquid marble" and "liquid plasticine". It should be added that for some liquid plasticine with complex shape or large size, it is inappropriate to use "droplet system" or "complex droplet" to describe it, because the word "droplet" itself also contains specific shape factors, so this paper uses "liquid system" or "complex liquid" to describe it in the corresponding place.
In view of the fact that the establishment and development of liquid plasticine technology is mainly based on the single-layer nanoparticle structure liquid system, at the same time,The liquid plasticine of this system has obvious advantages in transparency, ease of preparation and accuracy of shaping. The following will review the liquid plasticine of monolayer nanoparticle structure and other types of liquid plasticines.
2 Monolayer nanoparticle structured (mNPc) liquid plasticine
2.1 Liquid surface covered by a monolayer of nanoparticles
The fundamental reason for the formation of liquid plasticine is surface solidification, which is usually caused by particle clogging. Obviously, the smaller the particle size, the smoother the liquid surface and the more precise the shape shaping. As the most commonly used particle source, powder naturally has a problem of particle agglomeration, and the smaller the particles, the greater the degree of agglomeration, and the more uneven the particle size. Therefore, although the average particle size of the powder composed of nanoparticles is smaller than that of the micron-sized powder, it can only reach the submicron level, which leads to a large roughness of the composite interface formed even when the powder source nanoparticles with the smallest initial particle size are used to coat the droplet, as can be seen from the environmental scanning electron microscopy image (Fig. 2-a)
[58]. By using the superhydrophobic film with weak binding force as the particle source, the transfer of monolayer nanoparticles can be realized, and the size of particles that can be attached to the surface of droplets in air environment can be directly promoted to the level of 20 nm, which lays an important foundation for obtaining complex and precise liquid shapes. It is worth mentioning that due to the small size, the particles can not be observed even by using the environmental scanning electron microscope, but only the wrinkles caused by the particle blockage (Fig. 2-b), which is in sharp contrast to the liquid surface covered by the nanoparticles derived from the powder
[59].
2.2 Preparation and principle of mNPc liquid plasticine
In the report of 2013, the author not only demonstrated that the bare droplet could be permanently deformed by squeezing it with a plating tool (Fig. 1-d), but also demonstrated that the deformed droplet could be further deformed by pulling, picking, rubbing and other operations (Fig. 3A, B). This kind of liquid shaping based on single droplets is the germination of liquid plasticine technology, but at that time, the mechanism of droplet deformation was not clear
[43]. Through collaborative research, the authors reported in 2016 that the cause of this phenomenon was that during extrusion, the outermost particles on the surface of the film were transferred to the surface of the droplet and the interface was blocked
[44]. The reason for the occurrence of monolayer particle transfer is that the bonding force between the outermost layer and the second outer layer of the sol-gel film is very weak, which is determined by the interface effect in the process of evaporation-induced film formation. The reason why the interface is blocked is that in the process of removing the pressure, the droplet retracts under the action of surface tension, and the surface area decreases, so that the packing density of the particles transferred to the surface of the droplet increases, and when the density increases to a certain extent, the surface is blocked (Fig. 3-c). In this report, the method of particle transfer and droplet deformation by extrusion is officially named Imprinting method.
It should be pointed out that the whole surface of the droplet does not need to be covered with particles for stable deformation, and local blockage can cause deformation, which can be reflected by some typical droplet shapes. For example, although the droplets in Fig. 3A and B are obviously non-spherical, they still maintain the shape characteristics of bare droplets. The author's report in 2016 gives more intuitive evidence. This work uses laser confocal technology to solve the problem that the particles on the liquid surface are too small to be observed, and finds that there are often exposed areas on the surface of the deformed droplet (Fig. 3-D), which directly confirms that local blockage can cause droplet deformation. In addition, the work also explains why partially-jammed particles do not separate from each other in the case of an empty peripheral region through mechanical and energetic analysis. In short, the interaction between two nanoparticles on the liquid surface is mainly determined by van der Waals attraction and electrostatic repulsion, and there is an energy barrier when the distance between the particles is a few nanometers (Fig. 3-e). This means that when the particle spacing is reduced to the left of the barrier position due to the retraction of the liquid surface, it is difficult for the two particles to separate again
[44].
