An Argument for the use of Pressure Driven Flow in a Microfluidic System

22 Jun An Argument for the use of Pressure Driven Flow in a Microfluidic System

Posted at 08:04h in Engineering Challenges, Problem Solving by Key Tech

When you are dealing with fluid volumes hundredths that of a rain drop or flow rates that would take decades to fill a beer bottle, the performance of conventional fluid handling methods used on the macro scale can be adversely affected by factors that often dominate on the microfluidic scale. A prime example of this is the significant effect temperature has on the performance of positive displacement pumps when trying to control extremely low flow rates commonly used in microfluidics.

What is a PD pump?

A positive displacement pump’s function is aptly described by its name: a volumetric change caused by the positive displacement of a piston creates movement of a fluid volume equal to the volumetric displacement. A familiar positive displacement pump is a syringe. As the piston is pulled or pushed, a volume of fluid equal to the volumetric change of the syringe’s chamber is pulled into or pushed out of the syringe. There are currently PD pumps on the market that are intended for microfluidic applications that claim to control flow rates down to picoliters per second using this simple principle. These devices use a finely threaded screw to slowly move a piston in a syringe with a small diameter chamber, thus creating very low flow rates. For some applications, these pumps can work very well, but as we discovered during some recent testing, sometimes unexpected effects such as temperature can wreak havoc on the performance of PD pumps in a microfluidic system.

The effect of temperature

The damaging effect of a small temperature shift to a microfluidic system reared its ugly head while we were attempting to use a PD pump to hold a stationary boundary between two fluids in a channel smaller than a human hair. During testing, the fluid boundary would suddenly move tens of millimeters down the channel, ruining the experiment. We finally noticed that the fluid motion would occur just seconds after someone would open the door to the lab, letting cold winter air envelop the system. Delving into a few textbooks and performing several scoping calculations, we determined that a -1°C change in the temperature of the tubing connected to the PD pumps would cause the tube material to constrict and enlarge the tube’s volume by roughly the same volume as the channel! Even though the change in air temperature in the lab was rather dramatic, even changes such as the air conditioning in the room turning on can cause gradual changes in actual flow rates, causing a problem when trying to control flow rates to single nanoliters per second.

Alternatively, temperature shifts can also create unwanted fluid displacements by causing volumetric changes in air bubbles within the fluid. Small bubbles can be introduced in the system fairly easily; for example, by not thoroughly degassing the fluid before introducing it to the system or making connections that are not completely wetted. The relationship between temperature, pressure, and volume (PV=nRT) causes the air bubble to grow or shrink from small variations in temperature. This effect is similarly produced by small differences in pressure across the line. The volumes of air in a bubble and the pressures necessary to have a negative impact on a microfluidic system are extremely small too.

Control the flow with pressure

The significant effect of small temperature shifts on the microfluidic scale and the difficulty of finely controlling the environment are both motivations to avoid the use of a PD pump in a microfluidic system. Instead, opting for the use of a pressure differential to control flow rates can prove much more successful.As the pressure differential across the fluid changes, so does the flow rate. A pressure control system can monitor pressures in the system and maintain a differential that produces the desired flow rate.If environmental temperature or pressure shifts occur, the pressure control system will observe and correct any changes in the pressure differential. Of course, in order to maintain flow rates accurate to nanoliters per minute in a microfluidic channel, the pressure control system has to be able to detect and adjust pressures on the order of 0.001psi. The hardware and design necessary for such a pressure control system are a subject for another day.