The ability to control the size, shape, and location of particulate deposits is important in patterning, nanowire growth, sorting biological samples, and many other industrial and scientific applications. It is therefore of interest to understand the fundamentals of particle deposition via droplet evaporation. In the present study, we experimentally probe the assembly of particles on superhydrophobic surfaces by the evaporation of sessile water droplets containing suspended latex particles.
Superhydrophobic surfaces are known to result in a significant decrease in the solid-liquid contact area of a droplet placed on such a substrate, thereby increasing the droplet contact angle and reducing the contact angle hysteresis. We conduct experiments on superhydrophobic surfaces of different geometric parameters that are maintained at different surface temperatures. The transient droplet shape and wetting behavior during evaporation are analyzed as a function of substrate temperature as well as surface morphology. During the evaporation process, the droplet exhibits a constant contact radius mode, a constant contact angle mode, or a mixed mode in which the contact angle and contact radius change simultaneously. The evaporation time of a droplet can be significantly reduced with substrate heating as compared to room-temperature evaporation.
To describe the spatial distribution of the particle residues left on the surfaces, qualitative and quantitative evaluations of the deposits are presented. The results show that droplet evaporation on superhydrophobic surfaces, driven by mass diffusion under isothermal conditions or by substrate heating, suppresses particle deposition at the contact line. This preempts the so-called coffee-ring and allows active control of the location of particle deposition.