An Introduction to Microfluidics

In this talk, we will present an introduction to microfluidics which consists of small scale fluid dynamics when the typical length scale of the flow is 10-1000 microns. Such dimensions are similar to the thickness of a human hair (~100 microns) and larger than all the molecules present in the system so that the continuum assumption is still valid. Such devices are useful in a broad range of applications, including chemical synthesis, biosensing, drug discovery and, more generally, “lab-on-a-chip”. The goal is to design and fabricate devices capable of the precise manipulation and control of small fluid volumes as well as the nano/micrometer sized particles present in such volumes. Precision, low footprint, portability, low power requirement, automation and high throughput of microfluidic devices bring significant advantages and have enabled technological advances. Microfluidics typically involve laminar flows as Reynolds numbers are low; multiphase flows since particles and/or droplets are present; and multiphysics due to the easy manipulation of both liquids and particles using external (e.g., electric) fields. We will present some examples of microflows and microfluidic devices, which can be categorized essentially into two types, the channel flow and droplet flow types. In the former, reactions take place in microchannels while in the latter they occur in individual droplets. We will also discuss some of the challenges microfluidics present, including the difficulty to induce mixing at low Reynolds numbers although mixing is a necessary condition for efficient reactions to occur, such as those required for biochemical assays. We will show how both active and passive mixing techniques can be utilized for efficient mixers. Other opportunities and challenges related to the use of electric field in multiphase flows in small devices will be presented.