Mastering thermocavitation microjets

This thesis explores strategies for controlling thermocavitation-driven microjets to advance their use in needle-free jet injection (NFJI) devices. Thermocavitation occurs when a continuous-wave (CW) laser is absorbed in a liquid confined within a microfluidic channel, producing localised explosive boiling, vapour bubble formation, and subsequent jetting due to bubble expansion. CW lasers operate at lower energy intensities; therefore bubble expansion transfers momentum gradually to the surrounding liquid, in a process termed inertia-driven jet formation. CW systems offer advantages of compactness, lower cost, and suitability for portable NFJI devices. The research systematically investigated key jetting parameters - velocity, diameter, trajectory, breakup time, and droplet size - by tuning liquid filling level, laser energy, fluid properties (using additives), channel geometry, and surface wettability. Experiments showed that channel filling - laser energy combinations dictate jet morphology, with specific conditions producing slender water jets. The effects of additives such as surfactants, viscosifiers, and viscoelastic polymers were examined to find that viscosifiers reduced jet velocities and ejected volumes, while dilute polymer solutions exhibited delayed jet tip breakup. All three additive types modified drop size distributions in distinct ways. Further increasing polymer concentration enhanced elastic stresses, which slowed jet tip displacement, resisted jet breakup, and in extreme cases caused jet retraction. Mapping these behaviours in Deborah–Weber space and modelling with the FENE-CR equation highlighted the roles of initial polymer stretch and finite polymer extensibility in predicting jet dynamics. Next, the study of different tapered channel geometries revealed that jet velocity and ejected volume outcomes depend strongly on the liquid filling level in each channel. This is due to the interplay of taper-induced acceleration, flow focusing, and fluidic resistance to bubble growth. Straight and parabolic tapers produced high-velocity jets (40–60 m/s) with volumes (15–20 nL) suitable for NFJI applications. Next, alternating wettability patterns (hydrophobic-philic coatings) on the channel wall surface enabled control over jet trajectory and stability: asymmetric patterns tilted jet trajectory towards the hydrophobic surface, while symmetric patterns maintained on-axis flow and reduced jet sway. These findings provide new physical insights to characterise and control thermocavitation-driven microjets, as well as design guidelines for the development of reliable CW-NFJI technologies.