Optical and acoustic study of nucleation and growth of bubbles at a liquid-solid interface induced by nanosecond-pulsed-laser heating

Temporal and spatial temperature modelling for understanding pulsed laser induced solution based nanomanufacturing

Recently, pulsed lasers have demonstrated great potential in targeted synthesis in solution based nanomanufacturing, realizing high precision and accuracy in space, time and energy input. The unique temperature history induced by pulsed lasers is indispensable to understand the related fundamentals and to realize the precision control of deposition. In this study a heat transfer model was developed and applied to predict the temporal evolution and the spatial distribution of pulsed laser induced temperature change across the reaction sites. Chemically deposited ZnO crystals were studied as an example, showing the relationships among laser parameters and heating conditions, and deposited crystal characteristics. Peak temperature and heat accumulation induced by pulsed laser were found to affect deposited crystal number density and size. The nucleation number density distribution was found to be correlated with the spatial temperature distribution and inversely proportional to the crystal size. The presented heat transfer model is a crucial tool to understand crystallization fundamentals and it is essential for facilitating pulsed laser as a new tool for research, design, manufacturing and control.

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 θ_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 (θ_c ∼ 99°) under air irradiation to nearly spherical (θ_c ∼ 167°) 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.

A Theoretical Model for Steam Laser Cleaning

A theoretical model for removal of tiny particles from solid surface covered with a thin liquid film by laser cleaning is established by taking adhesion force and cleaning force into account. When pulsed laser irradiates on the solid surface coated with a thin liquid film, a sheet of liquid near the liquid/substrate interface can be superheated through thermal diffusion. The rapid growth of vapor bubbles inside the superheated liquid can generate transient stress wave with high pressure, large enough to expel micron and sub-micron particles from the contaminated surface. By calculating the adhesion force and cleaning force, the cleaning threshold of laser fluence can be predicted from this theoretical model. The difference between cleaning force and adhesion force increases quickly along with the laser fluence and leads to higher cleaning efficiency.