The deep understanding of droplet deformation mechanism has promoted the regulation of single droplet morphology, and also opened the research of liquid shaping based on multiple droplets. It is found that the clogging degree and shape characteristics of the surface of a single droplet can be adjusted by injection/extraction (Fig. 3-f), and that the coalescence of multiple deformed droplets can inhibit the flow of liquid due to the sufficient particle density at the junction, so that the fusion body can reflect the shape of each original droplet (Fig. 1-f). It is these discoveries that led to the establishment of liquid shaping technology based on splicing method and the concept of liquid plasticine.
Subsequently, the preparation of mNPc liquid plasticine was improved by the establishment of the "partition method"
[45]. Compared with the process from small to large of the splicing method, the process of the segmentation method is characterized by the process from large to small. Specifically, a superhydrophobic film with weak binding force is deposited in a container such as a Petri dish, and then a large volume (usually on the order of milliliters) of liquid is poured into it to form a liquid cake, which is then shaken. During shaking, the viscous force between the liquid cake and the substrate is large, so the liquid cake will deform and the surface area will increase during shaking. After the shaking is stopped, the liquid cake retracts under the action of surface tension, and the surface area is reduced, resulting in the blockage of particles transferred to the surface of the liquid cake during the shaking process (Fig. 4-a). At this time, hydrophobic tools such as plastic droppers and pencil leads can be used to divide and shape the liquid cake according to the target shape (Fig. 4b-c). Because of the large contact area with the substrate and the fine wrinkles on the surface due to particle blockage, the liquid cake hardly moves due to the large static friction when the hydrophobic tool crosses, which provides an important guarantee for accurate and rapid shaping. If part of the liquid is extracted from the blocked liquid cake to enhance the degree of surface blockage, it is more conducive to subsequent shaping. Although the height and transparency of the liquid plasticine obtained in this way are low and the surface is not smooth enough, the shape and transparency can be optimized by subsequent liquid addition (Fig. 4-d)
[45].
Considering that the blocked liquid cake can be used as the parent of the complex shape liquid plasticine, we think it is necessary to call it the cake shape liquid plasticine. Starting from the cake-shaped liquid plasticine, the combination of the splicing method and the segmentation method is very beneficial to the construction of large-sized liquid plasticine with complex shapes (Fig. 4E, f).
The above is the preparation idea, method and principle of mNPc liquid plasticine. The formation of this kind of liquid plasticine is based on the use of superhydrophobic film with weak binding force. At present, the superhydrophobic film with weak binding force, which is composed of several layers of SiO
2 nanoparticles, is the only particle source that can be used to prepare mNPc liquid plasticine. The film can be deposited on the surface of glass, plastics and other materials, and is prepared by a sol-gel method, and the key of the preparation is the preparation of a monodisperse sol system
[60,61]. The addition of hexamethyldisilazane (HMDS) to the SiO
2 sol prepared by St Stöber method under appropriate conditions makes the SiO
2 nanoparticles in the sol fully alkylated and avoids crosslinking with each other, which is the core link in the preparation of this sol. Recently, Kano et al. Added a silane coupling agent containing tertiary amine groups to the sol on the basis of HMDS modification, and the final film formed was not superhydrophobic, but also had particle transfer characteristics.Moreover, the tertiary amine groups on the surface of SiO
2 nanoparticles can be protonated under acidic conditions, which enhances the hydrophilicity of the particles and immerses them in the liquid. As a result, they prepared mNPc liquid plasticine with pH response (Fig. 5)
[48]. It can be seen from this work that the nanoparticle film used to prepare mNPc liquid plasticine has a certain performance control space, and if the composition of this film particle source can be changed more, the types and functions of mNPc liquid plasticine are expected to be expanded to a greater extent.
2.3 Application of mNPc liquid plasticine
Like bare droplets and liquid marbles, the functional development of liquid plasticine is mainly based on the property of self-supporting liquid containers. Compared with spherical droplets and liquid marbles, the size of liquid plasticine is often much larger, but its height is usually limited by gravity to no more than 5 mm. Therefore, we believe that it should be determined on a case-by-case basis whether liquid plasticine is regarded as a microvessel or microreactor, or only as a self-supporting liquid vessel without emphasizing the word "micro". Being aware of this problem is helpful to not only learn from the research results of microreactors, but also not be bound by the concept of microreactions in application exploration. As a new thing, the application of liquid plasticine is still in its infancy. Among them, the earliest functional development of mNPc liquid plasticine is also the earliest application exploration in the field of liquid plasticine, which points to material transport and chemical reaction, and its purpose is to show some basic functions and characteristics through simple phenomena
[44]. Subsequently, mNPc liquid plasticine has been applied in environmental gas detection and protein analysis
[46][47]. The following is an overview of these application studies.
