Ultrahigh efficient emulsification with drag-reducing liquid gating interfacial behavior

Significance

Emulsification is ubiquitous in our daily life, as it is widely applied for food, cosmetics, coatings, biomedicine, material fabrication, and the petrochemical industry. Achieving energy-saving emulsification with mass production is attractive, but challenging. This work shows a drag-reducing liquid gating interfacial emulsification method, which enables the achievement of uniform and controllable droplets and the suppression of fouling. It also provides an appropriate environment for the temperature-sensitive biological components (e.g., proteins, enzymes, and bacteria) to avoid deactivation by exposure to high temperatures during emulsifying. This method could potentially spark further experimental and theoretical efforts to boost the development of economically sustainable emulsification applications.

Abstract

Emulsification is a crucial technique for mixing immiscible liquids into droplets in numerous areas ranging from food to medicine to chemical synthesis. Commercial emulsification methods are promising for high production, but suffer from high energy input. Here, we report a very simple and scalable emulsification method that employs the drag-reducing liquid gating structure to create a smooth liquid–liquid interface for the reduction of resistance and tunable generation of droplets with good uniformity. Theoretical modeling and experimental results demonstrate that our method exhibits ultrahigh efficiency, which can reach up to more than 4 orders of magnitude greater energy-saving compared to commercial methods. For temperature-sensitive biological components, such as enzymes, proteins, and bacteria, it can offer a comfortable environment to avoid exposure to high temperatures during emulsifying, and the interface also enables the suppression of fouling. This unique drag-reducing liquid gating interfacial emulsification mechanism promotes the efficiency of droplet generation and provides fresh insight into the innovation of emulsifications that can be applied in many fields, including the food industry, the daily chemical industry, biomedicine, material fabrication, the petrochemical industry, and beyond.

Nowadays, emulsions are extensively applied in progressive areas, ranging from food (1, 2), cosmetics (3), biomedicine (4–6), and material fabrication (7–9) to the petrochemical industry (10, 11). In 2021, the global emulsion market was over $300 billion (12). Emulsification is a powerful technique for its capability to break up and mix two or more immiscible liquids into droplets to meet the huge demand of the market (7). Conventional emulsification techniques are capable of high-speed production to satisfy the demands in experimental and industrial fabrication (13), which can be divided into noncontact and contact methods. For instance, ultrasound emulsification, a typical noncontact method, provides ultrasonic waves to break up liquids into polydisperse droplets and usually requires high energy inputs (14). Simultaneously, the majority of input energy dissipates into heat, in which the functional structures of the temperature-sensitive biological components, such as proteins, enzymes, and bacteria, would be damaged by exposure to high temperatures in emulsifying (15). Contact methods include homogenization, vortex mixing, microfluidics, porous matrix (PM) emulsification, etc. (16–19). Among the above methods, PM emulsification can consume less energy and easily scale up (20, 21). During the whole emulsification process of the PM method, the shear stress for dispersing liquids into droplets is supplied by the solid. Regardless of the number of immiscible phases in emulsifying, the initial emulsification process always occurs at the solid–liquid interface. As long as the solid exists, it is impossible to achieve a molecularly smooth interface like the liquid–liquid interface due to the high resistance at the solid–liquid interface, which cannot be ignored. Besides, the abrasion or fouling on the surface or inside of the solid matrix is inevitable, resulting in a decline in flux and shortened service life (22). The liquid interface is expected to be an ideal alternative to the solid interface for its smooth feature, but the flowability of the liquid has remained a major obstacle to obtaining a stable liquid–liquid interface (23). Recently, the idea of building a stable liquid–liquid interface has been realized by liquid gating technology, which has attracted increasing attention because it provides a special combination of dynamic and interface physicochemical behaviors (24–26).

