Effective colloidal emulsion droplet regulation in flow-focusing glass capillary microfluidic device via collection tube variation

Abstract

Colloidal emulsion droplets, created using glass capillary microfluidic devices, have been found in a myriad of applications, serving as subtle microcarriers, delicate templates, etc. To meet the objective requirements under varying circumstances, it is crucial to efficiently control the morphology and dimensions of the droplets on demand. The glass capillary collection tube is a crucial component of the flow-focusing microfluidic system due to its close association with the geometrical confinement of the multiphasic flow. However, there are currently no guidelines for the design of the morphology and dimensions of the glass capillary collection tube, which shall result in a delay in assessing serviceability until after the microfluidic device is prepared, thereby causing a loss of time and effort. Herein, an experimental study was conducted to investigate the effect of the geometrical characteristics of glass capillary collection tubes on the production of colloidal emulsion droplets. After characterizing the generated colloidal emulsion droplets, it was found that the geometrical variations of the glass capillary collection tube resulted in numerical disparities of droplets due to different degrees of flow-focusing effects. The stronger flow-focusing effect produced smaller droplets at a higher frequency, and the dimensional variation of colloidal emulsion droplets was more responsive to varying flow rates. Furthermore, the transformation from colloidal single-core double-emulsion droplets to multi-core double-emulsion droplets also changed with the flow rate due to the glass capillary collection tube morphology-determined varying flow-focusing effect. These experimental findings can offer qualitative guidance for the design of glass capillary microfluidic devices in the preliminary stage, thus facilitating the smooth production of desired colloidal emulsion droplets.

Fig. 1. Experimental and schematic illustration of the system for generating and characterizing colloidal emulsion droplets.

Fig. 2. (a) Schematic illustration of the flow-focusing glass capillary microfluidic device for colloidal single/double-emulsion droplet generation. (b) The microfluidic flow rate regulation results in the generation of colloidal single-emulsion droplets with a traditional conical glass capillary collection tube. (c) The physical illustration of the as-prepared glass capillary microfluidic device with three different morphologies of the collection tube. The scale bars in (b) are 500 μm.

Fig. 3. The integrated regulation results of the generation of colloidal single-emulsion droplets through the combination of flow rate adjustment and the variation of the morphology and the dimension of the collection tubes in glass capillary microfluidic devices. (a) Optical images of microfluidic devices with conical glass capillary collection tubes in three different dimensions. (b) The dimensionless diameters of the colloidal single-emulsion droplets generated with glass capillary microfluidic devices correspond to (a). (c) The generation frequencies of the colloidal single-emulsion droplets produced with glass capillary microfluidic devices correspond to (a). (d) Optical images of microfluidic devices with inner concave glass capillary collection tubes in three different dimensions. (e) The dimensionless diameters of the colloidal single-emulsion droplets generated with glass capillary microfluidic devices correspond to (d). (f) The generation frequencies of the colloidal single-emulsion droplets produced with glass capillary microfluidic devices correspond to (d). (g) Optical images of microfluidic devices with parallel straight glass capillary collection tubes in three different dimensions. (h) The dimensionless diameters of the colloidal single-emulsion droplets generated with glass capillary microfluidic devices correspond to (g). (i) The generation frequencies of the colloidal single-emulsion droplets produced with glass capillary microfluidic devices correspond to (g). In addition, the flow rate of the Q₁ phase is fixed at 0.5 ml h⁻¹ during the flow rate regulation of the Q₂ phase. Scale bars are 500 μm.

Fig. 4. In situ optical images of generated colloidal single-emulsion droplets produced with glass capillary microfluidic devices with three different collection tube morphologies and the same dimension of 300 μm. (a) Optical images of the glass capillary microfluidic device with a conical collection tube. (b) Optical images of the glass capillary microfluidic device with an inner concave collection tube. (c) Optical images of the glass capillary microfluidic device with a parallel straight collection tube. Scale bars are 500 μm.

Fig. 5. Dimensionless diameter comparison of the colloidal single-emulsion droplets generated with glass capillary microfluidic devices with the same collection tube dimension, but different collection tube morphologies.

Fig. 6. The integrated regulation results of the generation of colloidal double-emulsion droplets through the combination of three-phase flow rate adjustment and the morphology variation of the collection tubes in glass capillary microfluidic devices. (a–c) Relationships between the dimensionless diameters of the droplet shell D_o and the droplet core D_i of the colloidal double-emulsion droplets concerning the corresponding single-phase flow rate regulation in a glass capillary microfluidic device with a conical collection tube. (d–f) Relationships between the dimensionless diameters of the droplet shell D_o and the droplet core D_i of the colloidal double-emulsion droplets concerning the corresponding single-phase flow rate regulation in a glass capillary microfluidic device with an inner concave collection tube. (g–i) Relationships between the dimensionless diameters of the droplet shell D_o and the droplet core D_i of the colloidal double-emulsion droplets concerning the corresponding single-phase flow rate regulation in a glass capillary microfluidic device with a parallel straight collection tube. In addition, the flow rate of the Q₂ and Q₃ phases are fixed at 1 ml h⁻¹ and 10 ml h⁻¹ respectively during the flow rate adjustment of the Q₁ phase. The flow rate of the Q₁ and Q₃ phases are fixed at 0.5 ml h⁻¹ and 10 ml h⁻¹ respectively during the flow rate adjustment of the Q₂ phase. The flow rate of the Q₁ and Q₂ phases are fixed at 0.5 ml h⁻¹ and 1 ml h⁻¹ respectively during the flow rate adjustment of the Q₃ phase.

Fig. 7. Optical images of the generated colloidal double-emulsion droplets during the single-phase flow rate adjustment corresponding to Fig. 7(a)–(c), respectively. Scale bars are 100 μm.

Fig. 8. Statistical analysis of the integrated regulation results of the generation of colloidal double-emulsion droplets through the combination of three-phase flow rate adjustment and the morphology variation of the glass capillary collection tube in microfluidic devices. (a–c) Dimensionless diameter comparison of the colloidal double-emulsion droplets generated with glass capillary microfluidic devices with the same collection tube dimension, but different collection tube morphologies regarding each single-phase flow rate regulation.

Fig. 9. In situ optical images of the generation process of colloidal multicore double-emulsion droplets produced with the glass capillary microfluidic device with a conical collection tube morphology during single-phase flow rate adjustment and the summary of the morphological regime of the colloidal double-emulsion droplets produced by glass capillary microfluidic devices with different collection tube morphologies concerning the flow rate variation of the Q₂ phase and Q₃ phase, while the flow rate of the Q₁ phase is fixed at 0.8 ml h⁻¹. (a) The generation process of colloidal multicore double-emulsion droplets during the single-phase flow rate adjustment of the Q₃ phase. (b) The generation process of colloidal multicore double-emulsion droplets during the single-phase flow rate adjustment of the Q₂ phase. (c) Statistical analysis results of the morphological regime of colloidal double-emulsion droplets generated by the glass capillary microfluidic device with a conical collection tube. (d) Statistical analysis results of the morphological regime of colloidal double-emulsion droplets generated by the glass capillary microfluidic device with an inner concave collection tube. (e) Statistical analysis results of the morphological regime of colloidal double-emulsion droplets generated by the glass capillary microfluidic device with a parallel straight collection tube. Scale bars are 500 μm.