Three-dimensional internal flow evolution of an evaporating droplet and its role in particle deposition pattern
Physics of Fluids(2023)
摘要
The internal flow within an evaporating sessile droplet is one of the driving
mechanisms that lead to the variety of particle deposition patterns seen in
applications such as inkjet printing, surface patterning, and blood stain
analysis. Despite decades of research, the causal link between droplet internal
flow and particle deposition patterns has not been fully established. In this
study, we employ a 3D imaging technique based on digital inline holography to
quantitatively assess the evolution of internal flow fields and particle
migration in three distinct types of wetting droplets: water, sucrose aqueous
solution, and SDS aqueous solution droplets, throughout their entire
evaporation process. Our imaging reveals the three-stage evolution of the 3D
internal flow regimes driven by changes in the relative importance of capillary
flow, Marangoni flow, and droplet boundary movement during evaporation, each
exhibiting unique dynamics. The migration of particles from their initial
locations to deposition can be divided into five categories, with particles
depositing either at the contact line or inside the droplet. We observe the
changing migration directions of particles due to competing Marangoni and
capillary flows during droplet evaporation. We further develop an analytical
model that predicts the droplet internal flow and deposition patterns and
determines the dependence of the deposition mechanisms of particles on their
initial locations and the evolving internal flow field. The model, validated
using different types of droplets from our experiment and the literature, can
be further expanded to other Newtonian and non-Newtonian droplets, which can
potentially serve as a real-time assessment tool for particle deposition in
various applications.
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