2.3.1 Electrophoresis of nanogold
The preliminary exploration on the application of mass transport mainly shows that the liquid plasticine can be used as a self-supporting liquid pipeline for electrophoresis experiments
[44]. In this study, an aqueous solution of nanogold was first made into a "U" (Fig. 6-a) or "one" (Fig. 6-b) shaped liquid plasticine, and then electrodes were inserted at both ends of it. After electrification, it was found that the liquid near the positive electrode gradually turned black, indicating that the negatively charged gold nanoparticles in the solution were flowing to the positive electrode and aggregating. This electrophoresis phenomenon means that the liquid plasticine with pipeline structure can realize material transport like a solid pipeline. Not only that, the liquid plasticine pipe can also be easily divided into two parts, and at the same time, the liquid inside will not flow out, so as to achieve the effect of "cutting off water without water flow", which is difficult to achieve by conventional solid pipes.
2.3.2 Low speed chemical microreactor
The preliminary exploration of chemical application mainly shows that liquid plasticine as a tubular microreactor can achieve low-speed and controllable chemical reactions
[44]. In this study, FeCl
3 and NaOH solutions were first made into liquid plasticine separately, and then coagulated to form a "Z" shaped liquid plasticine (Fig. 6-c). After coagulation, the brown color appears near the junction, indicating the formation of Fe(OH)
3. After that, the color change only slowly advances to the FeCl
3 side, because the external force inducing coagulation only acts on the NaOH liquid plasticine, and the direction of the force determines the flow direction of the liquid after coagulation. It should be emphasized that the chemical reaction here is very slow, which is mainly attributed to two reasons, one is that the coagulation process of liquid plasticine does not produce violent oscillation under the restraint of surface particles, and the other is that the long and narrow shape makes the diffusion and mixing of internal substances slower. The characteristics of low effective rate and controllable direction of chemical reaction in liquid plasticine are expected to play an important role in chemical analysis, material synthesis and other fields.
2.3.3 Multifunctional gas sensor
Like the liquid marble, the particle shell on the surface of the liquid plasticine can not block the gas in and out. Based on this, the liquid plasticine has been successfully applied to gas sensing, and has shown more functional characteristics than the liquid marble
[46]. Specifically, in this work, phenolphthalein solution was made into a long strip of liquid plasticine (Fig. 7-a), and the relevant information of ammonia diffusion behavior was obtained through its color change. First, a drop of ammonia water was injected near the end of the liquid plasticine (Fig. 7-b), and then the color was immediately observed to be generated at one end of the liquid plasticine and spread along the length direction at a fast and then slow rate (Fig. 7c, d). The diffusion speed of the volatilized ammonia in this direction can be known by measuring the traveling speed of the color front, and the ammonia concentration reflected by the color front can be known by simulating the diffusion process in combination with Fick's second law (Fig. 7-e). In contrast, liquid marbles are limited by their shape characteristics and can only be used to reflect the presence or absence of the gas to be measured
[30⇓~32]. If the liquid marble is regarded as a zero-dimensional device, then the strip-shaped liquid plasticine can be regarded as a one-dimensional device, and the complex-shaped liquid plasticine can be regarded as a two-dimensional device. The increased dimensions lay the foundation for the expansion of functions.
2.3.4 Protein separation and analysis platform
At present, the most practical application of liquid plasticine is in the field of biomedicine. Li Fei et al. Cooperated with our research group to combine liquid plasticine with isoelectric focusing technology, successfully realized the separation and analysis of proteins, and demonstrated many advantages of this system
[47]. Protein separation belongs to the pretreatment process of protein analysis. The traditional protein separation technology relies on large-scale equipment, and the protein collection after separation and the subsequent analysis operation are relatively complex, which is difficult to meet the needs of rapid detection at any time and anywhere. In contrast, the liquid plasticine – isoelectric focusing (LP-IEF) protein analysis platform has the advantages of miniaturization, portability, ease of operation, and low cost. Specifically, a solution containing the protein to be analyzed, ampholytes, and hydroxyethyl cellulose was first made into a strip-shaped liquid plasticine, and then a square liquid plasticine was made on both sides of the length direction using phosphoric acid and sodium hydroxide solution, and finally a paddle-shaped liquid plastisol was obtained by coagulation (fig. 8-a). By connecting the two ends of the liquid plasticine to a battery, a pH gradient can be created in the middle of the pipeline, and proteins with different isoelectric points will gather at different positions in the pipeline, thus achieving the separation and enrichment of different kinds of proteins (Fig. 8 B, C). The high transparency of mNPc liquid plasticine allows the operator to observe the internal phenomena with the naked eye, while the cuttability and openness provide great convenience for subsequent analysis. After protein separation, the liquid plasticine tube can be converted into a plurality of liquid marbles containing only one protein by cleavability; Openness allows the syringe to be easily inserted into the liquid marble/liquid plasticine for substance addition and protein extraction (Fig. 8-d), thus making the upstream process of protein analysis simple and efficient.
