The heart signal: An acoustic signature observed during a second-bubble entrainment

Abstract

Passive acoustic signals from the free fall of water droplets onto a liquid free surface are reported experimentally. The signals are co-registered with video images, permitting the association of the signals with various physical-hydrodynamic events such as the initial impact, first bubble entrainment, fragmentation of small bubbles and the entrainment of a second bubble. An interesting acoustical signal is observed when a satellite drop hits a previously formed cavity, creating an oscillating air body which shows great symmetry about the time axis; its envelope is similar to analytical polar-cardioid or ‘heart’ function. The heart signals reach a maximum value when four cycles are complete..