Principles of Flow

Reynolds number, Re, characterizes flow. (Note at this chapter's end.) For many devices, D, the characteristic length, equals 4A/P, A being the cross-sectional area of the channel and P the wetted perimeter of the channel. In devices, Re is much less than one hundred and often less than ten, so flow remains laminar and non-turbulent. Laminar flows transport molecules predictably, but in bees at least, changes in the momentum of hemolymph probably alter these flows.

Pressure driven syringe pumps drive flows through the channels of microfluidic devices so that single layers of fluid contact the walls and remain stationary. Slow flow over the walls creates a parabolic profile of fluid velocities across the channel. The most centrally positioned flows encounter the least resistance and, therefore, travel the fastest. A second method for driving fluid is electro-osmotic pumping (EOP).

EOP distributes a double layer of oppositely charged ions along the walls of the channel. An electric field positioned across the channel encourages ions in this double layer to move toward their opposite polarities. Viscous forces impede and resist the ions moving along the walls. Convection drags the bulk of fluid in the center of the channel. Electro-kinetic flows require high, superimposed voltages and will not be considered.

0 0

Post a comment