flow in the direction of the arrows 6 due to the viscosity that is always present this effect is called the marangoni effect instead of a temperature gradient it is also possible to use a concentration gradient or an electrical charge gradient a concentration gradient can be achieved for example by introducing a surfactant such as a detergent to the interface 4 adjacent the lip of the feed tube 1 the surfactant reduces the surface tension on the interface 4 adjacent the feed tube causing an increasing tension gradient along the interface in the downward direction in fig1 this surface tension gradient gives rise to fluid flow through the marangoni effect as discussed alternatively the required surface tension gradient can be induced through an electrical charge gradient such a charge gradient could be achieved for example by generating a net positive charge on the interface of the conducting fluids such as on the interface between mercury and electrolyte h 2 so 4 using for example a battery and causing a potential difference between two electrodes located adjacent the lips of the feed and discharge tubes respectively the positive charges along the interface 4 will tend to migrate toward the negative electrode inducing a charge gradient between the feed tube and the discharge tube this charge gradient in turn causes a surface tension gradient along the interface giving rise to the marangoni effect fig6 illustrates as an example one embodiment of an apparatus for producing the electrical charge gradient in this figure.