Here, we present a drag-reducing liquid gating interfacial emulsification (DRLGIE) method with ultrahigh efficiency, controllability, and uniformity, as well as antifouling behaviors. The capillary-stabilized drag-reducing liquid gating structure is employed to form a stable liquid–liquid interface, which provides shear stress and reduces the resistance for easily dispersing liquids into droplets. Theoretical modeling and experimental analysis validate this method that processes the high-efficiency emulsification performance. In addition, it exhibits the ability to have a long-term antifouling behavior, presenting a huge potential for industrial scale-up. Droplets with controllable sizes and uniformity can be easily obtained by this method. For temperature-sensitive biological components (e.g., enzymes, proteins, and bacteria), it also offers a suitable environment to avoid deactivation by exposure to high temperatures during emulsifying. This method presents the possibility for broad applicability in numerous areas ranging from the food industry, the daily chemical industry, biomedicine, and material fabrication to the petrochemical industry and beyond. Given the large demand for emulsions, we believe that our ultrahigh-efficient emulsification technique can be scaled up to industrial fabrications to improve industrial productivity and facilitate the global transition to a more sustainable society. Overall, this technique with ultrahigh efficiency, energy-saving, controllability, uniformity, and antifouling benefits is sufficiently attractive and can potentially provide access to less energy input and waste of fewer resources, including advanced mechanism, high energy efficiency, and eco-friendly and sustainable technology, to meet United Nations (UN) goals in Sustainable Development Goal (SDG) 7 (27).

Results

In commercial emulsification systems, the majority are operating through mechanical driving or high pressure, in which a great amount of input energy is needed to overcome resistance or dissipate to other forms, such as heat, during the long-term operation. For instance, the PM system, a typical non-drag-reducing liquid gating interfacial emulsification (non-DRLGIE) system, has the lowest energy input among the commercial systems, but still requires high pressure to consume the energy (20, 21). Because there is a high viscosity-resistance coefficient η_(S-L) between solid and liquid, the liquid flow through its rough solid surface is difficult, resulting in a higher drag force (Fig. 1A and SI Appendix, Eq. S12):

$$F_{\text{D}(\text{PM})}=3\pi {\eta }_{(\text{S}-\text{L})}{\mu }_{\text{D}}u\prime L,$$ [1]

Fig. 1.

Mechanism of the DRLGIE system. (A) Schematic illustration of drag-reducing liquid gating interfacial behavior of the DRLGIE system. For the non-DRLGIE system, the dispersed liquid flows hardly through a rough solid pore and disperses large droplets into continuous liquid, owing to its high drag force. For the DRLGIE system, a capillary-stabilized drag-reducing liquid gating structure creates a stable liquid–liquid interface, and the flow of dispersed liquid would be more conveniently generated by droplets with small sizes through the drag-reducing liquid–liquid interface, giving origin to its low drag force. There is a proportional relationship between drag force F_D and the viscosity-resistance coefficient η; that is, F_D ∝ η, and η_(S-L) > η_(L-L). (Scale bars, 100 μm.) (B) Different energy states of dispersed (oil) and continuous (water) phases for generating emulsions in non-DRLGIE (PM) and DRLGIE systems. The initial state is the lowest-energy state, due to the separated oil and water phases. Introducing the extra energy input leads to oil and water phases mixing and, finally, to a metastable state. Δt is the time difference in which the DRLGIE system reaches the final state faster than the non-DRLGIE (PM) system. (C) Comparison of energy-consumption densities between DRLGIE and commercial systems [e.g., non-DRLGIE (PM) (33, 38–40), vortex mixing (36, 37), ultrasound (29–33), and homogenization (29, 34, 35)]. To clearly present the energy-consumption densities of the DRLGIE system, the lowest energy-consumption density (dark star) and other measurements (light stars) are included.

where μ_D is the viscosity of the dispersed liquid, u′ is the flow rate of the dispersed liquid in the pore, and L is the pore size of the porous solid.

