Effect of hydrophilic-lipophilic balance on cavitation-driven emulsification near a liquid-liquid interface.

HYPOTHESIS: Cavitation-driven emulsification is governed by bubble collapse and microjet formation at the liquid-liquid interface, which in turn is correlated with the system's hydrophilic-lipophilic balance (HLB). Modifying the HLB value through the use of surfactants affects interfacial properties, one of which is the static interfacial tension between liquids. This might change emulsification pathways and consequently the type of formed emulsion. EXPERIMENTS: The HLB value of an oil-water system was systematically tailored using surfactants Tween 80 and Span 80, which were separately dissolved in demineralized water and silicone oil, respectively. The resulting systems were first characterized in terms of viscosity, surface tension, and static oil-water interfacial tension. Single laser-induced cavitation bubbles were then formed at various distances from the liquid-liquid interface, and their collapse dynamics were characterized using high-speed visualization. Bubble dynamics through bubble centroid displacement were correlated with the HLB value of the respective system. FINDINGS: In systems with higher HLB values (≥ 12.9), bubbles always collapsed away from the oil-water interface, following classical density-driven jetting behaviour. Intermediate values (between 10.7 and 6.4) produced bubble collapse both towards and away from the interface. At the lowest HLB value of 4.3, bubbles collapsed towards the oil-water interface, a reversal of the expected jetting direction. These indicate different emulsification pathways, which are consistent with typical HLB ranges for surfactant applications. This demonstrates that in cavitation-driven emulsification, the system's HLB value affects directly the collapse dynamics of cavitation bubbles, and consequently, the type of emulsion they produce.