Published online by Cambridge University Press: 31 January 2022
Acoustic microstreaming is a nonlinear response of a fluid that undergoes high-amplitude acoustic stimulation and tends to viscously absorb it. The present experimental study investigates the generation of acoustic microstreaming induced by an oscillating wall-attached bubble undergoing nonspherical shape modes. From a microscope top view, the formation of particular flow signatures is explored for the main classes of spherical harmonics $Y_{nm}(\theta, \phi )$: zonal ($m = 0 < n$), sectoral ($n = m > 0$) and tesseral ($0 < m < n$) oscillation. The microstreaming induced by a bubble animated by a sectoral mode alone reveals a pattern characterized by a $4n$-lobe flower shape. Tesseral modes give rise to 4m-lobe flower-shaped patterns. Finally, when sectoral and zonal modes coexist, two kinds of pattern stand out: $2n$-lobe flower shape and $n$-pointed star shape. The preferential emergence of one or another streaming pattern is discussed on the basis of the amplitude and phase shift between both shape modes.
High frame rate top-view visualization of the dynamics of a bubble animated by a zonal mode of degree n=4. Recording frame rate: 67065 images/s. Visualization frame rate: 20 images/s.
Low frame rate top-view visualization of the microstreaming induced by a bubble animated by a zonal mode of degree n=4. Recording frame rate: 2000 images/s. Visualization frame rate: 20 images/s.
High frame rate top-view visualization of the dynamics of a bubble animated by a sectoral mode of degree n=4. Recording frame rate: 67065 images/s. Visualization frame rate: 20 images/s.
Low frame rate top-view visualization of the microstreaming induced by a bubble animated by a sectoral mode of degree n=4. Recording frame rate: 2000 images/s. Visualization frame rate: 20 images/s.
High frame rate top-view visualization of the dynamics of a bubble animated by a tesseral mode of order m=1. Recording frame rate: 67065 images/s. Visualization frame rate: 20 images/s.
Low frame rate top-view visualization of the microstreaming induced by a bubble animated by a tesseral mode of order m=1. Recording frame rate: 2000 images/s. Visualization frame rate: 20 images/s.
High frame rate top-view visualization of the dynamics of a bubble simultaneously animated by a zonal and a sectoral mode of degree n=3. Recording frame rate: 67065 images/s. Visualization frame rate: 20 images/s.
Low frame rate top-view visualization of the microstreaming induced by a bubble simultaneously animated by a zonal and a sectoral mode of degree n=3. Recording frame rate: 2000 images/s. Visualization frame rate: 20 images/s.