In order to reduce the drag force between liquid and solid, our strategy is to create a smoother liquid–liquid interface by introducing a drag-reducing gating liquid Krytox 100 into the DRLGIE system, which provides a lower viscosity-resistance coefficient η_(L-L), then resulting in a lower drag force F_D(DRLGIE). These will contribute to more convenient transportation for dispersed liquid, dispersing smaller droplets into continuous liquid (Fig. 1A). The drag force F_D(DRLGIE) is related to the η_(L-L) as follows (SI Appendix, Eq. S35):

$$F_{\text{D}\left(\text{DRLGIE}\right)}=-{{\displaystyle \int }}^{\text{}}{\eta }_{\left(\text{L}-\text{L}\right)}{\left(\frac{\partial v_{\text{rz}}}{\partial r}\right)}_{\left(r\hspace{0.17em}=\hspace{0.17em}L/2\right)}\text{d}s,$$ [2]

where r and z are the radius axis and vertical axis of the cylindrical coordinate system, respectively. v_rz is the velocity component on the r–z plane, and at r = L/2, the integral of the shear stress along the cylindrical coordinate system is the drag force of the fluid during movement.

Fig. 1B exhibits the energy states of the oil and water phases in the emulsion-generating process with the PM and DRLGIE systems. Both systems have the minimum energy state at the initial stage, where the oil and water phases are normally immiscible, owing to liquid–liquid phase separation. In order to get the stabilized emulsions in the final state, external energy input is needed to overcome the energy barrier and active two phases to reach a high-energy medium state. The PM system has a high energy input to overcome its undesirable high resistance from the solid–liquid interface during the emulsification process. Conversely, the DRLGIE system needs less energy input to complete the emulsifying, based on its drag-reducing liquid–liquid interface to have a much lower transmembrane pressure. At a constant flow rate, the DRLGIE system can reach the final state faster than the PM system.

Fig. 1C contrasts the energy-consumption densities for preparing emulsions in the commercial systems and the DRLGIE system. The energy-consumption density [E_V = P/Q_E, where P is the energy input and Q_E is the volume of emulsion produced per unit time (28)] is used to evaluate the energy consumption per unit volume of emulsion generated by different emulsification systems. The distribution ranges of ultrasound and homogenization methods are similar, which are 1.80 × 10⁴ ∼1.62 × 10⁶ kJ/m³ (29–33) and 2.00 × 10⁴ ∼1.60 × 10⁶ kJ/m³ (29, 34, 35), respectively. For the vortex mixing, E_V is ranging from 8.20 × 10³ to 3.56 × 10⁵ kJ/m³ (36, 37). For PM, there is an obvious decline in E_V, as 3.96 × 10² ∼2.29 × 10⁵ kJ/m³ (33, 38–40). However, different from the above emulsification techniques, our work has the lowest E_V, with E_V = 3.86 × 10 ∼8.26 × 10 kJ/m³, and it can reach up to 4 orders of magnitude greater energy-saving than the PM method and can obtain 5 orders of magnitude greater energy-saving than the ultrasound and homogenization methods. The energy-saving of the DRLGIE system not only brings immeasurable economic benefits, but also causes a decrease in energy consumption through the improvement of energy-saving and energy efficiency to satisfy the UN goals stated in SDG 7 (27).

