Focused, High-Speed Liquid Jets Induced Via Low-Voltage Sparks in Capillary Tubes

When a high-voltage capacitor is discharged between two electrodes in a conducting liquid, the resulting spark can create a rapidly expanding cavitation bubble. We exploit this phenomenon in capillary tubes to create high-speed liquid jets that could be used for several applications, including drug delivery, surface coating, inkjet printing, etc. We conducted a systematic study to understand the role of various parameters, such as fluid viscosity, location of the spark point, and discharge voltage, on the jet dynamics and show that all three parameters of the system have a statistically significant effect. Overall, we observed focused jets with diameters 𝒪(10^-4-10^-3) m, and speeds in the range 𝒪 (10 ^1 –10 ^2 ) m/s, yielding typical jet Reynolds numbers Re_j∼𝒪(10^3-10^4) . For viscous fluids and low voltages, we were able to produce discrete droplets. However, in the jet regime, the speed can be tuned by the discharge voltage and the location of the spark point relative to the center of the meniscus. In addition to basic jet hydrodynamics, we also report experiments in which this jetting mechanism is used to deposit liquid onto substrates placed below the open end of the capillary. We propose that this system can generate focused liquid jets useful for coating with viscous liquids, and potentially for needle-free jet injections.