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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.

Microfluidics refers to devices and methods for controlling and manipulating fluid flows at length scales less than a millimeter. Miniaturization of a laboratory to a small device, usually termed as lab-on-a-chip, is an advanced technology that integrates a microfluidic system including channels, mixers, reservoirs, pumps and valves on a micro scale chip and can manipulate very small sample volumes of fluids. While several flow control concepts for microfluidic devices have been developed to date, here flow control concepts based on thermally responsive polymer solutions are presented. In particular, flow control concepts base on the thermally triggered reversible phase change of aqueous solutions of the polymer Pluronic will be discussed. Selective heating of small regions of microfluidic channels, which leads to localized gel formation in these channels and reversible channel blockage, will be used to control a membrane valve that controls flow in a separate channel. This new technology will allow generating inexpensive portable bioanalysis tools where microvalve actuation occurs simply through heaters at a constant pressure source without a need for large external pressure control systems as is currently the case. Furthermore, a concept for controlled cross-channel transport of particles and potentially cells is presented that relies on the continuous regeneration of a gel wall at the diffusive interface of two co-streaming fluids in a microfluidic channel.