The dispersion of droplets is one of the essential properties for emulsions. In general, the droplets’ dispersity can be classified as polydispersity and monodispersity, depending on whether their radii are uniform. The droplet uniformity is crucial, which allows the samples to be divided into equal volumes, and remarkably increases the repeatability and reliability of microscale experiments in numerous aspects, such as life-science research and diagnostics (41, 42). Commercial systems and the DRLGIE system were evaluated to create droplets (Fig. 2A). Droplets in the commercial systems (e.g., ultrasound, homogenization, and vortex mixing) have broad radius ranges. While utilizing the PM or DRLGIE systems, highly monodispersed droplets can be obtained. More surprisingly, the radii of droplets in DRLGIE are about 33.3 μm, which are much smaller than those of PM (73.5 μm). The pore size L and the angles β or θ (β and θ are the angles between γ_CD and the horizontal directions in the PM and DRLGIE systems, respectively) will lead to the difference. As shown in Fig. 2B, the droplet diameter D in the PM and DRLGIE systems can be expressed by D_P = L_P/sinβ and D_D = L_D/sinθ, respectively. Within the dynamic liquid-lining layer in the DRLGIE system, it shows that L_P > L_D and β < θ (SI Appendix, Fig. S1), resulting in D_P > D_D. Moreover, the total surface area S of droplets in the DRLGIE system increased faster than that in the PM system (Fig. 2C). The difference between the diameter D_P and D_D and the time difference Δt (which is the time difference derived from the pressure between two systems to reach the threshold [SI Appendix, Fig. S4]) directly affect the S, in which S_(PM) = 3Q(t − Δt)/2D_P, S_(DRLGIE) = 3Qt/2D_D (where Q is the flow rate and t is the time to produce droplets). In Fig. 2D, it was found that the DRLGIE system can reduce by 33.7% the input transmembrane pressure ΔP compared to that of the PM system at the pore size of 1 μm. By theoretical analysis of the balance of dynamic lift force (F_L), inertia force (F_I), pressure force (F_P), interfacial tension (F_γ), and drag force (F_D), ΔP in the PM and DRLGIE systems is as follows, respectively (Fig. 2B and SI Appendix, Eqs. S20 and S39):

$$\text{Δ}{P}_{\left(\text{PM}\right)}=\frac{288{\mu }_{\text{D}}^2}{121L_{\text{P}}^2{\rho }_{\text{D}}}{\eta }_{\left(\text{S}-\text{L}\right)}^2+\frac{4{\gamma }_{\text{CD}}\sin \beta }{11L_{\text{P}}},$$ [3]

where ${\mu }_{\text{D}}$ is the viscosity of the dispersed liquid, and ${\rho }_{\text{D}}$ is the density of the dispersed liquid.

$$\text{Δ}{P}_{\left(\text{DRLGIE}\right)}=\frac{32{\mu }_{\text{R}}}{11L_{\text{D}}}{\eta }_{\left(\text{L}-\text{L}\right)}+\frac{4({\gamma }_{\text{CD}}\sin \theta +{\gamma }_{\text{DR}}\sin \left(\theta +\psi \right)+{\gamma }_{\text{CR}}\sin \varphi )}{11L_{\text{D}}},$$ [4]

Fig. 2.

Properties and advantages of DRLGIE. (A) Dispersibility of droplets prepared by commercial systems (e.g., ultrasound, homogenization, vortex mixing, and PM) and DRLGIE systems. Ultrasound, homogenization, and vortex mixing generated polydisperse droplets. PM and DRLGIE created monodispersed droplets. (Scale bars, 100 μm.) (B) Schematic of the droplet formation of the PM and DRLGIE systems at the triple-phase interfaces. (C) The time-dependent total surface area of droplets generated by the PM and DRLGIE systems at a flow rate of 0.5 mL/min. Δt is the time difference between the PM and DRLGIE systems starting to produce droplets. (D) The transmembrane pressure of the PM and DRLGIE systems. Compared with PM, DRLGIE can reduce pressure input by 33.7%. (E) Infrared images and their corresponding temperatures of emulsions during the emulsifying process prepared by the ultrasound and DRLGIE systems. (Scale bar, 1 cm.)

