Influence of surfactants on an evaporating drop: Fluorescence images and particle deposition patterns

The insoluble surfactant pentadecanoic acid is spread at the interface of aqueous droplets, which are then deposited on a well-defined substrate and allowed to evaporate. The surface state of the surfactant is imaged as the drop evaporates using fluorescence microscopy, and the mean rate of evaporation is calculated from successive digitized silhouettes of the sessile drops. The drops contain suspended microspheres that act as tracers in the flow within the drop and deposit on the substrate in patterns that vary with the surfactant surface state. Patterns observed include circular mounds of particles at the original drop periphery termed coffee rings, created by an outward flow to the three-phase contact line connected polygons formed by Marangoni-Benard flow; and uniform monolayers of microspheres deposited on the substrate. These results establish that surfactants can be used to alter flow fields and deposition patterns from evaporating aqueous drops. This is potentially useful in the organization of nanoparticles and in the deposition of materials functionalized with specific binding sites whose folded structures might be destroyed by other solvents. The deposition of biologically functionalized materials is demonstrated by depositing streptavidin-labeled microspheres, which retain their ability to bind biotin after deposition.