Micro and Nano Fluidics:

Buoyant Microfluidics

It is not surprising that the use of buoyancy as a driving force in microfluidic systems has attracted little or no attention. Buoyant forces are proportional to the volume and do not scale favorably as the device size is reduced. Nevertheless, in certain biotechnological applications, one can produce sufficiently large buoyancy forces to generate fluid motion at velocities on the order of mm/s even in conduits with equivalent diameters of a few hundreds of microns. One example is the thermal polymerase chain reaction (PCR) for DNA amplification. In this process, the reagents’ temperature varies from about 55?C to 94?C. Such large temperature variations can induce significant buoyant forces. Another class of systems that can be driven by buoyant forces is rotating laboratories on a chip (lab on a CD). In such laboratories, large centrifugal accelerations may induce significant buoyant forces even when the temperature variations are relatively small. These temperature variations can be used to propel and control fluid flow.

We have successfully demonstrated a self-actuated, continuous flow (SAFC) PCR reactor for DNA amplification. The common PCR process requires the cycling of sample’s temperature from about 94C (denaturation) to 55?C (annealing) to 72?C (extension). In most bench-top PCR reactors, the sample is maintained stationary while the temperature is repetitively alternated, which makes it necessary to heat and cool both the reagents and the heating block. This process results in considerable thermal inertia, and it is relatively slow and energy-intensive. Some of these shortcomings can be overcome through miniaturization and the use of continuous flow reactors. In continuous flow PCR reactors, the temperatures of the three thermal zones are maintained fixed while the reagents are circulated continuously through these thermal zones. Continuous flow reactors allow for significantly shorter heating and cooling times with reduced energy consumption since it is not necessary to combat the thermal inertia of the apparatus. The continuous flow reactors require, however, a means for propelling the sample. In recent years, various means for propelling the fluid, ranging from pressure to peristalsis to magneto-hydrodynamics, have been proposed. Here, we put forward an intriguing alternative.
The relatively large temperature variations that are needed for PCR induce significant variations in the fluid’s density, which we use to generate fluid motion. We constructed a triangular shaped, closed loop in the vertical plane (Fig. 1). The vertical leg was heated to 94?C (denaturation zone), part of the diagonal leg was maintained at 55?C (annealing zone), and the horizontal leg was maintained at 72?C (extension zone). The upper part of the tube was exposed to the ambient temperature to facilitate cooling from 94?C to 55?C. The closed loop formed a thermosyphon, and the fluid’s density variations facilitated counter-clockwise circulation. Flow velocities as high as 5mm/s were attained in a conduit with a 700?m diameter. Interestingly, since for a given loop geometry both the buoyant and viscous forces are proportional to the length of the loop, the flow velocity is only weakly dependent on the actual length of the loop, and the loop can be readily scaled down in length with only moderate impact on the fluid velocity. This assertion was demonstrated through experiments and theoretical calculations.

The reagents circulated continuously among the various heated zones as required for DNA amplification. Successful amplifications of 700-bp and 305-bp fragments of Bacillus cereus genomic DNA were demonstrated.
We speculate that by judiciously heating various branches of a network, we can use buoyant forces to direct the flow along a desired path without the need for mechanical valves.

PUBLICATIONS RELEVANT TO BUOYANT MICROFLUIDICS

• Chen, Z., Qian, S., Abrams, W., R., Malamud, D., and Bau, H., H., 2004, Thermosyphon-based PCR Reactor: Experiment and Modeling, Analytical Chemistry, 76, 3707-3715