where μ_R is the viscosity of the drag-reducing gating liquid, $\varphi$ is the angle between γ_CR and the horizontal direction, and $\psi$ is the angle between γ_CD and γ_DR. Both equations give the dependence with respect to the viscosity-resistance coefficient and the vertical interfacial tensions, differing in their power law of the viscosity-resistance coefficient and the values of interfacial tensions. According to theoretical calculations, the vertical interfacial tensions in the DRLGIE system are higher than those in the PM system (SI Appendix, section S2), whereas ${\eta }_{\left(\text{L}-\text{L}\right)}<{\eta }_{(\text{S}-\text{L})}\ll {\eta }_{(\text{S}-\text{L})}^2$, $\frac{32{\mu }_{\text{R}}}{11L_{\text{D}}}{\eta }_{\left(\text{L}-\text{L}\right)}$ is much lower than $\frac{288{\mu }_{\text{D}}^2}{121L_{\text{P}}^2{\rho }_{\text{D}}}{\eta }_{(\text{S}-\text{L})}^2$, eventually resulting in ΔP_(DRLGIE) < ΔP_(PM). Furthermore, with the dynamic liquid–liquid interface, the DRLGIE system presents the antifouling property. The DRLGIE system has arrested the risk for fouling, such as the Fe₃O₄ nanoparticles solution, whereas the PM system suffered from heavy pollution and the contaminated matrix that was quite difficult to clean (SI Appendix, Fig. S5A). It was found that the permeability in the PM system declines even after rinsing (SI Appendix, Fig. S5B), while the flux recovery in the DRLGIE system will improve the energy-saving behavior in long-term operation, as well as offer potential opportunities in complex emulsions and applications.

Additionally, for the majority of commercial techniques, a large amount of energy input is normally dissipated as heat during the long-term emulsifying operation. For example, ultrasound, a typical emulsification method, makes extensive use of generating droplets, but the procedure still faces high energy-cost limitations, especially causing the temperature of the solution to increase rapidly from ambient temperature (25.5 °C) to 95.7 °C after operating for 10 min (Fig. 2E). When the operation stopped, its temperature declined to ambient temperature, and the heat was lost into the environment. However, the temperature in the DRLGIE system keeps at the ambient temperature, and no additional energy is lost in the form of heat. This indicates that the DRLGIE system has the energy-saving property and also provides a suitable environment for temperature-sensitive biological ingredients, like enzymes, proteins, and bacteria, to keep performance or activity (15) in food, cosmetics, and medicine (43).

Deeply exploring the DRLGIE system, the diameters of droplets are controllable in the appropriate operation conditions, which are convenient and easier compared with the conventional systems. The pore size, the flow rate of dispersed liquid, and the concentration of surfactant solutions in the DRLGIE system play significant roles in emulsion generation, affecting the transmembrane pressure and determining the diameters of the droplets. When increasing the pore sizes of membranes, the lower transmembrane pressure (SI Appendix, Fig. S9) and larger-sized droplets (Fig. 3A) can be obtained. It is worth noting that the energy-saving rates [(P_(DRLGIE) − P_(PM))/P_(PM)] were enhanced as pore sizes increased from 0.22 μm to 1 μm, and then decreased as pore sizes rose, further from 1 μm to 10 μm. Simultaneously, the energy-saving efficiency reached the highest as the pore size was 1 μm. At the pore size of 0.22 μm, the pressure in the DRLGIE system was higher than that of the PM system, owing to the thickness of the drag-reducing gating liquid layer in pores that cannot be ignored (44). The flow rate of dispersed liquid further impacts the transmembrane pressure and the diameters of the droplets. For the flow rate, previous studies have found that it is an important experimental parameter for the process of multiphase separation (24, 45, 46), but the contribution of the liquid–liquid interface itself has not been studied in depth. Most of them are from the discussion of the transport behavior of fluid inside microscopic pores, rather than the analysis of microscopic liquid–liquid interface drag-reducing behavior. Unlike the separation process for the study of the emulsification process, a rising in the flow rate would increase the transmembrane pressure on the basis of Darcy’s law (Fig. 3B and SI Appendix, Eq. S40) (47),

$$\mathrm{\Delta }P=\frac{Q\mu l}{k{A}_{\text{tot}}},$$ [5]

