A passive microfluidic pump comprising: (a) a fluid inlet structured to fluidly connect to an upstream fluidic device; (b) a resistive region dimensioned to control fluid flow; (c) an absorbent region ; and (d) an evaporation barrier to minimize evaporation from the pump and enclosing the resistive region and the absorbent region ; characterized in that the resistive region comprises a first porous medium , and a fluidly non-conducting boundary defining a path for fluid flow through the first porous medium from the fluid inlet to the absorbent region ; wherein the absorbent region comprises a fluidly non-conducting boundary defining a volume of a second porous medium sized to absorb a predetermined volume of fluid imbibed from the resistive region , wherein the resistance to fluid flow through the resistive region is at least ten times greater than the resistance to fluid flow through the absorbent region , and wherein the resistive region produces and maintains a predetermined fluid flow rate of from 1 nL/min to 100 μL/min, as determined when the fluid is water, as the fluid advances through the second porous medium from the resistive region and for substantially all of the time during which the predetermined volume of fluid advances through the second porous medium from the resistive region . (a) a fluid inlet structured to fluidly connect to an upstream fluidic device; (b) a resistive region dimensioned to control fluid flow; (c) an absorbent region ; and (d) an evaporation barrier to minimize evaporation from the pump and enclosing the resistive region and the absorbent region ; characterized in that the resistive region comprises a first porous medium , and a fluidly non-conducting boundary defining a path for fluid flow through the first porous medium from the fluid inlet to the absorbent region ; wherein the absorbent region comprises a fluidly non-conducting boundary defining a volume of a second porous medium sized to absorb a predetermined volume of fluid imbibed from the resistive region , wherein the resistance to fluid flow through the resistive region is at least ten times greater than the resistance to fluid flow through the absorbent region , and wherein the resistive region produces and maintains a predetermined fluid flow rate of from 1 nL/min to 100 μL/min, as determined when the fluid is water, as the fluid advances through the second porous medium from the resistive region and for substantially all of the time during which the predetermined volume of fluid advances through the second porous medium from the resistive region . the resistive region comprises a first porous medium , and a fluidly non-conducting boundary defining a path for fluid flow through the first porous medium from the fluid inlet to the absorbent region ; wherein the absorbent region comprises a fluidly non-conducting boundary defining a volume of a second porous medium sized to absorb a predetermined volume of fluid imbibed from the resistive region , wherein the resistance to fluid flow through the resistive region is at least ten times greater than the resistance to fluid flow through the absorbent region , and wherein the resistive region produces and maintains a predetermined fluid flow rate of from 1 nL/min to 100 μL/min, as determined when the fluid is water, as the fluid advances through the second porous medium from the resistive region and for substantially all of the time during which the predetermined volume of fluid advances through the second porous medium from the resistive region . <EOS>