Thermo-Fluid Dynamics of Laser-Irradiated Droplet Using a Fully Coupled Optics-CFD Approach

Laser-droplet interactions drive critical advancements across numerous applications, such as bioengineering and nanolithography. While extensive experimental research has been conducted, numerical studies still largely rely on simplified models to capture the complex underlying dynamics. This study investigates the deformation dynamics of a transparent water droplet following its interaction with a laser pulse. Three-dimensional simulations are performed using for the forst time a fully coupled framework that integrates a ray-tracing model, which initialises the laser-induced temperature field, with a GPU-accelerated compressible multiphase flow solver able to handle evaporation, condensation, and shock propagation. The coupled approach demonstrated very good agreement with published experimental results. The ray-tracing model showed that the droplet acts as a lens, focusing rays onto specific points to rapidly increase temperature and initiate explosive evaporation. Sequentially, based on the mass transfer rate, the droplet's hydrodynamic deformation is categorized into three distinct stages. During the primary stage, massive mass transfer rate occurs over a very short duration, where rapid evaporation causes a sharp local pressure rise, generating intense compression shocks and inertial forces that begin stretching the droplet. This transitions into an intermediate stage characterised by a reduced but non-negligible mass transfer rate. Here, propagating shocks reflect as rarefaction waves, leading to the formation of a large cavitation bubble and its subsequent collapse generating secondary shocks. The droplet continues to stretch under decreasing pressure gradients and momentum. In the final stage, mass transfer approaches zero, and residual inertial forces cause the droplet to further stretch, driving it toward complete fragmentation.