Experimental study on the formation of two axial jets of cavitation bubbles near soft membranes with different thicknesses

A cavitation bubble collapses non-spherically, splits into two smaller bubbles, and induces two axial jets flowing in opposite directions near a soft membrane. The soft membrane is deformed and even pierced after the impact of the downward jet. In this paper, we experimentally studied the formation of two axial jets of a spark-induced bubble near Ecoflex membranes with different thicknesses. Assuming that the millimeter-sized bubble is filled with an ideal gas and collapses adiabatically in water, the dominant dimensionless parameters, the stiffness (d′) and stand-off distance (h*), determining the formation of the two axial jets, are obtained, and the value of d′ is varied by changing the membrane thickness. Two parameter plots for the regimes of bubble jetting and no jetting were obtained. The critical h* of the bubble jetting increased exponentially with increasing d′, reaching a maximum of 1.1 at d′ = 0.045 56 (the thickness d = 10 mm). This was because the counteracting force induced by the rebounding of the deformed membrane grew in strength. After that, the h* remained constant since the counteracting force was unchanged due to the feature changes of the membrane deformation. Consequently, we obtain a function of the boundary line between the regimes. The sub-regime of the membrane piercing was obtained in the regime of bubble jetting. It was found that the velocity of membrane deformation induced by the jet impact was an important factor in membrane piercing.