a reaction reservoir having at least one reaction reservoir sidewall thereby defining a reaction reservoir volume, wherein the reaction reservoir initially contains the first fluid;

a microchannel having at least one portion fully enclosed by at least one sidewall, in fluid communication with the reaction reservoir, having a proximal end with a proximal end opening, a distal end with a distal end opening, a length, the at least one microchannel sidewall defining a cross-sectional area, wherein the length and cross-sectional area are selected to obtain a predetermined rate of molecular diffusion between the first fluid and the second fluid, and wherein the microchannel is configured such when the microchannel is filled with the second fluid, and the second fluid is a liquid, that flow of the second fluid through the microchannel is laminar and the capillary action of the microchannel and the second fluid is such that the second fluid does not flow into the reaction reservoir unless the pressure of the second fluid is increased by an external source; and

a pressure equalizing vent in fluid communication with the reaction reservoir and a fluidic external environment, wherein the pressure equalizing vent is capable of equalizing pressure within the reaction reservoir with that of the external environment, wherein the pressure equalization vent has a distal end with a distal end opening in communication with the external environment, a length, at least one sidewall, having a sidewall thickness, and defining a cross-sectional area, and a proximal end with a proximal end opening in communication with the reaction reservoir, and wherein the pressure equalizing vent is configured such that when it is filled with a third fluid, and the third fluid is a liquid, that flow of the third fluid through the pressure equalizing vent is laminar and the capillary action of the pressure equalizing vent and the third fluid is such that the third fluid does not flow out of the distal end opening unless the pressure of the third fluid is increased by an external source;

wherein the reaction reservoir has a maximum first dimension in a primary axis, a maximum second dimension in a secondary axis, and a maximum third dimension in a tertiary axis, and the pressure equalizing vent is a continuous microchannel configured lying wholly exterior to the reaction reservoir and having at least one component in the primary axis with a magnitude greater than the maximum first dimension, at least one component in the secondary axis with a magnitude greater than the maximum second dimension, and at least one component in the tertiary axis with a magnitude greater than the maximum third dimension.