Pickering multiphase materials using plant‐based cellulosic micro/nanoparticles

Pickering multiphase systems stabilized by solid particles have recently attracted increasing attention due to their excellent stability. Among various solid stabilizers, natural and renewable cellulosic micro/nanoparticles that are derived from agricultural and forestry sources have become promising candidates for Pickering stabilization due to their unique morphological features and tunable surface properties. In this review, recent progress on forming and stabilizing Pickering multiphase systems using cellulosic colloidal particles is summarized, including the physicochemical factors affecting their assembly at the interfaces and the preparation methods suitable for producing Pickering emulsions. In addition, relevant application prospects of corresponding Pickering multiphase materials are outlined. Finally, current challenges and future perspectives of such renewable Pickering multiphase systems are presented. This review aims to encourage the utilization of cellulosic micro/nanoparticles as key components in the development of Pickering systems, leading to enhanced performance and unique functionalities.

their separation.Over a century ago, Ramsden and Pickering discussed Pickering stabilization, wherein colloidal particles assemble at interfaces to form and stabilize multiphase systems. [9,10][13] This dual protection of the Pickering droplet surface enables high stability of droplets against screening of their surface properties and changing the surrounding environment.This indeed results in less susceptibility of the droplets to environmental factors, for example, pH, [14] shear, [15] temperature, [16] and salinity, [17] which offers a route on endowing Pickering emulsion-derived products with longer self-life and novel functions.Developed by Binks and co-workers in recent twenty years, synthetic particle stabilizers, for example, SiO 2 , [18] Fe 3 O 4 , [19] etc., have been widely investigated for their capability to create Pickering emulsions owing to their uniform structure and chemical properties.Meanwhile, a range of functionalities derived from Pickering multiphase systems have been designed and developed, for example, a delivery carrier, the precursor of porous constructs, a template for emulgels, etc.However, owing to the growing demands on sustainable and green production, more recent researches have shifted their interests to replacing synthetic particles with naturally derived ones, [20][21][22][23] greatly extending the application range of Pickering systems. [24,25]lant-derived cellulosic micro/nanoparticles, especially nanocelluloses, are prime examples of natural, sustainable, and efficient (nano)materials. [26,27][40][41][42][43][44][45][46][47][48] Cellulose nanocrystal (CNC) is isolated by acid hydrolysis of cellulose fibers, wherein the amorphous regions are removed while retaining the crystalline portion intact during hydrolysis, thus resulting in highly-crystallite, rodlike CNC with 10-20 nm in width and hundreds of nanometers in length. [49]Compared with CNC, cellulose nanofibril (CNF) is more flexible, fibrillar, and has less crystallinity.[52][53][54] The surface hydroxyl groups presented on cellulosic particles are typically considered non-surface-active; however, the weak amphiphilic nature of crystalline cellulose, which contains both hydrophilic and hydrophobic crystalline faces, [55] dictates their behavior at the interfaces. [56]This effect also controls the formation and stabilization of Pickering systems.Meanwhile, the advantages of shape and colloidal features of CNF enable the structuring of the textural attributes of Pickering systems via self-associating and forming particulate networks in the aqueous medium. [57]Collectively, it is of great interest to better understand the stabilizing capability of different types of cellulosic particles, which can promote their applications in developing novel Pickering systems.
The general structure of this review is schematically illustrated in Figure 1.Several recent reviews have considered to use of nanocelluloses for Pickering stabilization, mostly focusing on Pickering emulsions using specific types of nanocellulose (e.g., cellulosic nanocrystals) or for specific applications (e.g., foods). [58,59]Here, we offer an insight to comprehensively analyze the current status of cellulosic particles from the angle of Pickering stabilization (both for emulsions and foams), which is rarely reported.The theoretical considerations of Pickering multiphase systems are first highlighted, including common methods for generating Pickering systems.We then focus on the advantages of using plant-based cellulosic nano/microparticles as Pickering stabilizers, as well as the factors affecting the system stability.Finally, we provide an overview of novel materials and applications derived from cellulosic particle-stabilized Pickering multiphase systems, with an emphasis on the unique merits of a synergistic combination of Pickering stabilization and cellulosic nanomaterials.

Fundamental aspects of cellulosic particles in Pickering stabilization
In principle, one prerequisite has to be fulfilled to obtain stable Pickering systems: the surface of particles should be partially wetted by both water and oil, which can be defined by the contact angle (θ ow ) that is measured between the water and oil phases (Figure 2A). [60]Solid particles with θ ow < 90 • preferentially wet the water phase when absorbed at the oilwater interface, resulting in an oil-in-water (O/W) emulsion.Conversely, particles with θ ow > 90 • are primarily located in the oil phase, leading to a water-in-oil (W/O) emulsion.63] The capability of particles absorbed at the interface is influenced by the particle surface chemistry and interfacial tension between the phases.This relationship can be described in terms of the adsorption energy, which is directly related to the interfacial area replaced by the surface of a particle.Numerous studies have shown that the morphology of particles plays a crucial role in determining the stability of Pickering systems.For spherical particles, the adsorption energy can be calculated using the following Equation (1) [64] : where R represents the radius of the particles,  ow is the interfacial tension between oil and water phases, and  represents the equilibrium three-phase contact angle reflecting the degree of partial wetting of the particles by the two fluids.When utilizing anisotropic, rodlike cellulosic micro/nanoparticles to achieve Pickering stabilization, additional considerations regarding particle shape, for example,  [55,67] orientation and packing at the interface, should be taken into account.These factors influence the available adsorbing surface of the particles.Therefore, the adsorption energy when considering these effects can be calculated using Equation (2) [65] : where a and b represent dimensions of long and short semiaxes, respectively.Accordingly, several key points relating to these fundamental theories can be obtained.First, the irreversible interfacial adsorption of particles with the proper size and wettability is due to the large energy or barrier required for the particle detachment from the interfaces compared to thermal energy (k B T).This indicates that the stability of Pickering emulsion systems is relatively high owing to the formation of a mechanically robust barrier on the droplet surface.Furthermore, when comparing the cross-sectional areas at the interface between spherical and rodlike particles, it can be observed that for any value of the  between 0 and 180 • , the inequality condition, ΔG rod > ΔG sphere applies. [65]This implies that anisotropic cellulosic micro/nanoparticles can generate even stronger physical adsorption and more easily assemble at the liquid interfaces compared to the equivalent spherical particles, providing advantages in achieving and maintaining the higher stability of Pickering emulsions.Finally, the more significant shape effect of anisotropic particles can result in a lower loading level than that of spherical ones, which might be caused by the larger surface coverage and denser surface layer.As a consequence, these collectively imply the importance of particle shape on the stabilization of Pickering systems. [66]More importantly, the rodlike, anisotropic cellulosic micro/nanoparticles assembled at the interfaces through the Pickering mechanism theoretically make the system become extremely stable against droplet breakage (Figure 2B), which is key for realistic applications.

