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Author:

Liu, Haihu (Liu, Haihu.) (Scholars:刘海湖) | Ba, Yan (Ba, Yan.) | Wu, Lei (Wu, Lei.) | Li, Zhen (Li, Zhen.) | Xi, Guang (Xi, Guang.) (Scholars:席光) | Zhang, Yonghao (Zhang, Yonghao.)

Indexed by:

SCIE EI Scopus

Abstract:

Droplet dynamics in microfluidic applications is significantly influenced by surfactants. It remains a research challenge to model and simulate droplet behaviour including deformation, breakup and coalescence, especially in the confined microfluidic environment. Here, we propose a hybrid method to simulate interfacial flows with insoluble surfactants. The immiscible two-phase flow is solved by an improved lattice Boltzmann colour-gradient model which incorporates a Marangoni stress resulting from non-uniform interfacial tension, while the convection-diffusion equation which describes the evolution of surfactant concentration in the entire fluid domain is solved by a finite difference method. The lattice Boltzmann and finite difference simulations are coupled through an equation of state, which describes how surfactant concentration influences interfacial tension. Our method is first validated for the surfactant-laden droplet deformation in a three-dimensional (3D) extensional flow and a 2D shear flow, and then applied to investigate the effect of surfactants on droplet dynamics in a 3D shear flow. Numerical results show that, at low capillary numbers, surfactants increase droplet deformation, due to reduced interfacial tension by the average surfactant concentration, and non-uniform effects from non-uniform capillary pressure and Marangoni stresses. The role of surfactants on the critical capillary number (Ca-cr) of droplet breakup is investigated for various confinements (defined as the ratio of droplet diameter to wall separation) and Reynolds numbers. For clean droplets, Cacr first decreases and then increases with confinement, and the minimum value of Ca-cr is reached at a confinement of 0.5; for surfactant-laden droplets, Ca-cr exhibits the same variation in trend for confinements lower than 0.7, but, for higher confinements, Ca-cr is almost a constant. The presence of surfactants decreases Ca-cr for each confinement, and the decrease is also attributed to the reduction in average interfacial tension and non-uniform effects, which are found to prevent droplet breakup at low confinements but promote breakup at high confinements. In either clean or surfactant-laden cases, Ca-cr first remains almost unchanged and then decreases with increasing Reynolds number, and a higher confinement or Reynolds number favours ternary breakup. Finally, we study the collision of two equal-sized droplets in a shear flow in both surfactant-free and surfactant-contaminated systems with the same effective capillary numbers. It is identified that the non-uniform effects in the near-contact interfacial region immobilize the interfaces when two droplets are approaching each other and thus inhibit their coalescence.

Keyword:

breakup/coalescence capillary flows computational methods

Author Community:

  • [ 1 ] [Liu, Haihu; Li, Zhen; Xi, Guang] Xi An Jiao Tong Univ, Sch Energy & Power Engn, 28 West Xianning Rd, Xian 710049, Shaanxi, Peoples R China
  • [ 2 ] [Ba, Yan] Northwestern Polytech Univ, Sch Astronaut, 127 West Youyi Rd, Xian 710072, Shaanxi, Peoples R China
  • [ 3 ] [Wu, Lei; Zhang, Yonghao] Univ Strathclyde, Dept Mech & Aerosp Engn, James Weir Fluids Lab, Glasgow G1 1XJ, Lanark, Scotland
  • [ 4 ] [Liu, Haihu]Xi An Jiao Tong Univ, Sch Energy & Power Engn, 28 West Xianning Rd, Xian 710049, Shaanxi, Peoples R China
  • [ 5 ] [Li, Zhen]Xi An Jiao Tong Univ, Sch Energy & Power Engn, 28 West Xianning Rd, Xian 710049, Shaanxi, Peoples R China
  • [ 6 ] [Xi, Guang]Xi An Jiao Tong Univ, Sch Energy & Power Engn, 28 West Xianning Rd, Xian 710049, Shaanxi, Peoples R China
  • [ 7 ] [Ba, Yan]Northwestern Polytech Univ, Sch Astronaut, 127 West Youyi Rd, Xian 710072, Shaanxi, Peoples R China
  • [ 8 ] [Wu, Lei]Univ Strathclyde, Dept Mech & Aerosp Engn, James Weir Fluids Lab, Glasgow G1 1XJ, Lanark, Scotland
  • [ 9 ] [Zhang, Yonghao]Univ Strathclyde, Dept Mech & Aerosp Engn, James Weir Fluids Lab, Glasgow G1 1XJ, Lanark, Scotland

Reprint Author's Address:

  • 刘海湖

    Xi An Jiao Tong Univ, Sch Energy & Power Engn, 28 West Xianning Rd, Xian 710049, Shaanxi, Peoples R China.

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Source :

JOURNAL OF FLUID MECHANICS

ISSN: 0022-1120

Year: 2018

Volume: 837

Page: 381-412

3 . 1 3 7

JCR@2018

3 . 6 2 7

JCR@2020

ESI Discipline: ENGINEERING;

ESI HC Threshold:108

JCR Journal Grade:2

CAS Journal Grade:2

Cited Count:

WoS CC Cited Count: 54

SCOPUS Cited Count: 80

ESI Highly Cited Papers on the List: 0 Unfold All

WanFang Cited Count:

Chinese Cited Count:

30 Days PV: 3

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