Acoustically induced bubble jets

Abstract

A detailed experimental study on acoustically induced bubble jets produced far from any solid wall is presented. This particular type of bubble jet is observed when a laser-induced gas cavity is seeded in a liquid by an optical dielectric breakdown while applying a strong ultrasonic field. This generation method allows a high degree of control over jet dynamics, achievable by changing critical experimental parameters such as the laser pulse energy (i.e., the size of the bubbles), the driving signal frequency, the seeding phase (relative to the sound field), the acoustic pressure amplitude, and, in particular, the pressure gradient at the bubble inception location. The temporal evolution of the characteristic dimensions of the bubble jets is analyzed by means of numerical simulations of spherical bubble dynamics and their dependence on the local acoustic field. The jet strength is presented in terms of the jet elongation ratio, defined as the axial length over the maximum width. The most relevant parameters related to the jet strength are the maximum radius reached by the bubble prior to the jet formation and the pressure gradient during the bubble collapse phase. The existence of optimum driving parameters and the bubble seeding position to obtain the strongest jets achievable in a given acoustic pressure distribution is discussed. This study is a first step to shed some light on the jet formation process due to known time-variable pressure gradients. The work can be extended to further understanding the role of bubble jets in sonochemical reactions and biological or medical applications.