Surface nano-patterning using the coffee-stain effect

Abstract

Addition of nanopacticles in a base solvent leads to suspensions with enhanced physiochemical properties, compared to base solvent. This new type of suspensions is called nanofluids, with applications ranging from biomedicine to automotives. As a consequence extensive research is being conducted in the field, in particular, on the evaporation of these fluids as it leads to well-defined and highly ordered coffee-rings. However, the exact physics governing this phenomenon remain elusive. The goal of this experimental investigation is to elucidate how various parameters affect the progression of nanofluid coffee-stain formation. Examination of the coffee-ring structuring, produced by the free evaporation of sessile droplets containing nanoparticles, revealed an unexpected, disordered region at the exterior edge of the ring. A self-assembly mechanism with two components, particle velocity and wedge constraints, was proposed to describe the deposition of particles at contact lines of evaporating drops. Environmental pressure was used as a method to control particle crystallinity in the coffee-rings. Essentially, evaporation rate and pressure were found to be inversely proportional. Macroscopically, lowering pressure led to a transition from “stick-slip” to constant pinning. Nanoscopically, lowering pressure promoted crystallinity. Findings supported the proposed, in this thesis, particle self-assembly mechanism. Particle aspect ratio and flexibility were subsequently examined. Pinning strength was found to be a function of particle aspect ratio and rigidity, leading to constant pinning. The proposed, in this thesis, particle self-assembly mechanism was found to be applicable to a variety of aspect ratios and flexibilities. Lastly, particulate crystals grew following different pathways depending on particle flexibility.