The formation of ordered two dimensional particle arrays from the evaporation of a liquid suspension offers important uses in creating desired surface structures in materials, and sensors in biomedicine. The notion of using evaporative forces in this manner of particle self-assembly was first put forth in the 1990s. The use of evaporation from a droplet offers arguably a high level of simplicity and appeal in understanding the interesting interplay of evaporation, fluid flow, and particle deposition behavior. The important insight gained in the case of droplets was the heightened evaporation rate at the stationary outer rim that enabled the creation of a flow within the droplet to cause copious migration of particles there to engender the well-known coffee stain effect.

• Particle hydrophobicity affects deposition

  

  The prevailing exposition of the particle deposition mechanism in an evaporating droplet for self-assembly does not lead one to expect any deviation in deposition behavior to occur for particles of differing hydrophobicity. It is well known that if the contact angle that the droplet makes with the surface is sufficiently low a “coffee stain” pattern results, and that if the angle is sufficiently high few deposits are left behind except a dot at the center of the dried out droplet. We explore the behavior in the large intermediary region, and find that it depends strongly on particle properties. Droplet evaporation experiments that we conducted with micron sized polystyrene and silica particles on glass coated with silane suggest that the basic deposition mechanism outlined can be strongly influenced by a balance between capillary forces and the drag arising from flow patterns in the droplet, and inter-particle capillary force factors

  F.F. Shao, A. Neild, T.W. Ng, Hydrophobicity effect in the self assembly of particles in an evaporating droplet, Journal of Applied Physics. 108 (2010) 034512.

• Deposition is affected by porous media evacuation (NEW)

  

  As evaporation does not incur energy introduction, the droplet coffee-stain patterning approach is attractive for biochemical tests conducted in the field or in third world environments. A practical strategy uses chemically functionalized microbeads for the coffee stain deposition process. From an application perspective, it will be necessary to minimize the coffee stain deposition time, as evaporation, depending on the volume of the droplet, can be a slow process. The introduction of a porous media will generate a capillary flow (or wicking) that removes any remnant liquid in the droplet, thus permitting it to be done inexpensively and in the field. Using optical profilometry, we were able to establish that polystyrene microspheres developed more copious and defined single ring coffee depositions than silica of the same size and concentration in a suspension. In analyzing the droplet capillary evacuation process with a porous media, we found the liquid bridge formed during the later stages to rupture and leave behind some liquid material for a second stage evaporation process. This was responsible for a two ring structure that was more visible with silica microspheres. A high degree of hysteresis of the contact angle was found to develop at the contact line in which values below 5o could be achieved. Dynamic observations showed the copious and dense packing of polystyrene particles to be more resistant to ring break up from the evacuation flow. Nevertheless, erosion of the back array portions of the ring was evident notwithstanding either type of microsphere used.

  F. Shao, T.W. Ng, J. Efthimiadis, A. Somers, W. Schwalb Evaporative micro-particle self assembly influenced by capillary evacuation Journal of Colloid and Interface Science. Accepted 8/2/2012.