where ΔP is the transmembrane pressure, Q is the flow rate, μ is the dynamic viscosity, l is the film thickness, k is the permeability (SI Appendix, Eq. S46), and A_tot is the cross-sectional area at the macroscopic view. The theoretical predictions forecast the transmembrane pressures at a series of flow rates (Fig. 3B, lines), which also match experimental results (Fig. 3B, squares). At the same time, rising flow rate is accompanied by an increased frequency of droplet production and accelerating the coalescence of small droplets to larger ones after generating over the droplet-generation membrane surface. It indicates that increasing the flow rate may be at the expense of droplet size distribution. Besides, surfactant concentration also plays an essential role in droplet formation. By increasing the concentration of surfactant, the smaller droplets could be obtained (Fig. 3C), but there was no difference in transmembrane pressures (Fig. 3D). At the lower concentrations of surfactants (0.01 mM), there is hardly any emulsion, which is brought about by only a few surfactant molecules adsorbed at the oil–water interface, and the surfactant films work weakly to prevent the oil droplets from amalgamating into the larger ones. With more surfactant molecules in the interface to decline the interfacial tension γ_CD at the high concentration of surfactants, the diameters of droplets are reduced. Consequently, droplets with controllable behavior can be easily achieved by the DRLGIE system, which would be further preferred when delivering drugs (5) or generating a matrix with the desired size (7).

Fig. 3.

Controllability and performance of DRLGIE. (A) The energy-saving behavior with different pore sizes. (A, Upper Insets) Optical micrographs of droplets. (Scale bars, 100 μm.) (A, Lower Insets) Declining the pore sizes causes smaller droplets. (B) Droplet size and transmembrane pressure versus flow rate of dispersed liquid. The theoretical model (lines) fit well with experimental results (squares). (B, Inset) The higher flow rate of dispersed liquid leads to bigger droplets. (Scale bars, 100 μm.) (C) Relationship between interfacial tension and surfactant concentration. Increasing concentrations of surfactants lead to smaller droplets. (D) The influence of surfactant concentrations on transmembrane pressure. (D, Inset) Optical images of emulsions. (Scale bar, 1 cm.)

Integrating the drag-reducing liquid gating interface into the emulsification mechanism has offered opportunities to harness this ultrahigh-efficient emulsification technique with superior properties for high energy-saving in numerous applications, ranging from food, personal care, biomedicine, material, and pesticide to industry (Fig. 4A). In 2020, the worldwide polymer-emulsion market size was about $25.2 billion (48), in which microparticles are one of the fundamental products in emulsion polymerization. In Fig. 4B, we present the potential of DRLGIE for energy-saving in large-scale particle fabrication. At a certain applied pressure P (P_{critical (DRLGIE)} < P < P_{critical (PM)}), the dispersed liquid would transport through in the DRLGIE system, whereas it would be blocked in the PM system. Under the equal energy input with the same applied pressure, the DRLGIE system can create more emulsions. Furthermore, the emulsions generated by DRLGIE can be used as a template for dynamic interfacial reactions in particle polymerization. DRLGIE potentially has great utility as the large-scale fabrication method in the polymer-emulsion market for shaped materials that are utilized in rubbers, coatings, textile, and smart micromachines (48–50).

Fig. 4.

Applications of the DRLGIE system. (A) Application scenarios display, ranging from food, personal care, biomedicine, material, and pesticide to industry. (B) Material fabrication of DRLGIE. (B, Top) When applying a constant pressure that is higher than P_{critical (DRLGIE)}, but lower than P_{critical (PM)}, only DRLGIE can generate emulsions, and the droplets can be utilized as templates to form particles. (B, Middle) Optical micrographs of droplets. (Scale bars, 100 μm.) The optical images of emulsions. (Scale bars, 1 cm.) (B, Bottom) Scanning electron micrograph of particles made from polymerized droplets containing vinyl dimethicone and hydrogen silicone oil. (Scale bar, 100 μm.) (C) Enzymatic biphasic reaction of DRLGIE and ultrasound. (Scale bar, 1 cm.) (C, Upper) Infrared images of emulsions prepared by ultrasound and DRLGIE. (C, Lower) Enzyme activity of lipases emulsified by ultrasound and DRLGIE. With DRLGIE, the lipase can keep active. (D) Drug release of DRLGIE. The oil-phase coated Nile Red simulated drug loading, and the dyes would release after droplet contact with the ethanol. (Scale bars, 100 μm.)

