A flow controller which uses a combination of hydrostatic pressure and electroosmotic flow to control the flow of a fluid. A driving fluid (1204) whose flow rate is dependent on both hydrostatic pressures and electroosmotic flow can be used (a) directly as a working fluid in an operable device, for example a chromatograph, or (b) to displace a working fluid (1203) from a storage container (625) into an operable device (1301), or both (a) and (b). The driving fluid (1204) can be composed of one or more fluids. Part or all the driving fluid (1204) is passed through an electroosmotic device (100) so as to increase or decrease the flow rate induced by hydrostatic pressure.