Additives influence thermocavitation-induced microjets

The study of thermocavitation jetting is relevant for several applications, including needle-free injection and 3D printing. A laser focused on the closed end of a glass microchannel filled with liquid induces cavitation (bubble growth and collapse) and forms microjets traveling outside the microchannel. The maximum bubble size is proportional to the amount of absorbed optical energy and also depends on the liquid filling volume. We studied a nozzle-free rectangular channel that can produce jets with distinct shapes and breakup behaviour. The interplay between instabilities such as axis-switching, bubble collapse, and capillary instability led to a slender jet of ~50 µm diameter and ~17 m/s velocity, which was identified as suitable for needle-free injection and investigated further. We also tested solutions of three different types of additives: surfactants, viscous, and viscoelastic additives of varying concentrations to assess their influence on the formation and breakup of the jet. We observed that viscous and viscoelastic additives delay jet breakup, but surfactants had no measurable effect. Moreover, the presence of the viscous additives led to slower (~13 m/s) and thinner (~30 µm) jets.