In this study, the reliability and validity of LP-IEF system for protein analysis were also verified by combining different analysis methods. For example, by combining with colorimetry, the quantitative analysis of microalbumin in urine of diabetic patients was realized, and the deviation of the measurement results was less than 9.4% compared with the conventional clinical immunoturbidimetry. Successful identification of a diabetes marker (α-1-microglobulin/bikunin precursor) in urine samples was achieved by coupling with matrix-assisted laser desorption ionization time-of-flight mass spectrometry
[47]. This study shows the application value and unique advantages of liquid plasticine in the biomedical field for the first time, and also provides an important reference for how to tap the application potential of liquid plasticine and make it practical.
3 Other types of liquid plasticine
3.1 Liquid plasticine coated with nanometer powder particle
Powders are the most accessible source of particles, but it is not easy to use powder particles to achieve controlled and precise liquid shaping due to their large size and other reasons. Inspired by the initial research work of mNPc liquid plasticine, Liu Jianlin et al. Found a solution to this problem
[43,44][49]. They chose the powder composed of hydrophobic SiO
2 nanoparticles as the particle source, first prepared the liquid marble, and then extracted more than 90% of the liquid from it to achieve effective interfacial plugging, and successfully constructed liquid plasticine with different shapes on this basis (Fig. 9). It should be emphasized that the nanoparticles in the powder exist in the form of uneven agglomerates, which makes the particle arrangement on the surface of the formed liquid marble very uneven. In addition, when we repeated the experiment, we found that particles often fell off the surface of liquid marbles during the pumping process. These phenomena may explain why so much fluid needs to be pumped to produce a high enough degree of clogging to achieve plasticity. From the results, although the shape accuracy and transparency of this powder source liquid plasticine are not as good as that of mNPc liquid plasticine, it should be enough to meet many application requirements.
3.2 Liquid plasticine wrapped by a single layer of millimeter-scale polygonal sheet
In the current reports on liquid marbles and liquid plasticine, the particle size of the droplet/liquid surface is basically in the micro/nano order, and the particle shape is mostly three-dimensional configuration. The surface particles of liquid marbles prepared by candle ash coating are flaky, but the size is mainly distributed in the order of microns and is very uneven
[12]. Recently, Fujii et al. found that a millimeter-scale polygonal sheet (made of Hydrophobically treated terephthalate) could also be used to wrap a droplet, thus obtaining a polyhedral liquid marble (Fig. 10-a); Moreover, they also found that the polygonal sheet, which can be adsorbed on the surface of droplets in a single layer, is prone to interface blockage, and the larger the size, the easier the blockage, thus they successfully realized the preparation of liquid plasticine
[52,53]. For example, if a water drop rolls back and forth on a 2.48 mm long hexagonal sheet pile, the water drop will become an uneven strip under the constraint of the adsorbed surface sheet within a few seconds, and the complex shape of liquid plasticine can be obtained by splicing with this as a structural unit (Fig. 10-b).
It is worth noting that the larger flake size is also beneficial to produce sharp corners, but it is not beneficial to obtain a thinner pipe structure
[53]. Another important feature is that because the sheets used are flat and smooth, they scatter less when they cover the liquid surface evenly, so the liquid plasticine formed has good reflectivity or transparency (depending on whether the sheets themselves are reflective or transparent). However, in the construction process of complex shapes, the sheets are often stacked on the surface of the liquid and form an uneven distribution, so that the surface of the liquid plasticine is scattered with many blurred or highlighted areas (Fig. 10 C, d), which is very unfavorable for obtaining uniform optical properties. This work provides a new idea for the material selection of liquid plasticine, and the discovery of the phenomenon that large polygonal sheets are easy to clog on the liquid surface is of great significance for further understanding of the liquid plasticine system.