Assembly of cellulosic particles at the interfaces
Due to the hydrophilic nature of cellulose, the assembly of cellulosic particles at the interfaces is a dynamic process that is influenced by various factors.The question that arises is how these factors contribute to the particle assembly at the interfaces.As implied previously by studies from Capron and Dri, crystalline faces of cellulosic particles were structurally nonequivalent, that is, their hydrophobic edge (200) was expected to orient and bend toward the oil phase when adsorbing at the interfaces (Figure 2C). [55,67]This finding provides an evidence that unmodified, rodlike cellulosic particles can accommodate the curvature of spherical oil droplets, forming assembled structures at the interfaces.However, since no actual hydrophobic entities link to the surface of cellulosic particles, they come into contact with the oil phase primarily through their surface, rather than forming penetration into the bulk phase, which limits dynamic adjustment of adsorption and subsequent arrangement.
Taking advantage of the existence of hydrophobic crystalline faces in the cellulosic particles, as well as their morphological feature, they can self-assemble at the interfaces, while highly depending on their dimension, surface properties, interparticle interactions, etc.For instance, Capron and co-workers investigated the effect of the surface charge density of CNC on its adsorption at the oil-water interface. [55]n this study, the sulfated CNCs that were originally produced with sulfate acid hydrolysis were partially desulfated by mild acidic treatment (2.5 N hydrochloric acid or 5 N trifluorohydric acid) at different processing durations, resulting in a wide range of surface charge densities (0.123-0.017 e/nm 2 ) of CNCs while maintaining their well-defined nanostructures.When the surface charge densities of CNCs were <0.03 e/nm 2 , they could be effectively adsorbed at the oilwater interface, leading to fine droplets with high stability.On the contrary, when the surface charge densities of CNCs were >0.03 e/nm 2 , they could hardly be adsorbed at the interfaces, resulting less stable system.This was caused by the fact that an excessively high charge density would generate strong electrostatic repulsion among nanocrystals, hindering their adsorption, arrangement, and connection at the interfaces.Besides chemical treatment to change the surface charge density of CNC, adding a tiny amount of counterion (e.g., NaCl) into the continuous, aqueous phase could partially screen the negatively-charged surface of CNCs, promoting their adsorption and assembly at the interfaces.Indeed, this strategy is considered the easiest and most effective way to formulate CNC-based Pickering emulsions with high particle surface coverage. [68]In a study conducted by Zhao et al., the impact of surface sulfur content on formation and stabilization of O/W Pickering emulsions was investigated. [69]n this study, the sulfur content on CNC surface was controlled by adjusting the concentrations of sulfuric acid upon hydrolysis, resulting in tuneable diameter and crystallinity, which impacted the interfacial assembly of CNCs.As a result, the high-sulfur-content CNCs exhibited enhanced stabilizing capability in Pickering emulsion systems owing to their smaller diameter and higher crystallinity.
Beyond surface charge, the aspect ratio (length-to-width ratio) of CNC, a key factor for anisotropic particles, associated with the morphological effect of rodlike shape, greatly influences interfacial assembly. [70]Nanocrystals that exhibited low aspect ratio tended to align denser around the droplets, leading to high interfacial coverage (>84%), while droplets that were stabilized by high aspect-ratio (longer) nanocrystals showed low coverage ratio (<44%), forming interconnected droplets by longer nanocrystals (Figure 3). [71]his finding resulted in the fact that the shape of CNC indeed impacted the packing of nanocrystals at the droplet surface, which altered the stabilization form of droplets.Thus, this study reveals that when targeting different applications for cellulosic particle-based Pickering emulsions, one should consider selecting proper types of nanocellulose since the physicochemical performances of the emulsion products can be affected by the dispersion state and surface morphology of the droplets that are closely related to the stabilization form of cellulosic particles.
Besides oil-water interfaces, cellulosic particles can also be adsorbed onto other types of interfaces in colloidal systems.For instance, a water-in-water (W/W) interface that was generated between thermodynamically separated water solutions of poly(ethylene oxide) (PEG) and dextran in aqueous phase was able to be stabilized with CNC, forming W/W Pickering emulsions. [72]These W/W droplets consisted of a PEG-rich dispersed phase and dextran-rich continuous phase, wherein CNCs were simultaneously present at the interfaces and in the continuous phase.The addition of NaCl to the W/W emulsions resulted in the formation of weak gels in the continuous phase due to the cross-linking of non-adsorbed CNCs, which arrested the creaming of the dispersed phase droplets by forming a 3D network.In another study, the adsorption and distribution of CNC in the W/W Pickering emulsion could be regulated, forming systems with controllable gelation rate and stiffness. [73]The aggregation rate of CNC in the continuous, aqueous phase was tuned by adding different amounts of NaCl.At sufficiently high aggregation rates, emulsion gels were formed that were highly resistant to creaming, which was ascribed to the formation of a 3D-network of CNCs at the interfaces and in the continuous phase.
Although cellulosic particles can assemble at the interfaces, they hardly form strong interfacial adhesion due to the interfacial contact-oriented interaction.To this issue, physical modification has been considered as a tool to reinforce the interfacial assembly of cellulosic particles. [74]ne of the most efficient approaches was to use oppositely charged components to modify the surface chemistry of cellulosic particles, promoting their adsorption at the interfaces. [75,76]For instance, Bai and co-workers utilized cationic, small-molecular surfactant (ethyl lauroyl arginate, LAE) to modulate the assembly of CNCs at the oilwater interfaces by controlling the loading levels of LAE (Figure 4A), [77] which resulted in high emulsion stability against creaming and coalescence, even during long-term storage.This was because the hydrophobic part of LAE could be attached to the CNC surface via electrostatic attraction, resulting in enhanced hydrophobicity of the complexes to better contact with the oil phase.In a more sustainable manner, a combination of cellulosic particles with other types of nanopolysaccharide was developed to adjust the interfacial assembly of cellulosic particles, [78] wherein the mixed polysaccharide particles were more efficient for interfacial adsorption than those stabilized by a single polysaccharide particle.For example, Huan and co-workers developed an all-biomass system that used oppositely-charged fibrillike nanoparticles, chitin nanofibers (ChNF), to fine-tune the interfacial behavior of CNF. [79]Significantly, the location of CNF in the emulsions depended on the levels of ChNF, transferring from free CNF dispersing in aqueous phase at low ChNF concentration to its full coverage at the droplet surface at relatively high ChNF concentrations (Figure 4B).This F I G U R E 3 Scanning electron microscope images of polymerized styrene-in-water emulsions stabilized by cellulose nanocrystals from cotton (left panel), bacterial (middle panel), and Cladophora (right panel) revealing the coverage variation as a function of the aspect ratio of cellulose nanocrystal. [71]I G U R E 4 (A) Confocal images (merged) and schematic illustration of Pickering emulsion droplets stabilized by the cellulose nanocrystal/ethyl lauroyl arginate (CNC/LAE) complexes at different concentrations of LAE, showing dynamic stabilizing behaviors controlled by LAE loading. [77](B) Fluorescent images (merged) of cellulose nanofibril/chitin nanofibers (CNF/ChNF)-stabilized Pickering emulsions at different ChNF concentrations.The CNC and CNF/NCh complexes were dyed with Calcofluor white, and the oil droplets were stained with Nile red. [79]namic assembly process not only offered a route to facilitate the adsorption of CNF at the interfaces but also provided a way to modulate the structure and properties of Pickering droplets by controlling the interfacial arrangement and packing of CNF.In particular, when CNF/ChNF complexes fully covered the oil droplet surfaces, they became highly resistant to changes in environmental stresses (e.g., pH and ionic strength), which was crucial for ensuring the realistic applications of Pickering emulsions.
In some cases, specific applications of Pickering multiphase systems, for instance, using emulsion droplets as a processable element for advanced manufacturing, [80] require TA B L E 1 Pickering emulsification techniques suitable for cellulosic particles.

