Laser-induced motion in nanoparticle suspension droplets on a surface

The fluid and particle motion in a volatile colloidal nanoparticle suspension droplet (“nano-ink”) spreading on a flat surface upon local heating through a laser beam is investigated numerically. The laser diameter, laser intensity, and the absorption coefficient of the nano-ink as well as the substrate thermal diffusivity were varied. The simulations are conducted with a finite-element method discretization of the extended axisymmetric Navier-Stokes equations in Lagrangian coordinates, accounting for evaporation, thermocapillarity, and Young-force-driven wetting for the fluid phase as well as for inertia-controlled particle motion for the solid phase. An additional continuous particle coagulation model with a locally monodispersed particle distribution is solved on the locations of the discrete computational particles for example cases. The localized heating leads, through the action of thermocapillarity, to a displacement of the liquid in the radial (outward) direction. A dimple in the droplet center region is formed as a consequence, which becomes flattened for larger laser beam diameters due to a significant enlargement in spreading. Substrates with high thermal diffusivity or large thermal contact resistance to the liquid can inhibit the Marangoni-induced enlargement of the droplet footprint. The coagulation model predicts for large absorption coefficients particle clustering primarily at the free surface, which prevents the formation of structures (built by the coagulated nanoparticles) with a uniform thickness.