Surfactant aided bubble departure during pool boiling

The cyclic process of bubble nucleation, growth, departure, and rewetting of the heater surface is a typical sight during boiling. Early bubble departure at smaller sizes is known to improve boiling heat transfer. The passive strategy of addition of small amount of surfactant additives in water is a widely used technique in this regard. While several bubble departure models for pure fluid are present in the literature, they overestimate the bubble sizes during boiling with aqueous surfactant solutions. In this work, we perform pool boiling experiments at various heater orientations to elucidate the role of a lesser-known force of repulsion on bubble departure diameter. The same force was recently shown to facilitate bubble departure away from an inverted heater surface, even against the combined effect of surface tension and buoyancy. We perform a simple force balance analysis to demonstrate that this force originates from the repulsion between the surfactants adsorbed at the liquid-vapor interfaces of the thin liquid-film contained between neighboring bubbles. We show that this force is strong enough to facilitate early bubble departure from an upward facing heater, even at very small bubble sizes where buoyancy is negligible in comparison to the opposing forces of surface tension and drag. We further confirm the importance of this force during near-saturated boiling on vertical facing heaters, where surfactant-laden bubbles depart horizontally away from the heater surface, unlike boiling with pure water, where buoyancy simply slides the bubbles up along the heater surface. We believe that the new physical insights on bubble dynamics provided in this study are critical for heat transfer enhancements during boiling with aqueous surfactant solutions.