Ultrasonication emulsification
Ultrafast process, simple setup, suitable for a range of raw materials, and low cost Micron droplet size with broad distribution, the difficulty for industrial scale-up [4, 77]   Rotor-stator blending Rapid process, simple setup, industrial scalability, low cost Micron droplet size with broad distribution [94]   High-pressure homogenization Small droplet size with tunable range, large volume production, industrial scalability High instrumental cost, risk of over-processing [95]   strong adhesion of cellulosic particles at the interfaces.To this end, surface hydrophobization of cellulosic particles through covalent linking with synthetic chemicals, such as etherification, esterification, and acrylonitrile grafting, etc., has been used as a direct route to enhance the interfacial wettability. [81,82]This improvement promotes the assembly of cellulosic particles, even toward W/O Pickering systems. [83]For instance, Sarkar and co-workers explored the approach of grafting hydrophobic chains onto CNCs using octenyl succinic anhydride. [84]This modification process significantly increased the hydrophobicity of CNCs, which enhanced their capability of adsorbing at the interfaces, eventually resulting in highly stable Pickering emulsions.More importantly, although cellulosic particles were incapable of generating liquid foams owing to the low interfacial activity, [85] surface hydrophobization through the attachment of hydrophobic moieties onto particles could impart higher amphiphilic characteristics to achieve foam forming and stabilization. [86]esides to surface engineering of cellulosic particles by adsorbing oppositely charged components or hydrophobization, a more sophisticated nanoparticle-surfactant (NPS) approach has recently developed by dispersing cellulosic particles in water phase and dissolving ligands that can interact with cellulosic particles in the oil phase. [87,88]For instance, CNC-based NPS was generated (adsorbed) at the toluene/water interface via electrostatic interaction between charged groups of CNC and amine end-functionalized polystyrene originally dispersed in toluene. [89]This process led to a stable Pickering system with a robust barrier for exceptional mechanical properties.Beyond the water-oil system, CNC could be further assembled at the oil-in-oil (O/O) interface by using electrostatic interaction between CNC and amine-functionalized polyhedral oligomeric silsesquioxane, generating all-oil systems when jamming CNCSs at the interface. [90]Using such CNC-based assemblies, unique O/O high internal phase Pickering emulsions were prepared by a one-step homogenization process.In conclusion, these findings demonstrate the versatile capabilities of nanocelluloses in stabilizing and structuring a variety of unconventional, two-phase systems via interfacial adsorption strategies.

