The flow of particles (18) in an aperture (10) between two reservoirs (14) and (15) is controlled by suspending the particles (18) in a fluid (17) within the aperture (10), applying a potential difference across the aperture (10) so as to tend to electrophoretically transport the particles (18) between a region of higher potential field and a region of lower potential in the fluid (17), applying a pressure differential across the aperture (10) so as to tend to transfer the fluid (17) with the particles (18) therein though the aperture (10) from a high-pressure reservoir (14) to a low-pressure reservoir (15), and adjusting the potential difference and/or the pressure differential across the aperture (10) in order to achieve precise control over the translation of the particles (18) within the aperture (10). This permits precise control of velocity and displacement, and the measured delivery of particles in solution through an aperture from one reservoir to another by means of careful command over electrical potential and pressure differential over the aperture.