Evaporation-induced flow motion and particle deposition in sessile droplets

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Sessile evaporating droplets might appear simple systems but they hide surprisingly complex phenomena. The evaporation process induces internal and also interfacial flows that influence the shape and distribution of the deposits left behind once the liquid has vanished. A common example is given by the characteristic ring-shaped stain formed by an evaporated droplet of coffee. The control of the evaporation-driven flow inside the droplet as well as the particle distribution within the droplet area is currently one of the major technological challenges in this domain, with implications involving very different fields such as lithography, optoelectronics, paints, inkjet printing and medical diagnosis.The main objective of this proposal is to understand the role of the evaporation-driven flows in the mass transfer problem in sessile evaporating droplets. Mass transfer in this case refers to both vapor emission and particle motion within the evaporating droplet. First, the contribution of the interfacial flows emerging as a consequence of temperature gradients will be characterized. Second, the relationship between evaporated mass and evaporation-driven flow will be determined. Finally, the influence of particle size, hydrophobicity and charge on the final deposit will be evaluated. The experiments will be performed using state-of-the-art 3D particle-tracking-methods to measure systematically the flow and particle motion for the different operating conditions. The results obtained will settle the long-discussed debate on the role of evaporation-driven flow on the particle deposits in evaporating droplets.