In Situ Contact Angle Tuning of Silver Nanostructures by Laser Heating in Different Fluid Ambient

It has been theoretically suggested by Yan et al. (Colloids Surf, A 2013, 429, 142-148) that the contact angle theta(c) of a liquid droplet on any given surface can be controlled by immersing it in an appropriate surrounding environment. Here, we report the first experimental demonstration of such an in situ contact angle tuning of silver (Ag) nanoparticles on quartz substrates via nanosecond pulsed laser heating under various fluid ambients, like air, water, and glycerol. Nanosphere lithography (NSL) was used to deposit Ag nanopyramids on quartz substrates. This system was subsequently melted by laser heating inside the various fluids to form nanoparticles. By using a combination of top and side view scanning electron microscopy (SEM) imaging, we show that the contact angle of Ag nanoparticles could be increased by going from heating in air to heating under fluids, with a near hemispherical shape (theta(c) similar to 99 degrees) under air irradiation to nearly spherical (theta(c) similar to 167 degrees) under glycerol irradiation. The mechanism of this contact angle change could be explained qualitatively by the changing interfacial energies of the substrate and metal in the various fluids. Similar contact angle control was also achieved for nanoparticles created by dewetting of Ag thin films in the various fluids. One practical implication of this contact angle tunability is the ability to change the intensity ratio of the quadrupolar to dipolar localized surface plasmon resonances in the Ag nanoparticles. This work also has implications to those applications in which nanoparticles on a substrate are heated in various gas or fluid ambients, such as in catalysis, as the ensuing shape change can modify properties.