3.3 Liquid plasticine coated with stearic acid powder particles
Surface curing is the fundamental reason for the plasticity of liquid plasticine. In the above work, surface curing is achieved by surface blocking. Recently, Azizian et al. Found that surface curing can also be achieved by forming a gel layer on the surface of the liquid, and thus successfully prepared liquid plasticine
[50]. Stearic acid powder was used as the particle source in this work. Because stearic acid contains carboxyl groups, it can form hydrogen bonds with water molecules to produce strong interaction. Therefore, when droplets roll on stearic acid powder, some stearic acid molecules will be dissociated from the particles and immersed below the liquid level.The surface liquid, on the other hand, becomes quasi-continuously distributed under the dual action of hydrogen bonds and hydrophobic groups, and is constrained in the network formed by stearic acid molecules below the liquid surface and particles above the liquid surface, thus forming a gel layer on the surface of the droplet (Fig. 11-a). In the construction of complex shapes, the ellipsoidal/shuttle-shaped liquid plasticine formed by the reciprocal rolling of droplets for tens of seconds is mainly used as the initial sample (Fig. 11-b), which is shaped into the target shape by external force (Fig. 11-c).
The construction of this kind of liquid plasticine is limited by the volume and pH value of the droplet, and it is difficult to produce non-spherical shape by rolling if the volume is small, which is not conducive to the subsequent acquisition of complex shapes. When the pH value is high, the solubility of stearic acid will be increased, which will make the surface gelation difficult. In addition, there are many large particle agglomerates on the surface of this gel liquid plasticine, which is not conducive to obtaining higher molding accuracy and transparency. Although there are many limitations, the research in this paper shows that this kind of liquid plasticine can still be used for some applications based on pipeline structure and divisibility. The specific case is as follows: electrodes are inserted at both ends of the fusiform liquid plasticine containing cresol red.The polarization of the solution color was achieved by creating a pH gradient in the direction of the pipe through the electrolysis of water, and further, spherical liquid marbles with different colors were obtained by cutting (Fig. 11-d). The discovery that surface gelation can also cause liquid plasticity in this work increases people's understanding of the mechanism of liquid surface solidification, and is of great significance for the expansion of liquid plasticine types.
3.4 Liquid plasticine coated with sulfur powder particle
After the work of stearic acid gel liquid plasticine, Azizian et al. Tested another powder particle that can achieve liquid surface gelation, cyclooctasulfide (S
8,5~15μm)
[51]. After being attached to the surface of the liquid drop, the S
8 molecules on the surface of the particle and the water molecules can generate dipole-dipole interaction,The ions (H
+, HS
-, SO
42-) generated by the hydrolysis of some S
8 molecules can generate strong interactions with both S
8 molecules and water molecules (Fig. 12-a), and these interactions enable the formation of a gel layer on the droplet surface. As a result, they successfully prepared sulfur gel liquid plasticine. The surface morphology of this liquid plasticine is similar to that of stearic acid gel liquid plasticine, with many densely packed island-like aggregates (Fig. 12-b), which seems to be a common problem of droplet surface gelation. In terms of preparation, the two are the same in that the droplet is difficult to deform by rolling when the droplet volume is small; The difference is that sulfur gel liquid plasticine is less affected by pH, so it is suitable for a wider range of liquids. In addition, S
8 is a natural photocatalyst, which can be used for photodegradation of pollutants in water. Based on this, they made rhodamine B aqueous solution into fusiform sulfur gel liquid plasticine, and used it as a container for photodegradation experiments. The results showed that the degradation of rhodamine B in the fusiform sulfur gel liquid plasticine was more complete than that in the normal bare liquid droplet, which was attributed to the photocatalytic effect of the particles on the surface of the liquid plasticine on the one hand, and to the larger surface area of the abnormal liquid, which made full use of light on the other hand
[51]. It should be pointed out that this part of the application research and the gas sensing research of mNPc liquid plasticine both put forward that the high specific surface area of liquid plasticine is conducive to related applications, but there is no strict comparative experiment to verify it
[46].
4 Conclusion and prospect
In this paper, the research on liquid plasticine, a new soft matter system, is summarized and analyzed. Compared with liquid marbles, both of them are liquid systems coated by hydrophobic particles in air environment, and the main difference lies in the shape they present. In this paper, it is pointed out that the concept of liquid marble is mainly applicable to the system of quasi-spherical droplets, while the liquid cake blocked on the surface and the liquid system molded into various shapes should be collectively called liquid plasticine. The clarity and division of these two concepts are conducive to a deeper understanding of the two systems, promoting their respective development, and improving the rigor of scientific expression and analysis.
