Controllable preparation process of gas core double emulsion droplets via synergistic effect of backpressure and capillary number

doi:10.12034/j.issn.1009-606X.225306

Abstract

Abstract:

Gas core double emulsions serve as critical precursors for fabricating high-performance hollow microcapsules; however, precise control over their geometric characteristics remains a significant challenge due to the compressibility of the gas core and the complexity of multiphase interactions. To address issues of unstable encapsulation and the difficulty in independent dimension control, this study systematically investigates the controllable preparation of gas-oil-water emulsions under constant gas pressure using a dual flow-focusing microfluidic device. The research focuses on establishing a robust process strategy by manipulating key operating parameters, including oil phase viscosity, flow rates, and surfactant concentration. The experimental results demonstrate that the droplet formation process is governed by a distinct coupling mechanism involving the capillary number and junction backpressure. Increasing oil phase viscosity and flow rate elevate the backpressure at the flow junction, which effectively compresses the gas core and increases the shell thickness. The outer-phase parameters dominate the droplet breakup dynamics. By adjusting the surfactant concentration to alter interfacial tension, the overall droplet size can be tuned independently of the core size. Furthermore, based on the experimental data, a dimensionless empirical-physical model is developed to quantitatively predict the shell-to-radius ratio. This model functions as an effective inverse-design tool, enabling the reverse calculation of required flow conditions to achieve a target morphology. This work clarifies the fluid dynamic mechanisms from a process engineering perspective, providing theoretical guidance and a practical basis for the programmable and controllable fabrication of hollow microcapsules with specific structural characteristics.

Key words: gas core double emulsion droplet, microfluidics, capillary number, backpressure, fluid dynamics, controllable preparation

摘要：

气核双乳液是制备高性能中空微胶囊的关键前体，其尺寸和壳厚的精确调控是实现功能化应用的前提。针对微流控法制备过程中气核包封不稳定及壳厚难以独立调控的问题，本工作采用双流动聚焦微流控器件，在恒定供气压力下，系统研究了多相流参数对液滴形成动力学的影响。考察了油相黏度、流量比及外相表面张力等关键过程参数的影响，揭示了毛细管数与节点背压的耦合机制。研究发现，油相黏度和流量主要通过调节节点背压来控制气核体积，从而决定壳层厚度；而外相表面活性剂浓度则通过改变界面张力和外相毛细管数，主导液滴的剪切破碎过程。此外，基于实验数据，建立了一个包含毛细管数比和流量比的无量纲经验-物理混合模型，实现了对壳径比的定量预测。本研究阐明了气-油-水三相微流动过程中的流体动力学机理，为可控制备具有特定结构特征的中空微胶囊提供了理论指导和工艺设计依据。

关键词: 气核双乳液滴, 微流控, 毛细管数, 背压, 流体动力学, 可控制备

CLC Number:

O648.23

Jianhao ZHAO, Dongxiang WANG, Fangyang YUAN, Xinjun YANG, Jiyun DU, Wei YU. Controllable preparation process of gas core double emulsion droplets via synergistic effect of backpressure and capillary number[J]. The Chinese Journal of Process Engineering, 2026, 26(9): 987-997.

赵建浩, 王东祥, 袁方洋, 杨新俊, 杜继芸, 于伟. 基于背压-毛细管数协同效应的气核双乳液滴可控制备工艺[J]. 过程工程学报, 2026, 26(9): 987-997.

Figures/Tables 10

Fig.1 Experimental procedure for G/O/W double emulsion preparation

Fig.2 Schematic diagram of force analysis during the formation of gas core double emulsions

Table 1 Viscosity of the oil phase

Oil phase	100wt% PDMS	75wt% PDMS+25wt% silicone oil	50wt% PDMS+50wt% silicone oil
μ_o/mPa·s	280	195	62

Fig.3 Morphologies and size characteristics of G/O/W droplets under different oil phase viscosities and Qw/Qo (oil phase flow rate fixed at 0.25 mL/h) [(a) droplet diameter data; (b) gas core diameter data; (c) shell thickness data; (d) shell-to-radius ratio data; (e) optical micrographs of droplets generated using 100wt% PDMS oil phase at different traffic ratios: (e1) Qw/Qo=9.0, De=460 μm, Dg=410 μm; (e2) Qw/Qo=10.0, De=460 μm, Dg=360 μm; (e3) Qw/Qo=11.0, De=450 μm, Dg=360 μm; (e4) Qw/Qo=12.0, De=430 μm, Dg=340 μm]

