Nanomedicine, Volume I: Basic Capabilities
© 1999 Robert A. Freitas Jr. All Rights Reserved.
Robert A. Freitas Jr., Nanomedicine, Volume I: Basic Capabilities, Landes Bioscience, Georgetown, TX, 1999
184.108.40.206 Pressure Release Pumps
Some form of motive force must be applied to cause fluid to flow through a tube. The simplest possible motive force is pressurization. Consider a valved cylindrical nozzle of radius rtube and length ltube attached to a reservoir of volume Vreservoir maintained at a constant pressurization preservoir (even while being emptied). When the valve is opened, the fluid vents through the nozzle at a maximum velocity vmax into an external fluid environment having static pressure pexternal < preservoir, emptying the reservoir in a time tempty, both given by:
Taking pexternal ~ 1 atm, h = 0.6915 x 10-3 kg/m-sec for water at 310 K, rtube = 10 nm and ltube = 100 nm, then holding vmax <~ 10 micron/sec (the velocity of random thermal intracellular hydrodynamic flows; Section 220.127.116.11) for intracellular discharges requires a driving pressure of preservoir ~ 0.5 x 10-3 atm, which takes tempty ~ 0.3 sec to empty the reservoir at an energy cost (Eqn. 9.27) of Pflow tempty ~ 55 zJ (~13 kT). As a practical matter, normal variations in physiological pressure due to ~1 Hz heartbeat pulse waves are 0.5-10 x 10-3 atm in the tissues and veins and up to 50-70 x 10-3 atm in the aorta (Section 18.104.22.168), so either valve aperture or preservoir must be dynamically controlled if a steady exit velocity is required at such low discharge velocities. In addition, a minimum pressurization of ~pforce (Eqn. 9.24) must be used to overcome capillary forces and initiate the flow. Device cooling during rapid gas discharge is described in Section 5.3.3.
Last updated on 20 February 2003