Suppression of coffee-ring effect by freeze-drying, and its utilization for uniform coating technique

Abstract

The coffee-ring effect is the physical phenomenon explaining why the ring-like deposit is left behind along the perimeter of a colloidal drop. It has been renowned as one of the major obstacles in the processes which require homogeneous distribution of dried deposit such as printing or coating. Extensive attempts have been made to suppress this phenomenon, and the most general approach of these is to alter the flow to circulate, thereby redistributing the solutes back to the central region of the droplet. However, the evaporation of a liquid drop is a complicated non-equilibrium system, which is difficult to be controlled, thus much simpler and more effective approaches are needed. This dissertation explores the well-known drying technique, termed ‘freeze-drying’, as a powerful tool to avoid the coffee-ring effect, and describes its efficacy. The dried deposit prepared by the freeze-drying technique resulted in well-dispersed and locally homogeneous features. The method was applied to two following applications, thus confirming its potential use in the future: 1) analytes sampling by drop-deposition for surface-enhanced Raman spectroscopy, 2) nanoparticle deposition on a surface for its size measurement using electron microscopy. The findings in this dissertation have extended further options for uniform coating process by effectively eliminating the coffee-ring effect.