EFFECTS OF SURFACTANT ON LIFT COEFFICIENT OF ELLIPSOIDAL BUBBLES IN THE VISCOUS-FORCE DOMINANT REGIME

Abstract

Effects of surfactants on lift coefficients, <i>C_L</i>, of single ellipsoidal bubbles rising through linear shear flows were investigated. Two types of surface-active agents, 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 &#61; -5.5, where M is the Morton number. The range of the bubble Reynolds number was 0.1 &#60; Re &#60; 70. Bubble shapes were either spherical or ellipsoidal. Comparing with clean bubbles, less deformation of contaminated bubbles was confirmed. 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. A new correlation of drag coefficients was deduced, which agreed well with the experimental data. Both clean and contaminated <i>C_L</i> data showed a similar tendency: after a drastic decrease in <i>C_L</i>, <i>C_L</i> slightly increases and then decreases to a negative value while increasing the bubble Reynolds number, Re. A difference in concentration of Trion X-100 resulted in only a slight change in <i>C_L</i> at high Re regime. Different types of surfactant resulted in noticeably different values of <i>C_L</i> especially at low Re. The <i>C_L</i> of small 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, <i>C_L</i> can be expressed by relating the negative lift force due to shape deformation with the drag force.