Effects of surfactant on lift coefficient of ellipsoidal bubbles in the viscous-force dominant regime

Effects of surfactants on lift coefficients, CL, of single ellipsoidal bubbles rising through linear shear flows were investigated. Two types of surface-active agents, i.e. Triton X-100 and 1-octanol, were used. The liquid properties except for the surface tension were identical to those in a clean system of logM = -5.5, where M is the Morton number. The range of the bubble Reynolds number was 0.1 < Re < 70. Bubble shapes were either spherical or ellipsoidal. Comparing with clean bubbles, less deformation of contaminated bubbles was confirmed due to the fact that surfactant tends to accumulate on the bubble interface, making it behave like solid particles. A shape correlation without taking the dimensionless shear rate into account gave good evaluations of the bubble aspect ratio, which means that the shear rate is not a dominant factor causing the change of shape deformation. However, drag coefficients were affected by the shear rate. Making use of a correlation for bubbles in stagnant liquid, a new correlation of drag coefficients was deduced, which agreed well with the experimental data. Both clean and contaminated CL data showed similar tendency, i.e. after a drastic decrease to a local minimum, CL value slightly increases with increasing the bubble Reynolds number, Re, and then gradually decreases to negative values. A difference in concentration of Trion X-100 resulted in only a slight change in CL at high Re regime. Different types of surfactant resulted in noticeably different values of CL especially at low Re. The CL of small spherical bubbles in contaminated systems could be reproduced by a correlation for solid particles, supporting that fully-contaminated spherical bubbles behave like solid spheres. For deformed bubbles, the lift coefficients can be expressed by relating the negative lift force due to shape deformation with the drag force.