Emulsion is also adopted as the catalytic platform in biphasic enzymatic reactions in both laboratories and industry, with the advantage of the high surface areas to accelerate interfacial reactions (51, 52). Lipase is one of the powerful biphasic catalysts, whose activity can be affected by the temperature. Unfortunately, traditional emulsification methods, especially ultrasound, often cause increasing temperatures after processing for a long time. The bioactivities of lipase-included emulsions in ultrasound and DRLGIE systems were further investigated in Fig. 4C. In the ultrasound system, the activity of lipases has about 81.7% loss, while in the DRLGIE system, the lipases preserved high-efficient bioactivities, resulting from a constant temperature. Thus, an appropriate microenvironment can be provided by the DRLGIE system for temperature-sensitive biological components, like enzymes, proteins, and bacteria, to resist denaturation.

Superior long-term physical and chemical stability make emulsion a promising candidate for a carrier in the biomedicine field, especially in drug delivery and vaccine engineering (4, 5). The biomedicine market is estimated to be worth over $18 billion in 2028 (53), and our approach is expected to have lower production costs to fuel the growth of the biomedicine market. Here, we utilized the Nile Red loaded into the oil droplets to simulate the drug-release process of the droplets in the DRLGIE system (Fig. 4D). Without external interference, the dye can be coated in the oil droplets stably. Once the droplet contacted the interference, such as ethanol, the encapsulated dye was released. All the above results ensure that DRLGIE is a sustainable green fabrication technique, having the outstanding energy-saving behavior to be applied in the food industry, the daily chemical industry, biomedicine, material fabrication, the petrochemical industry, and beyond.

Discussion

In summary, we developed an ultrahigh-efficient liquid gating emulsification method that harnesses the capillary-stabilized drag-reducing liquid gating structure to form a smooth liquid–liquid interface. The liquid–liquid interface not only provides shear stress for breaking up liquids into narrow droplets, but also declines the resistance for saving energy in emulsifying. The unique interface enables the achievement of uniform and controllable droplets and the suppression of fouling. Besides, for the temperature-sensitive biological components, such as proteins, enzymes, and bacteria, DRLGIE would assist them in maintaining functional structures in an appropriate environment to avoid exposure to high temperatures. This method offers a sustainable, green, efficient, and controllable fabrication method and drag-reducing liquid gating interfacial mechanism for the global emulsification industry to prepare emulsions with lower energy consumption, which would be scaled up to a wide range of applications, especially saving energy for long-term operation. Given the large demand for emulsification in the whole world, we believe that our approach will be of benefit to reducing energy expenditure, as well as bring a more eco-friendly and sustainable environment to our society.

Materials and Methods

Materials.

Hydrophilic porous nylon membranes with the average pore size of 0.22, 0.45, 1, 8, and 10 μm were purchased from Haining Zhongli Filtering Equipment Factory, Ltd. Dodecane (>99.0%) was purchased from Shanghai Aladdin Bio-Chem Technology Co., Ltd. AKYPO RO 90 VG was provided by Kao Chemicals GmbH. DuPont Krytox GPL 100, 103, and 107 were purchased from MiDRIEr-Stephenson. Fe₃O₄ nanoparticles (diameter: 10 to 30 nm, 25% in H₂O) were purchased from Macklin. Vinyl dimethicone and hydrogen silicone oil were purchased from Zhejiang Xinan Chemical Industrial Group Co., Ltd. Nile Red was purchased from Tianjin HEOWNS Bio-Chem Technology Co., Ltd. Chloroplatinic acid (analytical reagent grade [AR]), toluene (AR), and ethanol (AR) were purchased from Sinopharm Chemical Reagent Co., Ltd. Lipase from Candida rugosa was purchased from Sigma-Aldrich. A lipase activity assay kit was purchased from Beijing Boxbio Science & Technology Co., Ltd. Milli-Q deionized (DI) water with a resistivity of 18.2 MΩ·cm was used in all experiments.