Methods for producing Pickering systems using cellulosic particles
As mentioned earlier, cellulosic particles can be irreversibly anchored at the oil-water interfaces upon adsorption, owing to the significantly high energy to detach them, which should be considered when preparing Pickering emulsions.To over-come this energy barrier, external force or alteration in solution condition are typically applied during Pickering emulsification. [91]Intense mechanical forces generated during emulsification help to surpass the energy barrier for desorption, enabling the formation of Pickering droplets that contain relatively small droplets.According to the energy required for the mechanical process, three types of preparation methods have been developed to generate Pickering systems (Table 1), including rotor-stator blending, ultrasonication emulsification, and high-energy homogenization.Moreover, the membrane emulsification and microfluidic technology that can be used to generate uniformly distributed droplets have also been utilized to produce Pickering emulsions.However, these two methods are hardly used for cellulosic particles owing to their large dimension and less surface activity.In this subsection, emulsification techniques suitable for applying cellulosic particles to form Pickering emulsions will be discussed.
Among different Pickering emulsification techniques, ultrasonication has been widely used to produce cellulosic particle-stabilized Pickering systems owing to its simplicity and efficiency (Figure 5A). [59,92]The acoustic cavitation generated by the sonicator results in the creation and collapse of air bubbles in the medium, which leads to various physical effects including turbulence, shock waves, highvelocity liquid jets, and high shear to break the bulk of oil phase into small droplets. [93]However, ultrasonication is typically more suitable for producing small quantities of samples at a laboratory scale, and more importantly, it is not easy to be transferred into large-volume-based industrial production.Rotor-stator blending that involves an impeller shaft with one or more rows of blades mounted onto it has also been applied to produce Pickering emulsions with cellulosic particles (Figure 5B). [94]As a result, the size of emulsion droplets is primarily determined by the shear forces generated within the turbulent region between the stator and rotor.More recently, Rojas and co-workers successfully applied high-pressure homogenization generated from a homogenizer or microfluidizer to prepare CNC-stabilized Pickering emulsions (Figure 5C), [95] which offered a way to achieve small oil droplets.Indeed, the smallest droplet size of CNC-stabilized Pickering emulsions was around 3 μm for ultrasonication, [4] while submicron for microfluidization, demonstrating the effectiveness of high-pressure homogenization at reducing the droplet size of Pickering emulsions.Owing to the high-volume flexibility, short processing time, simple setup and use, rotor-stator blending and high-pressure homogenization are capable of being used both in the laboratory and in the industry.However, the high instrumental cost of high-energy methods often limits their applications in specific fields, such as food emulsion products.It is noteworthy that the intensity and duration of the mechanical forces used to generate Pickering emulsions can have significant effects on both the structure of cellulosic particles and the resultant emulsions.If the mechanical forces were too intense or were applied for too long, the stability and uniformity of the emulsion might be compromised, leading to an increased polydispersity or even total demulsification of the droplets. [96]This often occurs when a high-energy homogenizer is used to produce Pickering droplets since over-processing of the samples is a common failure for this technique.Moreover, in some cases, the micron size in cellulosic particle-stabilized Pickering emulsion droplets is already sufficient, thus high-pressure methods are not favorable.Hence, the selection of a suitable high-energy method and appropriate operating conditions is of utmost importance in achieving controllable production of Pickering emulsions using cellulosic particles, which determines their functional properties.

APPLICATIONS OF PICKERING MULTIPHASE MATERIALS STABILIZED BY CELLULOSIC PARTICLES
Emulsion technology, particularly for the Pickering system, is a versatile route to create functional materials. [97,98]oreover, considering the inherent benefits of cellulosic particles, [99] a synergistic combination of Pickering stabilization and cellulosic particles therefore results in a range of materials bearing novel applications.This section provides a summary of how cellulosic particles show the potential to enhance the functionalities of Pickering multiphase materials.

