Patent ID: 8222049

Claim:
A method, comprising: flowing a first fluid from a first channel portion to a second channel portion in a microfluidic system, wherein a fluid path defined by the first channel portion has a larger cross-sectional area than a cross-sectional area of a fluid path defined by the second channel portion; flowing a second fluid in a third channel portion in the microfluidic system in fluid communication with the first and second channel portions, wherein the viscosity of the first fluid is different than the viscosity of the second fluid, and wherein the first and second fluids are substantially incompressible; without stopping the first or second fluids, causing a volumetric flow rate of the first and second fluids to decrease by a factor of at least 3 in the microfluidic system as a result of the first fluid flowing from the first channel portion to the second channel portion, compared to the absence of flowing the first fluid from the first channel portion to the second channel portion, wherein the volumetric flow rate of the first and second fluids is determined by Poiseuille's law; and effecting a chemical and/or biological interaction involving a component of the first or second fluids at a first analysis region in fluid communication with the channel portions while the first and second fluids are flowing at the decreased flow rate.