Morphology Characterization.

The scanning electron microscope (SEM) images of the porous nylon membranes were obtained by a field-emission SEM (Zeiss, GeminiSEM 500).

Selection of Drag-Reducing Gating Liquid.

In order to obtain a stable system of DRLGIE, there are two essential principles: 1) Liquids served as dispersed, continuous, and drag-reducing gating liquids are extremely immiscible; and 2) the density of drag-reducing gating liquid should be higher than that of both dispersed and continuous liquids (SI Appendix, Fig. S3A), which are not only crucial for the drag-reducing gating liquid to avoid the risk of being replaced from pores, but also to ensure the stability of creating emulsions. Based on the above principles, a series of drag-reducing gating liquids, including perfluoropolyether, silicone oil, and dichloromethane, were tested to obtain a suitable drag-reducing gating liquid. Results demonstrated that the perfluoropolyether exhibited superior stability in our study (SI Appendix, Table S1). Additionally, the transmembrane pressure declined when reducing the viscosity of the drag-reducing gating liquid (SI Appendix, Fig. S3B), and the transmembrane pressure was the lowest when utilizing Krytox 100 as the drag-reducing gating liquid. Hence, the Krytox 100 is appropriate to be selected as the drag-reducing gating liquid in the following exploration.

Preparation of DRLGIE.

Drag-reducing gating liquid was added to the surface of the nylon membrane by a micropipette. After the membrane was wetted by the drag-reducing gating liquid, we suspended the membrane vertically to remove the excess liquid to prepare DRLGIE.

Production of Emulsions by DRLGIE.

Dodecane was used as the dispersed liquid, and water in combination with nonionic surfactant oleth-10 carboxylic acid (90 VG) was employed as the continuous liquid. Dodecane was transported through the DRLGIE system and dispersed into surfactant solution to form an oil/water emulsion.

Transmembrane-Pressure Measurements.

The transmembrane pressures of systems were measured by a wet/wet current-output differential pressure transmitter (catalog no. PX273-100DI) from OMEGA Engineering Inc.

Antifouling Property Measurements.

Fe₃O₄ nanoparticle suspension (diameter: 10 to 30 nm, 25 Vol%) was diluted to an ∼0.5 Vol% suspension. The antifouling properties of the PM and DRLGIE systems were tested by infusing nanoparticles suspension through the membrane for 1 min at the flow rate of 0.5 mL/min, followed by rinsing of 3 mL DI water.

Surface Tension and Interfacial Tension Measurements.

The surface tension and interfacial tension of different liquids were measured by a contact-angle goniometer (DataPhysics OCA100) at 25 °C. The surface tension and interfacial tension data were recorded with droplet volume as large as possible. For dodecane droplets, the volume was about 10 μL. For Krytox 100, Krytox 103, and Krytox 107 droplets, the volume was about 3 μL. For aqueous droplets, the volume was about 25 μL. All experiments were repeated at least three times.

Characterization of Emulsions.

The micrographs of emulsions were recorded by an Olympus IX73 Microscope. The droplet diameter distribution of the emulsion was measured by Image-Pro Plus software.

Determination of Lipase Activity.

The lipase activity was measured with a Sucrose Synthase Activity Assay Kit (Boxbio). The activity of lipase was determined by ultraviolet visible near-infrared (LAMBDA 1050+).

Droplet Template for Generating Particles.

The mixture of vinyl dimethicone, hydrogen silicone oil, and chloroplatinic acid was used as the dispersed liquid. The droplets were prepared by the DRLGIE system and heated at 50 °C for 30 min to form particles.

Data Availability

All study data are included in the article and/SI Appendix.