Results from the direct shear tests show that the shear strength decreases slightly with increasing the NaCl solution. The sodium emission is most intense on the anode side of the bubble where orange anode spots are visible.Ī series of direct shear and nuclear magnetic resonance (NMR) tests were performed on a compacted weakly expansive clay saturated by sodium chloride (NaCl) solutions with different concentrations to study the effect of NaCl solution on the shear strength and its mechanism. A delay in the range of 0.1 s is observed between the emission of hydroxyl and sodium. The optical emission spectrum of a vapour bubble discharge consists of excited hydroxyl, hydrogen and sodium emission. Linear translation of bubbles in the cathode direction, coinciding with intense discharges inside the bubbles, is observed and can be explained by asymmetric heating due to the plasma. For larger bubbles, the reduced electrical field is smaller than the electrical field where electron attachment equals ionization, indicating that the discharge is a surface discharge. The breakdown electrical field decreases with bubble length. The electrical breakdown (corresponding to a corona-to-spark transition) of quasi-static vapour bubbles is discussed. The fluid columns on either side of the bubble serve as electrodes for the electrical discharges inside the bubble. A bubble is generated in a capillary filled with a NaCl solution due to Joule heating. DC-excited discharges in vapour bubbles in capillaries are studied.
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