Our facility is located on Rancho San Pedro, a land grant from King Carlos III to the Spanish soldier, Juan Jose Dominguez, in 1784. The Dominguez family continues to own and lease the property where our building is located. In fact, they once owned the entire Los Angeles river estuary which is now the Port of Los Angeles. Once oil was discovered in this region, it became an industrial area full of refineries and large-scale chemical operations…the stuff of classical chemical engineering.
During my drive into work I pass large holding tanks and arrays of large diameter pipes with valves and pumps that snake up and around in a mysterious confusion. From this roadside view, I enter our office park and into an environment where very small reservoirs and a complex array of channels with pumps and valves are managed with pneumatic controllers on small plastic cards the size of a credit card.
The mis-match in scales and the physics at these two scales cause me to wonder, what’s the same? what’s different? In one world the volumes being moved are many thousands of gallons, in the other a drop of water. Yet the purpose is the same: transport different fluid streams, combine them to create a new stream, and move them to a final output destination.
From a physics point of view, the key difference is that gravity is the main force acting on the liquid in a large volume fluid stream, while in the microfluidic regime, while gravity still has an influence, it is not the main force; surface tension and viscous drag play a larger role. As the dimensions get smaller, these surface forces become greater than gravitational forces. In short, surface area to volume is the key difference in the behavior of fluids in the macro and micro regime.
How do these forces at the macro and micro scale impact design and engineering considerations?
It turns out a great deal! We cannot transfer the same design and engineering principles across these two scales.
In the macro regime, the bulk properties of the materials drive the design and manufacturing considerations. Understanding the heat capacity, the coefficient of thermal expansion, the phase diagram of the liquids, and the thermal and material properties of the transfer pipes are among the many parameters critical for the design of the chemical plant. We don’t need to consider how we are going to make the pipes to meet the demands of the system, we can use well known design principles and manufacturing processes, depending on the service requirements of the system.
Not so in microfluidic device design considerations!