Continuous Visualization of Process of Positive Streamer Initiation, Propagation, Spark, and Bubble Expansion in Water

Introduction

Electrical discharge in liquids is a phenomenon which is seen in dielectric breakdown of insulating oils used in transformers and liquids for electric discharge machining, and has been investigated in many studies. Electrical discharge in water has attracted attention in recent years, and application to the environmental and medical fields is progressing, for example, in water purification and surgical operations. How the discharge propagates in the liquid is fundamental to these applications, but in spite of the fact that a bubble theory and an electrical theory (direct ionization theory) of breakdown have been proposed, the process of discharge propagation still has not been elucidated. As one reason for this, experimental clarification is difficult because the pre-breakdown phenomenon of spark discharge called streamers occurs at high speed in a microscopic region. However, with recent progress in recording technology, clarification of these phenomena is continuing. It is known that two types of underwater positive streamers generally occur, i.e., primary streamers and secondary streamers. In low conductivity water, primary streamers take a semi-spherical brush-shaped form and propagate at a speed of approximately 2 km/s, after which filament-like channels with a dendritic shape propagate. (1) Secondary streamers initially propagate at 20-30 km/s as dendritic filamentary channels, but then propagate at the same speed of about 2 km/s as primary streamers. The difference between primary and secondary streamers can be distinguished because a direct current-like component appears in the discharge current during high speed propagation. (2) When a streamer propagates to the counter electrode, the bubble channel joins the two electrodes and the process proceeds to spark discharge. It is also known that streamers occur in an electrical field of a certain minimum strength. As the mechanism responsible for forming a high electrical field, it has been found that a bubble cluster forms at the electrode tip before streamer inception, and a protrusion occurs on surface of the bubble cluster due to the accumulation of charge in the bubbles and concentration of the field. (3), (5) However, continuous recording of the series of changes in the bubble propagation process before streamer inception was difficult, as it is necessary to record a large number of images continuously under conditions of a high frame rate, high sensitivity, and short exposure time. The HPV-X2 high speed video camera used here enables high speed recording at a maximum 10 million frames/second (10 Mfps) and also has a high sensitivity sensor. This Application Note introduces the results of continuous recording of the process from the formation of the bubble cluster at the electrode tip to initiation of an underwater positive primary streamer, the condition of primary streamer propagation resulting in spark discharge, and the process of expansion of the bubbles.

December 8, 2025 GMT