Effect of bubble surface loading on bubble rise velocity

In this study, we report the rising behaviour of the millimetric size ellipsoidal shaped particle-laden bubbles (particle diameter dP ∼ 114 μm, bubble diameter dB ∼ 2.76 and 3.34 mm) in the range of bubble surface loading (BSL) from 0 to 0.6 both in absence and presence of a surfactant (Sodium Dodecyl Sulphate, 20% CMC). High-speed imaging was used to capture the trajectory of the particle-laden bubble and an image processing methodology was developed to quantify the bubble surface loading. Three different regimes were observed - bubble shape transition (nearly spherical to ellipsoidal), particle detachment (at bubble rear end), and steady (for high BSL) or expansion (for low BSL) of the particle surface covered zone. A threshold for bubble surface loading (BSL ∼ 0.40) was determined which had reasonable agreement with the experimental observations. Bubble rise velocity was observed to decrease with bubble surface loading but this trend was less steep in presence of surfactant. It was noted that loss of bubble surface mobility was higher in presence of surfactant, however in absence of surfactant, bubble surface loading contributed significantly to surface immobility. Finally, a correction factor to Schiller-Naumann drag coefficient model was proposed accounting for the bubble surface loading both in presence and absence of surfactant.