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Rheology dictated spreading regimes of a non-isothermal sessile drop
Published online by Cambridge University Press: 11 November 2022
Abstract
In the present work, within the framework of thin film theory, we delineate the interaction between the interfacial dynamics of thermal Marangoni flow and non-Newtonian rheology by considering a spreading droplet over a non-isothermal substrate. The numerical simulations, performed at different equilibrium contact angles $(\theta _e)$, dimensionless thermocapillary strengths
$(\beta )$ and shear-dependent viscosities
$(n)$, reveal that the fluid rheology nonlinearly influences the mechanisms of disjoining pressure and Marangoni stress. Accordingly, three distinct spreading regimes for non-Newtonian drops arise. Results indicate that the Marangoni film regime, having an approximate linear drop shape, sustains at lower
$\theta _e$, higher
$\beta$ and
$n$ ranges. Also, shear-thickening drops display an early onset of thermocapillary time scale and a steeper advancing front, while their shear-thinning counterparts retain a significant curvature for a much longer time. Contrastingly, the droplet regime is identified by fixed shape and uniform speed
$(U)$ at higher
$\theta _e$ and lower
$(\beta$,
$n)$ combinations. Here, an intricate interplay between
$\beta$ and
$n$ realizes a sharp increase in
$U$ for shear thinning compared with its invariance for shear-thickening droplets. The transition regime appears as an intermediate regime between the other two and involves multiple ruptured droplets. In all the regimes, we observe slower (faster) spreading of shear-thinning (thickening) droplets than the Newtonian droplets. In addition, the variations in
$n$ cause intense characteristic modulations to spreading attributes like droplet morphology and transient spreading behaviour, and also act as a switching mechanism between different spreading regimes. These unique results may be utilized for superior control of non-isothermal biofluid droplets in microfluidics.
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- © The Author(s), 2022. Published by Cambridge University Press
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