The effects of surfactant on the multiscale structure of bubbly flows

Abstract

It is well known that a bubble in contaminated water rises much slower than one in purified water, and the rising velocity in a contaminated system can be less than half that in a purified system. This phenomenon is explained by the so-called Marangoni effect caused by surfactant adsorption on the bubble surface. In other words, while a bubble is rising, there exists a surface concentration distribution of surfactant along the bubble surface because the adsorbed surfactant is swept off from the front part and accumulates in the rear part by advection. Owing to this surfactant accumulation in the rear part, a variation of surface tension appears along the surface and this causes a tangential shear stress on the bubble surface. This shear stress results in the decrease in the rising velocity of the bubble in contaminated liquid. More interestingly, this Marangoni effect influences not only the bubble's rising velocity but also its lateral migration in the presence of mean shear. Together, these influences cause a drastic change of the whole bubbly flow structures. In this paper, we discuss some experimental results related to this drastic change in bubbly flow structure. We show that bubble clustering phenomena are observed in an upward bubbly channel flow under certain conditions of surfactant concentrations. This cluster disappears with an increase in the concentration. We explain this phenomenon by reference to the lift force acting on a bubble in aqueous surfactant solutions. It is shown that the shear-induced lift force acting on a contaminated bubble of 1 mm size can be much smaller than that on a clean bubble.

Two-Phase Bubble Columns: A Comprehensive Review

“…Under these conditions, an increase in T c , reduces the viscosity and weakly promotes turbulence; this promotes collision and increases film drainage speed and thus, coalescence. Grover et al [78] [13][14][15][16][17][18] ) observed the same decrease in the homogeneous flow regime, but no effect in the heterogeneous flow regime. Yang et al [190] (d c = 0.041 m, H 2 , CO/Paraffin) observed a decrease in ε G with an increase in both T c (293-523 K) and p c (1-3 MPa).…”

“…Conversely, if "non-coalescing" solutions are employed, the transition flow regime is identified by the appearance of "clusters of bubbles", as defined in refs. [15,16] and observed by [7]. The flow regime transitions from the homogeneous flow regime towards the transition flow regime concept are displayed in Figure 3.…”

Motion and coalescence of a pair of bubbles rising side by side

“…The model of elastic collision among contacting bubbles, which is derived under the potential flow assumption, results in a formation of horizontal bubble clusters (Sangani and Didwania, 1993;Smereka, 1993). The horizontal clusters are observed experimentally (Zenit et al, 2001;Figueroa-Espinoza and Zenit, 2005;Takagi et al, 2008) by adding a small amount of electrolyte or surfactant in order to reduce the bubble coalescence. In the usual bubble column, however, coalescence frequency occurs.…”

Influence of Surfactants on the Motion of a Pair of Bubbles in Quiescent Liquids

“…気泡流には様々なスケールがあり、気泡が生成 する乱れは多くの研究者の注目を集めている。そ の乱れは、流れ場と相互作用し、単相流とは異な る複雑な流動構造を示す [1][2][3][4][5][6]。特に気泡クラスタ ーは、乱流の秩序渦構造よりも大きく、大規模な 流動構造そのものを変化させる [7,8]。 そのため気 泡クラスター形成の有無について様々な報告が 行われている [9][10][11]。 気泡のクラスター形成は、まず理論的に予測さ れた。ポテンシャル流れを仮定し、複数の気泡が 上昇する際、気泡間相互作用によって水平面に気 泡 が クラスターを 形成することが報告された [12,13]。一方で、実際の気泡流では、気泡同士の 合体によって気泡径差が生まれ、通常気泡のクラ スター構造は観察されない。さらに前述のポテン シャル流れを用いた理論でも、気泡径差を考慮す るとクラスターは形成されない [14]。しかし、実 験的研究において、界面活性剤や電解質を混入さ せることで気泡クラスターの形成が報告されて いる [15,16]。 界面活性剤や電解質の混入は、気泡表面の境界 条件を大きく変化させる。界面活性剤では気泡の 抗力の増加や合体を防ぎ、電解質ではやクリーン な気泡を保ちながら気泡合体を防ぐ [17][18][19][20] /01 02 0/%/ü ! 1 ö 3 4 5 6 7 8 9 6 % !ú ÷ ù ' ü ü ö !…”