Evaporative micro-particle self assembly influenced by capillary evacuation

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 5° 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.