Pickering emulsions for food applications
Owing to its sustainable and label-friendly nature, cellulosic particles are particularly suitable for the development of food products, [100][101][102] herein food Pickering emulsions. [96,103]he stability of food emulsions during storage is a critical factor that significantly influences their effectiveness and usability. [104]However, Pickering emulsions that were stabilized by unmodified cellulosic particles often exhibited insufficient storage stability owing to the relatively large droplet size. [82]To solve this problem, a range of approaches has been developed, [40,105] including decreasing the droplet size, increasing the viscosity of the continuous phase, and forming a crosslinking network for emulsion gels.Recently, Rojas and co-workers developed an all-cellulose Pickering emulsion system wherein a sequential addition of CNC and CNF was used to create highly stable food emulsion systems. [4,106]This formulation developed a unique stabilizing approach where CNC selectively adsorbed onto the oil droplet surfaces, mitigating droplet coalescence, and CNF facilitated the formation of a three-dimensional, weak network of aggregated oil droplets and nanofibrils in the aqueous phase, inhibiting gravitational separation (Figure 6A).The function of non-adsorbing CNF in the aqueous phase originated from the depletion effect, which was caused by the change of osmotic pressures of the systems.By changing the loading levels of CNF, this effect could induce different aggregation states of the CNC-stabilized droplets.At optimum CNF loading, the resultant emulsions displayed remarkable stability, remaining homogenized and free of any noticeable signs of separation for a period exceeding seven months.This extended stability period met the stringent shelf-life expectations associated with food products. [91]n order to extend the storage duration, Cranston and coworkers developed a dryable and rehydratable Pickering emulsion using a mixture of CNC, tannic acid (TA), and water-soluble cellulose derivatives (i.e., methyl cellulose or hydroxyethyl cellulose). [107]The incorporation of TA into all-cellulose Pickering emulsions improved their stability against freeze-drying.Subsequently, the emulsion powder exhibited excellent rehydration properties, requiring no intensive mixing.Upon reconstitution, the emulsion demonstrated minimal alteration in droplet size, indicating its ability to maintain stability throughout the dehydration and rehydration process (Figure 6B).The mechanism underlying this system originated from the interaction between cellulose derivatives and TA, leading to the formation of complexes at the surfaces of the oil droplets (Figure 6C), which resulted in the creation of a protective shell around them.These results demonstrated the capability of cellulosic particles to form Pickering emulsions suitable for food applications.
As well known, cellulosic particles are non-digestible within the human gastrointestinal tract (GIT), [108,109] thus they may endow additional functions to Pickering emulsions that can be designed to improve human health and wellbeing.For instance, cellulosic particles possessed the ability to act as a physical barrier that encased the oil droplets, F I G U R E 6 (A) Fluorescent images of cellulose nanocrystal (CNC)-stabilized Pickering emulsions containing 1.0 wt% oil and 0.05 wt% cellulose nanofibril (CNF) in the continuous, aqueous phase.The far-right panel shows the schematic illustration of stabilizing mechanism of the emulsion with CNC and CNF.The CNC (as contour of the droplet) and CNF was stained with Calcofluor white, and the oil droplet was stained with Nile red. [4](B) Photographs showing the effect of freeze-drying and redispersion on the appearance of emulsions stabilized by CNC, methyl cellulose, and tannic acid (TA).Bottom panel shows the image of the initial emulsion, the scanning electron microscopy (SEM) image of the same emulsion after freeze-drying, and confocal image of the emulsion powder after redispersing in water.(C) Schematic showing of the coverage of CNC-based complexes at the oil droplet surface generating a protective shell. [107]sociating with their non-digestibility, impeding digestive enzymes from accessing macronutrients. [110]Additionally, they could also lower the activity of digestive enzymes, bile acids, or other gastrointestinal components by binding to them, enabling food nutrients to be protected and gradually released in the digestive system. [111]In a specific study conducted by Bajka and co-workers, the uptake of digested CNC-stabilized Pickering emulsions by murine intestinal mucosa was explored. [112]This result indicated that CNCs became entrapped within the intestinal mucus layer and did not penetrate to reach the underlying epithelium, irrespective of the duration of the experiment.Consequently, the CNCs were unable to be absorbed into the system, which might be likely caused by their large particle dimension and hydrophilic nature.In another study, an in vitro GIT model was employed to simulate the digestion process involving mouth, stomach, and small intestine phases, evaluating the gastrointestinal destiny of CNC-stabilized corn oil-in-water Pickering emulsions. [113]The results clearly demonstrated the protective role of CNC, as the quantity of free fatty acids released from the emulsion was nearly 40% lower when the lipid droplets were coated with CNC, compared with the sample with gum arabic (100% release) (Figure 7A).This is presumably attributed to the inhibition of lipid droplet digestion by bile salts and lipase via forming irreversible CNC coating to block their adsorption to the oil-water interfaces.Moreover, when using lauric acid-adsorbed CNC to form Pickering emulsions that underwent GIT digestion, the lipid droplets retained physical stability throughout the GIT phases, showing that only approximately 50% of droplets were digested. [114]This phenomenon was attributed to the presence of adsorbed lauric acid on CNC, which enabled the complexed stabilizer to uphold emulsion phase stability by preserving the size of lipid droplets, decreasing lipid digestibility, and prolonging the release of lipophilic nutrients.Overall, these findings indicate that cellulosic particles have the potential to effectively modify the digestion of Pickering-emulsified lipids, thereby offering opportunities for the development of functional foods with tailored digestion profiles.
Besides simply adjusting lipid digestion, targeted delivery using Pickering emulsions has also been achieved.For instance, hydrophobically-modified CNCs were employed to create Pickering emulsions that served as a delivery system for short-chain fatty acids, which offered benefits for colon health. [115]The presence of a coating on CNC hindered the digestion of lipid droplets during simulated gastrointestinal conditions, leading to an increased concentration of short-chain fatty acids reaching the colon (Figure 7B).On the other hand, the cellulosic particles also exhibit a significant impact on the bioavailability of micronutrients that are pre-encapsulated into the emulsions.For example, McClements and co-workers explored the bioaccessibility of vitamin D3 in CNF-based Pickering emulsions through GIT model stimulating, showing that the bioaccessibility of vitamin D3 decreased as CNF concentration increased in the emulsions. [116]This was attributed that the lipid phase was not fully digested with stimulated GIT in the presence of CNF, and thus some of the vitamins were not released into the continuous phase (Figure 7C).These findings hold promise for the application of cellulosic particle-stabilized Pickering emulsions in food-based treatments, as they demonstrate the F I G U R E 7 (A) Free fatty acids released as a function of time during simulated small intestinal digestion of corn oil-in-water Pickering emulsions that were stabilized with different loading levels of cellulose nanocrystals (CNCs).The emulsion that was stabilized with 0.75%-wt% gum Arabic was used as a reference. [113](B) Release of individual short-chain free acids as a function of time during simulated intestinal digestion of Pickering emulsions that were stabilized with hydrophobically modified CNCs. [115](C) Influence of cellulose nanofibril (CNF) concentration on stability and bioaccessibility of vitamin D3 in CNF-stabilized Pickering emulsions through a simulated digestion model.The emulsion that was stabilized with 0.7 wt% whey protein isolate was used as a reference. [116]I G U R E 8 Procedure of forming an edible biobased film containing oil droplets using cellulose nanofibril (CNF)-stabilized Pickering emulsions wherein flexibility and transparence of the films could be achieved by evaporating the solvent.The obtained composite film can be dissociated in the water, forming emulsion droplets.[119] ability of these emulsions to withstand the harsh conditions of the stomach, small intestine, and colon.
Beyond the direct use of cellulosic particle-stabilized Pickering emulsions in their liquid form, they have been applied as processable components in formulating food packaging materials. [117]For instance, the functional properties of whey protein films were enhanced by incorporating CNC-coated bergamot oil droplets. [118]In this concept, the incorporation of highly stable essential oil droplets enhanced the mechanical properties and water-vapour permeability of the films by forming 3D-networked, tightly-packed inclusions from CNC-based droplets in the film.Additionally, these droplets contributed to improved antimicrobial and antioxidant activity, ultimately producing active packaging films.In a more recent work, Velikov and co-workers developed CNF-based edible composite films by direct solvent casting of CNFstabilized soybean oil-in-water Pickering emulsions that were produced using microfluidization (Figure 8). [119]The droplet size of the CNF-stabilized emulsions could be well controlled by the high-energy emulsification process.At the optimal oil loading levels, flexible, transparent, and thermally stable composite films were fabricated, showing multilayered structures with oil phases effectively trapped between two layers of CNF.As a result, the mechanical and wetting properties of the films were significantly improved as such double layers could further form inter-layer penetration structures.Moreover, these films were biodegradable since they could dissociate when placed in water.The inclusion of curcumin within these composite films led to a significant enhancement in their antioxidant and antimicrobial activity.In summary, through structural design and application-oriented optimization, cellulosic particle-stabilized Pickering emulsions show great promise as raw components for the construction of functional, active packaging materials.

