A method is provided for transporting a plurality of cells through a flow chamber, wherein the cells are initially immobilized on an internal surface of the flow chamber. The method comprises: selectively releasing the cells from the internal surface of the flow chamber; and flowing liquid through the flow chamber such that the released cells travel with the liquid, thereby transporting the cells through the flow chamber. Cells can be immobilized on or selectively released from the internal surface by applying or removing a trapping potential. The trapping potential can arise from an electric field gradient or an optical field gradient. Alternatively, cells can be selectively released from the surface using photocatalysis. Selective release allows cells to be individually observed or analyzed downstream, and can be based on a signal detected from one or more cells immobilized on the surface.