Deposition of colloidal particles during sessile drop evaporation

A major challenge in developing nanoscale devices lies in controlling the position of such small materials on a substrate. Recently, there has been a lot of interest in utilizing sessile drop evaporation method to deposit colloidal particles and nanoparticles. This method is attractive because of the simplicity of the experimental procedure which includes placing a sessile drop of particle suspension on a substrate, letting the solvent evaporates, and the subsequent deposition of the particles on the substrate. Researchers have observed experimentally that different types of deposition profiles can be obtained by using this method. This finding stimulates the theoretical understanding of this drying process, which may lead to the understanding on how to control the particle deposition during drop drying. In this study, we have investigated numerically the evaporation of a sessile drop of colloidal particle suspension to understand the dynamics during evaporation and its influence on the particle deposition profile. In the first part of the study we compared the performance of two mesh generation methods, spine and elliptic mesh methods, in computing the fluid dynamics inside an evaporating sessile drop. The fluid flow profile was solved for a constant uniform evaporation flux along the drop interface. The fluid is flowing outward to compensate the evaporated solvent while pinning the contact line of the drop. We found that the elliptic mesh method is preferred for this problem. It is easier to control the mesh concentration, and the elements are more flexible. In the second part of the study, we studied the sessile drop evaporation problem using a more realistic evaporation flux profile. We simultaneously solved the fluid flow profile inside the drop and the vapor concentration profile surrounding the drop. The evaporation flux, calculated from Fick\u27s Law, is found to be diverging closer to the contact line, inducing higher fluid surface velocities closer to the contact line. In the third part of the study, we studied the effect of evaporation and fluid flow profiles on the particle deposition profile. The evaporation flux along the drop interface, the induced fluid dynamics inside the drop and the particle deposition profile on the solid substrate were solved simultaneously. Several particle deposition profiles, such as a ring-shaped deposit and uniform particle distribution, were obtained from the numerical simulations. The particle deposition profile is found to be determined by the competition of the mass transfer (both convective and diffusive) and the particle deposition rate