title: Stain effects studied by time-resolved infrared imaging.
abstract: Pattern formation in evaporating colloidal droplets is an important phenomenon that is commonly observed in several solute-solvent systems; this is also an emerging technique for obtaining fine patterning through controlled conditions of drying. After evaporation of the solvent a ring-like pattern remains on the solid substrate under the condition of contact line pinning. We have used a new analytical technique, time-resolved infrared imaging, to investigate the formation of patterned structures with droplet drying, which is a typical time-dependent phenomenon. We have coupled the technique with optical imaging to follow the evolution of the droplet shape and dimension in correspondence with the chemical images. The main advantage of the technique is represented by the possibility to have simultaneous spatial and time-resolved information; we have applied the method to a water-methylene blue system that has been studied during drying. We have monitored the droplet profile change, in terms of water and methylene blue variation with time and space, at the droplet edge. The analysis has allowed a detailed reconstruction of the evaporating droplet profile with a micrometer scale resolution and of the change in concentration of the dye as a function of evaporating time. A uniform ring-like pattern, after evaporation in controlled relative humidity, is observed. The data are consistent with a constant evaporation model, whose conditions are realized when a constant evaporation is achieved along the entire droplet. The technique has allowed elucidating the evaporation phenomenon close to the contact line in a dye solute-solvent system, which is very difficult to study with other techniques.
authors.affiliation: Laboratorio di Scienza dei Materiali e Nanotecnologie, D.A.P., Universita di Sassari, CR-INSTM, Palazzo Pou Salid, Piazza Duomo 6, 07041 Alghero (Sassari), Italy.
authors.forename: Plinio
authors.initials: P
authors.lastname: Innocenzi
authors.forename: Luca
authors.initials: L
authors.lastname: Malfatti
authors.forename: Massimo
authors.initials: M
authors.lastname: Piccinini
authors.forename: David
authors.initials: D
authors.lastname: Grosso
authors.forename: Augusto
authors.initials: A
authors.lastname: Marcelli