Effect of surfactant on flow patterns and draining films created by a static horizontal liquid jet impinging on a vertical surface at low flow rates

Abstract The flow behaviour and wetting patterns generated by a liquid jet impinging on a surface is important for cleaning and coating operations. The flow patterns generated by a static, coherent horizontal jet impinging on a vertical surface were studied for three nozzle sizes (1 mm, 2 mm and 3 mm diameter) on surfaces with different wetting characteristics, namely borosilicate glass and polymethylmethacrylate (Perspex). Experiments were performed using water and three different aqueous solutions of an anionic detergent (Tween 20) across the temperature range 20–60 °C. At the flow rates studied, which lie at the lower end of the range employed in industrial static spray ball systems (corresponding to jet mean velocities of 0.8–3.3 m s −1 ), an important series of phenomena are observed. After impingement, the liquid flows radially outwards over the vertical surface. The liquid flowing upwards from the point of impingement forms a feature resembling a hydraulic jump. The location of this film jump is sensitive to surface tension and contact angle at lower flow rates 0.65–10 g s −1 ), but at higher flow rates (greater than 11 g s −1 ) the jump location is insensitive to substrate nature. The model of Wilson et al. (2011) , predicting the film jump location, is here modified and gives good agreement with the experimental data at the lower flow rates, using the contact angle for water on each substrate. At the higher flow rates the modified model suggests an effective contact angle of 90°. The draining films exhibit the two forms reported by Wilson et al., namely rivulet and gravity flows, and the transition between these is described well by the criterion of Hartley and Murgatroyd (1964) across the range of temperatures and surfactant concentrations (above and below the critical micelle concentration) studied. Unlike the film jump location, the shape of the draining film was strongly affected by surfactant, which promoted gravity flow.