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Viscoelastic effects on the deformation and breakup of a droplet on a solid wall in Couette flow
Published online by Cambridge University Press: 16 May 2023
Abstract
The deformation, movement and breakup of a wall-attached droplet subject to Couette flow are systematically investigated using an enhanced lattice Boltzmann colour-gradient model, which accounts for not only the viscoelasticity (described by the Oldroyd-B constitutive equation) of either droplet (V/N) or matrix fluid (N/V) but also the surface wettability. We first focus on the steady-state deformation of a sliding droplet for varying values of capillary number ($Ca$), Weissenberg number (
$Wi$) and solvent viscosity ratio (
$\beta$). Results show that the relative wetting area
$A_r$ in the N/V system is increased by either increasing
$Ca$, or by increasing
$Wi$ or decreasing
$\beta$, where the former is attributed to the increased viscous force and the latter to the enhanced elastic effects. In the V/N system, however,
$A_r$ is restrained by the droplet elasticity, especially at higher
$Wi$ or lower
$\beta$, and the inhibiting effect strengthens with an increase of
$Ca$. Decreasing
$\beta$ always reduces droplet deformation when either fluid is viscoelastic. The steady-state droplet motion is quantified by the contact-line capillary number
$Ca_{cl}$, and a force balance is established to successfully predict the variations of
$Ca_{cl}/Ca$ with
$\beta$ for each two-phase viscosity ratio in both N/V and V/N systems. The droplet breakup is then studied for varying
$Wi$. The critical capillary number of droplet breakup monotonically increases with
$Wi$ in the N/V system, while it first increases, then decreases and finally reaches a plateau in the V/N system.
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- © The Author(s), 2023. Published by Cambridge University Press
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