INFLUENCE OF SURFACTANTS ON A CREEPING FREE-BOUNDARY FLOW-INDUCED BY 2 COUNTER-ROTATING HORIZONTAL THIN CYLINDERS

Surfactant is an impurity whose excess concentration at an interface i s capable of decreasing the local surface tension. This effect can pla y a key role in those flows which have a convergent stagnation point a t an interface where high concentration of surfactants is expected. In the present paper the influence of insoluble surfactants on a creepin g flow induced by two infinite thin cylinders placed symmetrically ben eath the free boundary and counter-rotated about horizontal parallel a xes, is analysed. Assuming that the flow is two-dimensional and the cy linders can be represented by two vortices with certain intensities, t he problem is solved by using complex variable techniques. When the tw o vortices merge to form a vortex dipole and gravity is neglected, an exact solution is found. In the general case some numerical simulation s showing typical deformation of the free boundary, are implemented.