<div>In many natural and industrial processes, bubbles are exposed to surface-active contaminants (surfactants) that may cover the whole or part of the bubble interface. A partial coverage of the bubble interface results in a spontaneous self-propulsion mechanism, which is yet poorly understood.</div><div>The main goal of this study is to enhance the understanding of the flow and interfacial mechanisms underlying the self-propulsion of small surfactant contaminated bubbles. The focus is on characterizing the self-propulsion regimes generated by the presence of surface-active species, and the influence of surfactant activity and surface coverage on the active bubble motion. </div><div>The study was developed by simultaneously solving the full system of partial differential equations governing the free-surface flow physics and the surfactant transport on the deforming bubble interface using multi-scale numerical simulation. </div><div>Results show in microscopic detail how surface tension gradients (Marangoni stresses) induced by the uneven interfacial coverage produce spontaneous hydrodynamics flows (Marangoni flows) on the surrounding liquid, leading to bubble motion. Results also establish the influence of both surfactant activity and interfacial coverage on total displacement and average bubble velocity at the macroscale. </div><div>Findings from this research improve the fundamental understanding of the free-surface dynamics of self-propulsion and the associated transport of surface-active species, which are critical to important natural and technological processes, ranging from the Marangoni propulsion of microorganisms to the active motion of bubbles and droplets in microfluidic devices. Overall, the findings advance our understanding of active matter behavior; that is, the behavior of material systems with members able to transduce surface energy and mass transport into active movement.</div>