Pickering emulsions for stimuli-responsive materials
The materials that show responsiveness to different stimuli, for example, pH, salinity, temperature, CO 2 , magnetic field, etc., have attracted great attention recently owing to their capability to adapt to different application ranges. [120,121]aking advantage of the two-phase nature of Pickering F I G U R E 9 Schematic illustration of pH-responsive behavior of Pickering emulsions stabilized by (A) benzyl-polyethyleneimine-modified cellulose nanocrystals (CNCs) [124] and (B) poly [2-(dimethylamino)ethyl methacrylate] (PDMAEMA)-g-CNC. [107](C) Schematic showing dual-responsive Pickering emulsions (temperature and CO 2 ) that are stabilized by Jeffamine M2005-Modified CNC. [127]stems and the tunability of cellulosic particles, their Pickering emulsions show promise in forming stimuliresponsive materials.Among different types of external stimuli, pH-responsiveness, relying on introducing changeability of cellulosic particles under acid or base conditions, is one of the most common and easily achievable stimuli for cellulosic particle-based Pickering emulsions. [122]mong different approaches, surface carboxylation of cellulosic particles is a simple route to enable pH-responsiveness.For instance, Kašpárková and co-workers utilized carboxylated CNC that was oxidized from microcrystalline cellulose with ammonium persulfate to form O/W Pickering emulsions at pH values of 2, 4, and 7. [123] The pH of the continuous, aqueous phase affected the formation and stabilization of the emulsions, with the largest droplet size being at pH = 2, which was related to decrease of charge on CNC with decreasing pH.Surface functionalization by introducing ionizable entities offers another way to endow pH-responsiveness of cellulosic particles.A series of benzylpolyethyleneimine-modified CNC via chemical reaction of periodate oxidation and reductive amination displayed pHresponsive amphiphilicity due to the presence of hydrophilic, ionizable amino groups and hydrophobic benzyl groups. [124]ickering emulsions stabilized by such novel CNCs were sensitive to pH changes, triggering alternating states of emulsification and demulsification with adjusting the pH 3 to 7 (Figure 9A).In another study, surface of CNC was modified by grafting poly [2-(dimethylamino)ethyl methacrylate] (PDMAEMA) via radical polymerization, which simultane-ously improved stabilization of O/W Pickering emulsions and endowed them with pH-responsiveness. [125]When Nile Red was used as an indicator for heptane-in-water emulsions, their color under ultraviolet light was changed from fuchsia to yellow as increasing pH, indicating the change of polarity of the oil phase.This was caused by the extent of PDMAEMA chain penetrating into the oil phase at different pH values, which changed the degree of protonation of amine groups (Figure 9B).This phenomenon was not observed in toluene-in-water emulsions owing to its less polar nature.
Owing to the use of acids or bases to adjust the aqueous medium environment to achieve responsiveness, the main limitation of such pH-responsible systems is their stability against increased ionic impurities trapped in the system.This is because the surface charge of cellulosic particles can be screened, causing aggregation of droplets and eventually oiling-off.Thus, the pH-responsive Pickering emulsions are vulnerable to multiple pH adjustments, restricting realistic applications.Compared with pH-responsive systems, magnetically responsive Pickering emulsions that are stabilized by cellulosic particles are in growing demand in applications since the magnetic field belongs to a non-invasive yet highly efficient stimulus, which is suitable for precise and targeted release.The magnetic stimulus to Pickering emulsions shows a promising opportunity to extend their use in the human body.For instance, Tang and co-workers applied magnetism-responsive Fe 3 O 4 @CNC to stabilize Pickering emulsions containing model drug curcumin. [126]Such emulsion showed controlled drug release and high in vitro anti-colon cancer efficiency triggered by a magnetic field.Under magnetic exposure, the adsorbed Fe 3 O 4 @CNC particles at the oil-water interfaces turned magnetized, which were attracted towards the field direction.After concentrating around the source of magnetic force, the nanoparticles induced a dilatational deformation of the fluid interface and then were released into the aqueous phase.This resulted in the de-stabilization of Pickering emulsions, exposing the curcumin-loaded, uncovered oil droplets to the buffer medium, which thus led to the controlled release of curcumin.The drug release profiles showed that exposure of the emulsion to an external 0.7 T magnetic field could initiate the release of bioactive substances from the emulsion, reaching half of the initial loading within 4 days.This result demonstrated the capability of cellulosic particles to function as magnetism-responsive stabilizers for Pickering stabilization.
Beyond responsive Pickering emulsions using single stimulus, dual stimuli systems have also been developed using cellulosic particles, wherein the secondary stimulus should not only endow additional function to the materials but also exhibit minimal effect on interfering with the initial properties of the materials.In this regard, hydrophobically-modified CNC using thermosensitive Jeffamine M2005 via simple electrostatic interaction was produced to stabilize O/W Pickering emulsions that were responsive to temperature and CO 2 simultaneously. [127]Stable emulsions were successfully achieved at a temperature of 20 • C, and the morphology and size of the droplets remained unchanged after 1 month of storage.As increasing the temperature to 60 • C for 20 min, demulsification occurred (Figure 9C), which was ascribed to the dehydration of polyethylene oxide and polypropylene oxide, causing aggregation of CNCs.In addition, demulsification was also achieved after bubbling CO 2 (Figure 9C), which was attributed to the loss of electrostatic interaction between CNCs and Jeffamine M2005.More importantly, when changing the temperature back to 20 • C or bubbling N 2 into the system, the water-oil mixture could be re-emulsified to stable Pickering emulsions.This result provided a route for the design and construction of dual-stimuli-responsive Pickering materials using cellulosic particles as colloidal stabilizers.It should be noted that such cellulosic particle-based dual-stimuli-responsive or switchable Pickering systems are designed to response to two independent stimuli, rather than forming sequential or successive responsiveness, which dilutes the application range of the system.This is likely caused by the difficulty of simultaneously integrating two or more responsive entities into the cellulosic particles prior to forming the emulsions.Thus, investigation on this topic should focus on finding appropriate design strategies and novel active structures to construct dual-responsive cellulosic particle-based Pickering emulsions that can response to different stimuli in a more controlled, orderly manner.
Although responsive cellulosic particle-stabilized Pickering emulsions are important and cause significant attention for the development of multifunctional emulsion products, the realistic applications of such systems are often limited by their incapability of large volume production, high production cost, and insufficient commercialization.Moreover, the reversibility and cycle life of the stimuli-responsive Pickering multiphase materials determine their application ranges.Therefore, future investigation on facilitating the deployment of responsive systems is to discover simple yet readily avail-able routes to produce cellulosic particle-based emulsions with efficient, robust, and adjustable responsiveness.

