SURFACTANT EFFECTS ON THE DYNAMICS OF A THIN LIQUID SHEET

The dynamics of a free-surface slender two-dimensional stream (liquid sheet) issuing from a nozzle in the gravitational field in still air, under the effect of surface-active agents, are analysed experimentally . The particular test section geometry (the liquid is forced to assume a bidimensional form between two vertical guides and a horizontal pla te placed at a certain variable distance from the nozzle exit section) employed in this study gives rise to various flow regimes depending o n the governing parameters: liquid flow rate, sheet height, surface pr essure, gravity. Two basic phenomena are observed: thinning of the she et (with recirculating motion inside it) and sheet-threadlines transit ion. For a certain surfactant (bulk) concentration, there exists a min imum critical flow rate value for which the sheet is seen to thin star ting at both of the sheet bottom corners. A ridge, usually referred to as a Reynolds ridge in the literature, separates the sheet from the t hin-film regions. The thin films exhibit recirculating flows (caused b y the onset of surfactant-induced surface-pressure-driven convection i n the gravitational field) and extend to the entire rectangular interf ace as the flow rate is reduced. At zero flow rate the thinned sheet r esembles a plane vertical soap him showing a recirculating cellular st ructure. These phenomena are linked to the presence of surface-active material adsorbed at the liquid-air interface and occur when the sheet height exceeds a critical value. Otherwise, at a critical flow rate v alue the liquid sheet breaks up into an array of (more or less regular ly distributed) discrete threadlines (vertical jets), whose spacing de pends on the surface tension of the test liquid.