Mastering thermocavitation microjets
Keerthana Mohan is a PhD student in the Department of Mesoscale Chemical Systems. (Co)promotors are prof.dr.ir. D. Fernández Rivas and prof.dr. J.G.E. Gardeniers from the faculty of Science & Technology.
This work explored methods for controlling thermocavitation-driven microjets to enable their use in needle-free jet injection devices (NFJIs). Thermocavitation involves localised explosive boiling of liquid contained in a microfluidic channel caused by continuous-wave (CW) laser absorption. This is followed by the subsequent formation and expansion of a vapour bubble producing a liquid microjet. The experiments performed in this thesis examined and controlled key jetting characteristics, such as diameter, velocity, trajectory, breakup time, and droplet size. This was controlled through adjustments to system parameters such as the liquid filling level and laser energy, modifying liquid properties through additives, and by tuning the channel geometry and wall wettability. CW lasers offer advantages over pulsed-laser systems, including lower cost, compactness, and suitability for portable NFJI devices.
Chapter 1 introduces the basis of thermocavitation-driven microjets generated by CW lasers within confined channels. Unlike pulsed-laser systems, where strong acoustic transients from high energy intensities drive jetting, CW systems operate at lower intensities. Therefore, jets are generated through slow bubble expansion that gradually transfers momentum to the liquid bulk. This process is termed inertia-driven jet formation. Using shadowgraphy and hydrophone measurements, the study distinguishes jet formation in CW systems from pulsed laser systems.
Chapter 2 examines two key aspects of CW jetting in conventional rectangular channels: the influence of initial liquid filling and laser energy on jetting regimes, and the effect of additives on characteristics of slender water jets. Four distinct jet shapes such as corrugated bodies, axis-switching jets, slender jets, and droplet trails, emerge from different filling-energy combinations. Additive studies show that surfactants mainly alter drop-size distribution producing larger sized droplets. Glycerol increases viscosity and produces slower and lower-volume jets. Dilute viscoelastic polymer solutions delay jet tip breakup, and lead to the formation of smaller droplets. This chapter also proposes new parameters for impulse jet characterisation.
Chapter 3 investigates highly inertial viscoelastic microjets using polyethylene-oxide (PEO) at concentrations from 0.025–0.75 wt.% under high strain rates (order of 104 s-1). Increasing polymer concentration enhances elastic forces, which resist breakup, decelerate the jet, and at high concentrations, even cause jet retraction. Jet behaviour is mapped in Deborah-Weber space and modelled using the FENE-CR equation, revealing the importance of initial polymer stretch and finite extensibility in predicting jet tip displacement dynamics. While the model captures most behaviours, it does not fully explain extreme ‘bungee-jumping’ cases, highlighting opportunities for further study.
Chapter 4 focuses on geometry-driven performance enhancement, comparing different tapered (straight, parabolic, hyperbolic) and rectangular channels. Comparison across different liquid filling levels reveal how fluidic resistance, flow-focusing, and taper-induced acceleration interact to control jet velocity and volume. Qualitatively across geometries, low liquid-filling levels (<40%) produce fast but low-volume jets, high liquid-filling levels (>80%) yield slower but larger plug-like flows of high-volume, and intermediate filling levels display distinct geometry-based behaviour. Straight and parabolic tapers achieve velocities of 40–60 m/s with 15–20 nL volumes, offering promising designs for needle-free jet injection (NFJI) applications.
Chapter 5 explores trajectory and stability control using patterned hydrophobic–hydrophilic coatings inside the channels. The presence of coatings was found to reduce hydraulic resistance in larger channels, increasing jetting velocity. Asymmetric coating patterns tilt jets toward hydrophobic regions via initial meniscus shaping, thereby modifying the jet trajectory. Symmetric patterns, with hydrophobic strips flanking a hydrophilic centre, help maintain on-axis stability. The findings demonstrate that surface wettability patterning is an effective method to fine-tune jet direction and minimise undesired deviation in NFJI devices.