Applications of Pickering foams
Comparing Pickering emulsions, Pickering foams have growingly received attention for material development in recent years owing to their lightweight, highly porous nature. [128]ore importantly, Pickering foams are an important supplement or sometimes substitute for Pickering emulsions since their design and formulation are relatively simpler than emulsions, resulting in more opportunities for developing complex structures using foams as precursors.Two types of Pickering foams, that is, liquid and dryable (solid) foam, have been achieved using cellulosic particles as stabilizers.In principle, cellulosic particle-stabilized liquid Pickering foams showed limited applications because their structure often totally collapsed upon removal of the liquid, [85] while they were still useful models for understanding the capability of cellulosic particles on adsorbing and arranging at air-water interfaces. [129]For dryable Pickering foams that are stabilized by cellulosic particles, they have shown a range of applications in the aspect of thermal management, cosmetics, pharmaceutical products, etc.
To avoid yielding fragile cellulosic networks upon removing the liquid phase in Pickering foams, a combination of cellulosic particles with other components to act as stabilizers was applied. [130]This is because most of the Pickering foams were stabilized with low solid concentrations of cellulosic particles (usually less than 2 wt%), which was insufficient to retain the whole structure intact against collapse.For instance, Svagan and co-workers developed a system that using a combination of metal-free semiconductor photocatalyst (g-C 3 N 4 ) and CNF to generate dryable Pickering foams (Figure 10A). [131]After drying, a large range of pores with comparative diameter to the liquid foam could be generated in the obtained solid foam.This unique structure of the solid foam enabled good sorption/uptake properties with model pollutant (Rhodamine B), without stirring and under mild lighting condition (low-power LEDs).This indicated that physicochemical properties and buoyancy of CNF-based solid foams played a crucial role in enhancing oxygenation, maximizing photon utilization and accelerating dye degradation.This study demonstrated the capability of using cellulosic particle-based Pickering foams to form solid materials for a range of applications, for example, waste stream adsorption, functional porous constructs, etc.
Besides to direct construction of Pickering foams, drying of Pickering emulsions offers another route to prepare solid foams that are constructed by cellulosic particles.Similar to Pickering foams, a combination of cellulosic particles with reinforcing agents is often used to achieve high mechanical performance after drying.For instance, a surface-active water-soluble polymer (methylcellulose) was used to modify the surface of microcrystalline cellulose, which can be used as a stabilizer for producing dryable Pickering emulsions (Figure 10B). [132]The solid Pickering foams obtained displayed porous, polyhedral, and tightly packed microstructure.In the meantime, these foams could be re-hydrated into a wet state without changing initial internal structures, which was ascribed to the mechanical robustness of the structure and  [131] (B) Schematic illustration of cellular foams derived from methyl cellulose (MCC). [132](C) Thermal insulation, photo-thermal conversion, and microwave absorption properties of CNF-based hybrid foam generated from Pickering emulsion. [133]I G U R E 1 1 (A) Wet Pickering foam formed in the absence of cellulose nanofibril (CNF), showing no structuring (free form) (left panel), and formed in the presence of CNF (right panel), displaying an improvement in structuring.(B) Solid foam templated from wet Pickering foam in the absence of CNF, displaying as collapsed powder upon drying in the air (left panel).In the presence of CNF, the templated solid foam maintained the initial structure, leading to a porous solid.[134] hydrophilic nature of cellulosic particles.In another case, a synergistic combination of CNF/CNT/PLA/Fe 3 O 4 was used as stabilizers to develop Pickering emulsions that can be freeze-dried into solid foams.[133] Due to the randomly distributed porous structure and the synergy between CNT and Fe 3 O 4 nanoparticles, ultralight CNF/CNT/PLA/Fe 3 O 4 foams exhibited superior thermal insulation performance, showing comparative properties to commercially available poly(vinyl alcohol) and polyurethane foams.The obtained foams also exhibited high light-to-heat conversion properties, with a surface temperature of up to 97 • C after 5 min of exposure to a heat source (Figure 10C).Due to its large spacing and porous features, the random porous structure in solid foams was favorable for scattering incident electromagnetic waves, which might be used as a microwave absorber.These results indicated another way to produce cellulosic particlebased Pickering foams, which can be designed to achieve high-performance, multifunctional applications.
[136] In comparison to foams stabilized solely by hydrophobic silica particles, the addition of CNF exhibited a substantial improvement in foamability, with an increase of up to 350%, as well as extending the lifetime of the foams.The incorporation of CNF facilitated the formation of a fibrillar network within the wet foam, which played a crucial role in regulating interparticle interactions, further effectively delaying or even preventing drainage, coarsening, and bubble coalescence (Figure 11A).In the meantime, this complex fluid could undergo a controllable transformation, resulting in the formation of lightweight and robust architectures upon drying (Figure 11B).The properties of such solid foams were influenced by the surface energy of the CNF precursor.This Pickering system showed the capability of universally forming super-stable complex fluid foams and offered a promising approach for producing strong and lightweight composite structures directly from their liquid counterparts.

