Substrate Wettability Influences Internal Jet Formation and Mixing during Droplet Coalescence

Sykes, Thomas C.; Harbottle, David; Khatir, Zinedine; Thompson, H. M.; Wilson, Mark C. T.

doi:10.1021/acs.langmuir.0c01689

Cited by 9 publications

(7 citation statements)

References 43 publications

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“…To represent the influence of dynamic apparent contact angle θ d , we compiled the Kistler model in the interFoam solver. The algorithm is used for every time step and can be written as where θ e is the equilibrium contact angle and f H –1 is the inverse function of the Hoffman empirical function, which can be obtained by and Ca is the capillary number defined as where v cl is the contact line velocity.…”

Section: Methodsmentioning

confidence: 99%

“…To represent the influence of dynamic apparent contact angle θ d , we compiled the Kistler model 56 in the interFoam solver. The algorithm is used for every time step and can be written as…”

Section: ■ Introductionmentioning

confidence: 99%

“…where θ e is the equilibrium contact angle and f H −1 is the inverse function of the Hoffman empirical function, which can be obtained by 56…”

Section: ■ Introductionmentioning

confidence: 99%

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How Superhydrophobic Grooves Drive Single-Droplet Jumping

2022

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Rapid shedding of microdroplets enhances the performance of self-cleaning, anti-icing, water-harvesting, and condensation heat-transfer surfaces. Coalescence-induced droplet jumping represents one of the most efficient microdroplet shedding approaches and is fundamentally limited by weak fluid–substrate dynamics, resulting in a departure velocity smaller than 0.3u, where u is the capillary-inertia-scaled droplet velocity. Laplace pressure-driven single-droplet jumping from rationally designed superhydrophobic grooves has been shown to break conventional capillary-inertia energy transfer paradigms by squeezing and launching single droplets independent of coalescence. However, this interesting droplet shedding mechanism remains poorly understood. Here, we investigate single-droplet jumping from superhydrophobic grooves by examining its dependence upon surface and droplet configurations. Using a volume of fluid (VOF) simulation framework benchmarked with optical visualizations, we verify the Laplace pressure contrast established within the groove-confined droplet that governs single-droplet jumping. An optimal departure velocity of 1.13u is achieved, well beyond what is currently available using condensation on homogeneous or hierarchical superhydrophobic structures. We further develop a jumping/non-jumping regime map in terms of surface wettability and initial droplet volume and demonstrate directional jumping under asymmetric confinement. Our work reveals key fluid–structure interactions required for the tuning of droplet jumping dynamics and guides the design of interfaces and materials for enhanced microdroplet shedding for a plethora of applications.

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Section: Methodsmentioning

confidence: 99%

Section: ■ Introductionmentioning

confidence: 99%

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How Superhydrophobic Grooves Drive Single-Droplet Jumping

2022

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“…However, a key limitation in VOF approaches -and Navier-Stokes based approaches in general -is that the dynamic contact angle θ d usually has to be prescribed as an input. For relatively well-behaved systems, empirical relations between θ d and the contact-line velocity U can be used (Yokoi et al 2009;Sykes et al 2020b). However, for a complex 3-D geometry evolving in time, as considered here, this is not straightforward.…”

Section: Computational Methodologymentioning

confidence: 99%

Morphologies and dynamics of micro-droplet impact onto an idealised scratch

et al. 2021

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As inkjet technology develops to produce smaller droplets, substrate features such as accidental scratches or manufacturing defects can potentially affect the outcome of printing, particularly for printed electronics where continuous tracks are required. Here, the deposition of micro-droplets onto a scratch of commensurate size is studied. The scratch is considered as a groove of rectangular cross-section, with rectangular side ridges representing material displaced from the substrate, and seven equilibrium morphologies are identified as a result of inertial spreading, contact-line pinning, imbibition into the scratch and capillary flow. A regime map is constructed in terms of scratch depth and width, and theoretical estimates of the regime boundaries are developed by adapting droplet spreading laws for flat surfaces to account for liquid entering the scratches. Good agreement is seen with numerical results obtained using a graphical processing unit-accelerated three-dimensional multiphase lattice Boltzmann model validated against published experiments, and the influences of Reynolds number, Weber number and advancing and receding contact angles are explored. Negative and positive implications of the results for printing applications are discussed and illustrated via multiple-droplet simulations of printing across and along scratches.

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“…For 'open-surface' droplet-based microfluidics, a wettability gradient surface [24][25][26] is a common way to promote the droplet mixing. Recently, Sykes et al [27] found that the mixing could be improved by the formation of an internal jet. The influence of substrate wettability, the volume ratio and droplet viscosity on the formation of the jet were studied by means of experiments and numerical simulations.…”

Section: Introductionmentioning

confidence: 99%