Fig.4 Morphologies and size characteristics of G/O/W droplets under different oil phase viscosities and oil phase flow rates (water phase flow rate fixed at 2.25 mL/h) [(a) droplet diameter data; (b) gas core diameter data; (c) shell thickness data; (d) shell-to-radius ratio data; (e) optical micrographs of droplets generated using 100wt% PDMS oil phase at different oil phase flow rates: (e1) Qo=0.26 mL/h, De=420 μm, Dg=320 μm; (e2) Qo=0.27 mL/h, De=425 μm, Dg=300 μm; (e3) Qo=0.28 mL/h, De=425 μm, Dg=260 μm; (e4) Qo=0.29 mL/h, De=400 μm, Dg=250 μm]

Table 2 Viscosity of the aqueous phase

Aqueous phase	3wt% PVA	3wt% PVA+0.10wt% SDS	3wt% PVA+0.15wt% SDS	3wt% PVA+0.20wt% SDS
μ_w/mPa·s	14	37	50	60

Table 3 Interfacial tension between the 75wt% PDMS oil phase and the aqueous phases

Aqueous phase	3wt% PVA	3wt% PVA+0.10wt% SDS	3wt% PVA+0.15wt% SDS	3wt% PVA+0.20wt% SDS
γ_ow/(mN/m)	20	16	14	12.5

Fig.5 Morphologies and size characteristics of G/O/W droplets under different outer-phase SDS concentrations and traffic ratios (Qw/Qo) (oil phase flow rate fixed at 0.25 mL/h) [(a) droplet diameter data; (b) gas core diameter data; (c) shell thickness data; (d) shell-to-radius ratio data; (e) optical micrographs of droplets generated using 3wt% PVA+0.20wt% SDS aqueous phase at different traffic ratios: (e1) Qw/Qo=9.5, De=400 μm, Dg=325 μm; (e2) Qw/Qo=10.0, De=390 μm, Dg=300 μm; (e3) Qw/Qo=10.5, De=340 μm, Dg=235 μm; (e4) Qw/Qo=11.0, De=360 μm, Dg=230 μm]

Fig.6 Morphologies and size characteristics of G/O/W droplets under different outer-phase SDS concentrations and oil phase flow rates (water phase flow rate fixed at 2.25 mL/h) [(a) droplet diameter data; (b) gas core diameter data; (c) shell thickness data; (d) shell-to-radius ratio data; (e) optical micrographs of droplets generated using 3wt% PVA+0.20wt% SDS aqueous phase at different oil phase flow rates: (e1) Qo=0.26 mL/h, De=390 μm, Dg=300 μm; (e2) Qo=0.27 mL/h, De=350 μm, Dg=255 μm; (e3) Qo=0.28 mL/h, De=340 μm, Dg=235 μm; (e4) Qo=0.29 mL/h, De=330 μm, Dg=225 μm]

Fig.7 Comparison between predicted and measured values of the shell-to-radius ratio (T/R)