The formation of liquid plasticine is mainly based on surface solidification, which is usually caused by surface particle clogging, and can also be caused by surface gelation. For particle clogging, the key is to increase the surface particle density of the droplet/liquid cake, which is often achieved by adjusting the surface area. Two strategies can be summarized: one is to increase the surface area first and then decrease it under the condition of constant liquid volume.In the process of increasing, more particles are adsorbed on the surface, and in the process of decreasing, the density of surface particles increases and then clogging occurs. Typical examples include the process of droplet imprinting and liquid cake rolling in the preparation of mNPc liquid plasticine. The second is to reduce the liquid volume, thereby reducing the surface area and increasing the surface particle density. Typical examples include the process of extracting liquid from droplets/liquid cakes in the preparation of mNPc liquid plasticine and nano SiO2 powder liquid plasticine. The size and agglomeration of particles have an important influence on the occurrence and degree of clogging. According to the results reported so far, it can be generally concluded that the larger the particle size is, the easier the clogging is, and the more uniform the particle size is, the easier the clogging is. For surface gelation, the key is to select particles that can produce strong affinity with the liquid but are not easy to disperse into the liquid. At present, there are two kinds of particles reported: stearic acid (stearic acid) and S8 powder particles.It is speculated that myristic acid (myristic acid), palmitic acid (palmitic acid) and other powder particles with similar structure to stearic acid may also be used for surface gelation. The use of surface gelation to prepare liquid plasticine is more limited by liquid properties, such as pH, than the use of surface particle plugging.
The complex shape of most liquid plasticine is mainly reflected in the top view. To obtain the abnormal shape in the side view (as shown in Figure 3A, B), it is necessary to use a droplet with a small volume (less than 100 microliters) to provide a large curvature.With extrusion, friction and other operations, the surface particles are closely packed locally. In addition, the particle size needs to be small enough to ensure that the small volume droplets show a clear side-view deformation. In this regard, mNPc liquid plasticine prepared on the basis of imprinting is the most typical representative, and it is also the only type of liquid plasticine that systematically exhibits complex side view shapes. In contrast, the liquid plasticine from the top view is easier to prepare, although its shape is often more complex. There are two main ways to prepare it, one is bottom-up splicing method, the other is top-down segmentation method, which can be used in combination. In theory, the smaller the surface particles, the more conducive to the realization of high-precision liquid shaping, and through the comparison of examples, it can be seen that mNPc liquid plasticine with the smallest particles in all liquid plasticine has obvious advantages in smooth contour and accurate details. From the point of view of optical properties, mNPc liquid plasticine has the highest transparency and the closest appearance to bare droplets. In contrast, the transparency of nano-sized SiO2 powder liquid plasticine was slightly lower, but still significantly higher than that of micro-sized powder plasticine such as stearic acid and S8, while the transparency of liquid plasticine prepared by millimeter-sized transparent flakes was also significantly higher than that of micro-sized powder plasticine when the flakes were uniformly distributed. Compared with mNPc liquid plasticine, other liquid plasticine has a general advantage, that is, the particle source is easy to obtain, while the particle source represented by powder is not only easy to obtain, but also easy to functionalize, which needs to be paid attention to in future research.
From the application point of view, the representative work at present includes particle transport, gas sensing, protein analysis and so on, among which protein analysis is the most practical. These application studies show that the utilization of liquid plasticine pipe structure is the key to its functional development. Compared with conventional solid pipes such as capillaries, liquid plasticine pipes have the advantages of plasticity, openness and easy cutting, which can be used as an important starting point for new functional design. In general, there is little application exploration of liquid plasticine at present, and the limited research on application (gas sensing, protein analysis) is carried out around mNPc liquid plasticine, while the application exploration of other types of liquid plastisol is still at the level of simple phenomenon demonstration.
Although there are few reports on liquid plasticine, the field has gradually entered a stage of rapid development, and there is great potential for future research. Preparation methods, variety expansion, physical properties, field drive, function development and other issues are worth exploring in depth. In addition, from the point of view of sample composition and self-supporting liquid container, liquid plasticine is similar to liquid marble. From the point of view of pipeline structure, liquid plasticine is similar to microfluidic chip, so we can fully learn from the related work in these two fields when selecting topics and researching.
National Natural Science Foundation of China(11974280)
National Natural Science Foundation of China(51672224)
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