A recent addition to the techniques of jets, wetted spheres and wetted wall columns, whereby a cylindrical droplet or 'slug' of the lighter liquid phase is maintained stationary in downflow of the denser liquid phase in a vertical cylindrical tube is described. Ordinarily the tube can be made of glass, and the denser liquid is an aqueous phase which wets the glass in preference to the light (organic) phase. This apparatus is experimentally very simple and has the added advantage that it can be used to obtain measurements of the effective interfacial tension. (from paper)