Apparatus and methods for modulating flow rates in microfluidic devices are provided. The methods involve modulating downstream pressure in the device to change the flow rate of materials in an upstream region of the device. Such methods include electrokinetic injection or withdrawal of materials through a side channel and the use of an absorbent material to induce wicking in the channel system. The apparatus provided includes a prefabricated wick in the device to provide for flow rate control. Additional methods for determining velocity of a particle and cell incubation time are also provided.