Abstract
Curvature characteristics of the vertical oscillatory gas-liquid interface induced by normal wall-jet impingement were experimentally investigated for a gas-jet penetrating into a liquid sheet. The penetration parameters affecting the curvature characteristics were considered to be the flow rate, jet height and liquid sheet thickness. The effect of substrate properties (hydrophilic and hydrophobic substrates characterized by the average static contact angles of 24° and 116°, respectively) on dynamic oscillatory interfacial profiles accompanied with curvature variation was captured by a high-speed camera. Image processing was employed to extract gas-liquid interfacial profiles and rebuild them by using suitable polynomial functions, and then the interfacial curvatures were obtained mathematically. The specific position of the maximum curvature on this vertical interface with an inscribed circle inside the liquid phase was considered to be the position of Rayleigh instability occurrence, whose vertical height from the bottom substrate was analyzed as the characteristic height. The maximum curvature at that particular position for two extreme profiles (the fattest and slimmest profiles) exhibited periodic development in oscillatory periods. The variation between the close-by peak and trough of the maximum curvatures during oscillations was adopted to evaluate the interfacial deformation intensity, and the curvature variation rate was further introduced as a temporal physical factor to estimate the interfacial deformation rate. Experimental results revealed the violent and fast interfacial deformation caused by the wall-jet impingement always occurred at a low jet height and even above the hydrophobic substrate. Experimental cases with approximately 25% probability causing droplets splattering out of the vertical gas-liquid interface was considered as critical cases, wherein a natural exponential relationship between the curvature variation rate and the modified Weber number was adequately determined as the boundary for Rayleigh instability occurrence via fitting analysis. Curvature dependence of the mechanism of Rayleigh instability occurring on the vertical gas-liquid interface was interpreted in the context of interfacial elastic energy transfer. Overall, this work should be helpful to understand the gas-liquid interfacial behavior in scientific research and industrial applications.
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