We illustrate the importance of particle geometry on droplet contact line pinning, 'coffee-stain' formation and nano-structuring within the resulting rings. We present the fundamentals of pure liquid droplet evaporation and then discuss the effect of particles on the evaporation process. The resulting coffee-stain patterns and particle structuring within them are presented and discussed. In the second part, we turn our attention to the effect of particle geometry on the evaporation process. A wide range of particle shapes, categorised according to aspect ratio, from the simple shape of a sphere to the highly irregular shapes of platelets and tubes is discussed. Particle geometry effect on evaporation behaviour was quantified in terms of change in contact angle and contact radius for the stick-slip cases. Consequently the hysteretic energy barrier pinning the droplets was estimated, showing an increasing trend with particle aspect ratio. The three-phase contact line (TL) motion kinetics are complemented with analysis of the nano-structuring behaviour of each shape, leading to the identification of the two main parameters affecting nanoparticle self-assembly behaviour at the wedge. Flow velocity and wedge constraints were found to have antagonist effects on particle deposition, although these varied with particle shape. This description should help in understanding the drying behaviour of more complex fluids. Furthermore, knowing the fundamentals of this simple and inexpensive surface patterning technique should permit its tailoring to the needs of many potential applications.
Keywords: Coffee-stain; Droplet evaporation; Graphene; Nano-structures; Particle shape; Triple line motion kinetics.
Copyright © 2014 Elsevier B.V. All rights reserved.