Abstract
Microfluidic droplet technology has been developing rapidly. However, precise control of dynamical behaviour of droplets remains a major hurdle for new designs. This study is to understand droplet deformation and breakup under simple shear flow in confined environment as typically found in microfluidic applications. In addition to the Newtonian–Newtonian system, we consider also both a Newtonian droplet in a non-Newtonian matrix fluid and a non-Newtonian droplet in a Newtonian matrix. The lattice Boltzmann method is adopted to systematically investigate droplet deformation and breakup under a broad range of capillary numbers, viscosity ratios of the fluids, and confinement ratios considering shear-thinning and shear-thickening fluids. Confinement is found to enhance deformation, and the maximum deformation occurs at the viscosity ratio of unity. The droplet orients more towards the flow direction with increasing viscosity ratio or confinement ratio. In addition, it is noticed that the wall effect becomes more significant for confinement ratios larger than 0.4. Finally, for the whole range of Newtonian carrier fluids tested, the critical capillary number above which droplet breakup occurs is only slightly affected by the confinement ratio for a viscosity ratio of unity. Upon increasing the confinement ratio, the critical capillary number increases for the viscosity ratios less than unity, but decreases for the viscosity ratios more than unity.
Highlights
Droplet-based microfluidic technology has recently been exploited to perform microfluidic functions
Dynamical behaviour of droplets under simple shear has been systematically studied under a broad range of confinement and viscosity ratios
Confinement ratios have different effects on the critical capillary number: as the confinement increases, the critical capillary number increases for λ < 1 but decreases for λ > 1, and, for λ = 1, the critical capillary number is kept at a value of around 0.4, regardless of the confinement ratio
Summary
Droplet-based microfluidic technology has recently been exploited to perform microfluidic functions. Most available droplet-based microfluidic devices do not have integrated functions and are still far from full automation. One major challenge towards an automated, sample-in and answer-out system is to be able to control dynamical behaviour of droplets on-chip. Flow speed is usually low, which ensures a laminar flow, droplets are confined in microfluidic channels, and their interfacial dynamics are still difficult to predict. In microfluidic applications, biological fluids and surfactants are often used, so the fluids can exhibit non-Newtonian rheology. In such microfluidic systems, the flow behaviour of droplets can be very different from Newtonian–Newtonian systems
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