CHALLENGES AND PERSPECTIVES
Recent advances in the development of plant-based cellulosic micro/nanoparticles as stabilizers for Pickering multiphase materials have been reviewed.Cellulosic particles are promising stabilizers due to their unique morphological features and tunable surface properties.The self-assembly of cellulosic particles at the oil-water interface forms a steric and electrostatic barrier that prevents the coalescence of the dispersed phase, which is influenced by its morphology, surface charge, wettability, etc., which can be adjusted by physical or chemical modification methods.In the material aspect, cellulosic particle-stabilized Pickering multiphase systems exhibit unique compositional and structural characteristics, resulting in a variety of applications that closely rely on the features of Pickering multiphase systems.
Although great progress has been achieved to engineer cellulose micro/nanoparticles into Pickering multiphase materials, several challenges still exist regarding processability and performance.Addressing these challenges necessitates a comprehensive understanding of the interactions between cellulose particles with water and oil phases.First, it is difficult to monitor the dynamic adsorption and self-assembly behavior of cellulosic particles at the oil-water interfaces, which can offer a better understanding of how to control such behavior in constructing materials.Second, safety and toxicology studies of cellulosic particles are still unclear, and the verification of unknown risks of cellulose particlebased Pickering emulsions requires long-term experiments.Last, exploiting cellulosic particle-based Pickering systems as building elements to develop high-performance, multifunctional, and customizable materials in a novel and green yet high-valued manner is still challenging.More importantly, beyond direct materialization of Pickering multiphase systems, applying secondary processing to them, for example, 3D printing, casting, molding, etc., which can create more diverse and interesting structures and applications, is underdeveloped.
In the future, we believe that a deeper understanding of the features of cellulosic micro/nanoparticles and their dynamic behavior at the interfaces can facilitate their practical applications to corresponding Pickering multiphase systems since stabilizers are critical for the performances of Pickering multiphase materials.The complete replacement of conventional surfactants with sustainable and environmentally-friendly cellulosic particles requires a synergy of interdisciplinary contributions from material science, chemical engineering, and biological science.It is important to develop more effective emulsification methods that can be used to successfully prepare cellulosic particle-stabilized Pickering emulsions containing smaller oil droplets, which are against physical instability.In addition, there is a need to understand how cellulosic particle-stabilized Pickering multiphase systems behave when incorporated into the realistic application scenario, especially their interactions with other ingredients or response to being exposed to processing operations.Moreover, more in vitro and in vivo research on the gastrointestinal fate and toxicity of Pickering emulsions stabilized by cellulosic particles would be beneficial for realistic applications.More importantly, beyond the advantages described in previous sections, a strong driver for developing and utilizing cellulose micro/nanoparticles in Pickering multiphase materials is the shift from petroleum-based materials toward an ultimate circular bioeconomy in the future.This shift requires a deep transformation of current technologies and policies for cellulosic particles, which can thus offer great opportunities that fit many urgent needs.

C O N F L I C T O F I N T E R E S T S TAT E M E N T
The authors declare no conflict of interest.

F I G U R E 1
Structure of this Review, after the Introduction, introduces theoretical consideration of Pickering stabilization using colloidal particles (panel I, section 2) and cellulose nano/microparticles (panel II, section 2).The Review also covers the recent advances in multifunctional materials derived from cellulosic particle-based Pickering multiphase systems (panel III, section 3 ).F I G U R E 2 (A) Schematic illustration of adsorption of spherical particles at the oil-water interface with possible positioning and contact angle ( ow ) between water and oil phases.(B) Schematic showing of the adsorption of rodlike, anisotropic cellulosic particles at the oil-water interfaces with a contact angle smaller than 90 • toward O/W Pickering emulsion, resulting in an extremely stable system owing to the irreversible interfacial adsorption of particles.(C) Different planes of cellulose microfibril structure exhibiting hydrophilicity and hydrophobicity in the structure of cellulosic micro/nanoparticles.

F I G U R E 5
Schematic illustration of (A) ultrasonication, (B) rotor-stator blending, and (C) high-pressure homogenization on producing cellulosic particle-stabilized Pickering emulsions.

F
I G U R E 1 0 (A) Dried cellulose nanofibril (CNF)-based cellular foam (left panel), optical microscopy image of the cellular structure of the neat CNF-based foam (middle panel), and visual appearance of neat CNF and g-C 3 N 4 -CNF foam (right panel).
This work was supported by the National Natural Science Foundation of China (32071720, 32271814, and 32301513), the Natural Science Foundation of Jiangsu Province (BK20231296), Tianjin Excellent Special Commissioner for Agricultural Science and Technology Project (23ZYCGSN00580), the China Postdoctoral Science Foundation (2023M740536), and the Foundation (No. 2021KF02, No. 2021KF32, and No. 2023GXZZKF61) of Guangxi Key Laboratory of Clean Pulp & Papermaking and Pollution Control, College of Light Industry and Food Engineering, Guangxi University.