References 28

[1]	Choudhury N, Meghwal M, Das K. Microencapsulation: an overview on concepts, methods and applications [J]. Food Frontiers, 2021, 2(3): 327-344.
[2]	Silva B F B, Rodríguez-Abreu C, Vilanova N. Recent advances in multiple emulsions and their application as templates [J]. Current Opinion in Colloid & Interface Science, 2016, 25: 98-108.
[3]	Zhu J Y, He J Y, Zhou J, et al. Recent progress in microencapsulation technology and its applications in petroleum industry [J]. Journal of Molecular Liquids, 2024, 407: 125162.
[4]	Molley T G, Jiang S, Ong L, et al. Gas-modulating microcapsules for spatiotemporal control of hypoxia [J]. Proceedings of the National Academy of Sciences, 2023, 120(17): e2217557120.
[5]	Toprakcioglu Z, Hakala T A, Levin A, et al. Multi-scale microporous silica microcapsules from gas-in-water-in-oil emulsions [J]. Soft Matter, 2020, 16(13): 3082-3087.
[6]	Yoon D H, Hasegawa K, Kaneko Y, et al. Formation of polymeric hollow microcapsules and microlenses using gas-in-organic-in-water droplets [J]. Micromachines, 2015, 6(5): 622-633.
[7]	Chen R, Dong P F, Xu J H, et al. Controllable microfluidic production of gas-in-oil-in-water emulsions [J]. Lab on a Chip, 2012, 12(20): 3910-3916.
[8]	Duran M, Serrano A, Nikulin A, et al. Microcapsule production by droplet microfluidics: a review from the material science approach [J]. Materials & Design, 2022, 223: 111230.
[9]	Lobel B T, Baiocco D, Al-Sharabi M, et al. Current challenges in microcapsule designs and microencapsulation processes: a review [J]. ACS Applied Materials & Interfaces, 2024, 16(31): 40326-40355.
[10]	Datta S S, Abbaspourrad A, Amstad E, et al. Double emulsion templated solid microcapsules: mechanics and controlled release [J]. Advanced Materials, 2014, 26(14): 2205-2218.
[11]	Vladisavljević G T, Al Nuumani R, Nabavi S A. Microfluidic production of multiple emulsions [J]. Micromachines, 2017, 8(3): 75.
[12]	Yang L, Wang K, Mak S, et al. A novel microfluidic technology for the preparation of gas-in-oil-in-water emulsions [J]. Lab on a Chip, 2013, 13(17): 3351-3354.
[13]	Nekouei M, Vanapalli S A. Volume-of-fluid simulations in microfluidic T-junction devices: influence of viscosity ratio on droplet size [J]. Physics of Fluids, 2017, 29(3): 032007.
[14]	Utada A S, Lorenceau E, Link D R, et al. Monodisperse double emulsions generated from a microcapillary device [J]. Science, 2005, 308(5721): 537-541.
[15]	Yang S H, Song W L, Fan L L, et al. Microfluidic fabrication of monodisperse microcapsules with gas cores [J]. Lab on a Chip, 2024, 24: 3556-3567.
[16]	Anna S L. Droplets and bubbles in microfluidic devices [J]. Annual Review of Fluid Mechanics, 2016, 48: 285-309.
[17]	Dai Y C, Cha H T, Nguyen N K, et al. Dynamic behaviours of monodisperse double emulsion formation in a tri-axial capillary device [J]. Micromachines, 2022, 13(11): 1877.
[18]	Hu C Y, Jiang F, Yan J. Formation of double emulsion droplets in flow-focusing microchips: a numerical parametric study [J]. Journal of Flow Chemistry, 2024, 14: 641-653.
[19]	Calhoun S G K, Brower K K, Chandran Suja V, et al. Systematic characterization of effect of flow rates and buffer compositions on double emulsion droplet volumes and stability [J]. Lab on a Chip, 2022, 22: 2315-2330.
[20]	Cleve S, Lassus A, Diddens C, et al. Microbubble formation by flow focusing: role of gas and liquid properties, and channel geometry [J]. Journal of Fluid Mechanics, 2023, 972: A27.
[21]	He C X, Liu X Y, Dong Y P, et al. Droplet formation in a step-emulsification microdevice: effect of fluid's viscosity [J]. Chemical Engineering and Processing: Process Intensification, 2023, 185: 109309.
[22]	Wang S T, Wang H, Wu Y T, et al. Numerical study of double emulsion generation in a flow-focusing microchannel by multiple-relaxation-time lattice Boltzmann method [J]. Physics of Fluids, 2024, 36(3): 032010.
[23]	Deng B X, Schroën K. Dynamic interfacial effects investigated by microfluidics: formation and stability of droplets and bubbles [J]. Current Opinion in Colloid & Interface Science, 2024, 73: 101826.
[24]	Huang B, Ge X J, Rubinstein B Y, et al. Gas-assisted microfluidic step-emulsification for generating hollow-core double emulsions [J]. Microsystems & Nanoengineering, 2023, 9: 38.
[25]	Wang J W, Hahn S, Amstad E, et al. Tailored double emulsions made simple [J]. Advanced Materials, 2022, 34(5): 2107338.
[26]	Xie B S, Jiang F, Lin H J, et al. Numerical investigation of the effect of microfluidic flow parameters and physical properties on double emulsion droplet forming [J]. Meccanica, 2024, 59: 157-168.
[27]	Lee D, Shen A Q. Interfacial tension measurements in microfluidic quasi-static extensional flows [J]. Micromachines, 2021, 12(3): 272.
[28]	Brosseau Q, Vrignon J, Baret J C. Microfluidic dynamic interfacial tensiometry (μDIT) [J]. Soft Matter, 2014, 10(17): 3066-3076.