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The internal flow within an evaporating sessile droplet is one of the drivingmechanisms that lead to the variety of particle deposition patterns seen inapplications such as inkjet printing, surface patterning, and blood stainanalysis. Despite decades of research, the causal link between droplet internalflow and particle deposition patterns has not been fully established. In thisstudy, we employ a 3D imaging technique based on digital inline holography toquantitatively assess the evolution of internal flow fields and particlemigration in three distinct types of wetting droplets: water, sucrose aqueoussolution, and SDS aqueous solution droplets, throughout their entireevaporation process. Our imaging reveals the three-stage evolution of the 3Dinternal flow regimes driven by changes in the relative importance of capillaryflow, Marangoni flow, and droplet boundary movement during evaporation, eachexhibiting unique dynamics. The migration of particles from their initiallocations to deposition can be divided into five categories, with particlesdepositing either at the contact line or inside the droplet. We observe thechanging migration directions of particles due to competing Marangoni andcapillary flows during droplet evaporation. We further develop an analyticalmodel that predicts the droplet internal flow and deposition patterns anddetermines the dependence of the deposition mechanisms of particles on theirinitial locations and the evolving internal flow field. The model, validatedusing different types of droplets from our experiment and the literature, canbe further expanded to other Newtonian and non-Newtonian droplets, which canpotentially serve as a real-time assessment tool for particle deposition invarious applications.

Three-dimensional internal flow evolution of an evaporating droplet and its role in particle deposition pattern