Surface nano-patterning using the coffee-stain effect
View/ Open
Askounis2015.docx (18.05Mb)
Date
29/06/2015Author
Askounis, Alexandros
Metadata
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.