  The results of the flow structure visualization experiments conducted on the
surface of a single bubble streamlined by uniform flow are presented. It is
shown that, at certain critical values for bubble size, flow velocity, and
contamination level, the axial symmetry of the surface flow loses its stability
in a threshold manner, and the first instability mode in the form of two
vortices appears. Below the threshold, the stationary flow on the bubble
surface is impossible. The experimental results indicate that the assumption
about the axial symmetry of the motion on the bubble surface containing
surfactants, which is used in most theoretical and numerical studies, is
invalid. Analysis of the results has revealed the most likely reason for the
spiral form of the trajectory in the problem of a small rising bubble in the
surrounding fluid. For the surfactant-free surface realized in the experiments
with isopropyl alcohol, the rising trajectory was a straight line, and no
vortex structures were observed on the bubble surface. In the experiments with
water, a spiral rising trajectory was observed, and the first instability mode
was formed on the bubble surface.
