A system and method for controlling fluid flow within a microchannel includes a fluid circuit comprising a fluid outlet well and one or more fluid inlet wells, all in communication with a microchannel. A negative pressure differential is applied to the outlet well and fluid flow from an inlet well into the microchannel is controlled by opening or closing the inlet well to atmospheric pressure. To stop fluid flow from the inlet well, a negative pressure differential may be applied to the inlet well to equalize pressure between the inlet and outlet wells. By sequentially opening and closing different inlet wells to atmosphere, controlled amounts of different reagents can be serially introduced into the microchannel.