UBC Theses and Dissertations

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UBC Theses and Dissertations

Effect of substrate cooling and droplet shape and composition on the droplet evaporation and the deposition of particles Bazargan, Vahid


Sessile droplets are liquid droplets resting on a flat substrate. During the evaporation of small sessile droplets, the contact line of the droplet undergoes two different stages: pinned stage with fixed contact area and de-pinned stage with fixed contact angle. An evaporation with a pinned contact line produces a flow inside the droplet toward the contact line. This flow carries particles and deposits them near the contact line. This causes the commonly observed “coffee-ring” phenomenon. This thesis provides a study of the evaporation process and the evaporation-induced flow of sessile droplet and brings insights into the deposition of particles from colloidal suspensions. Here we first study the evaporation of small sessile droplets and discuss the importance of the thermal conductivity of the substrate on the evaporation process. We show how current evaporation models produce a significant error for droplet sizes below 500 μm. Furthermore, we study the evaporation of line droplets with finite sizes and discuss the complex behavior of the contact lines during evaporation. We apply an energy formulation and show that the contact line starts receding from the two ends of line droplets with a contact angle above the receding contact angle of spherical droplets. And then we show the evaporation-induced flow inside the line droplets. Finally, we discuss the behavior of the contact line under the presence of surfactant and discuss the Marangoni flow effects on the deposition of the particles. We show that the thermal Marangoni effect affects the amount of the particles deposited near the contact line, where a lower substrate temperature corresponds to a larger amount of particles depositing near the contact line.

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