One of the main goals in the new area of microfluidics is the development of a robust, accurate, and inexpensive "Lab-on-a-Chip." Such a device is a miniature chemical processing system for genetic assays, drug-discovery screenings, flow cytometry, chemical analyses, environmental water-quality tests, and other similar applications. Most current work in this area focuses on adapting the successful techniques for miniaturization used in microelectronics and MEMS to produce microchannels, microchambers, and micropumps for fluidic processing. However, at the length scale of interest to most microfluidic applications, i.e., 10 - 100 mm, the flow in a channel presents the inherent difficulty of a large pressure drop. Such pressures are difficult to produce and sustain in these small devices.
A different direction for the development of microfluidics is the containerless processing of fluids using optically controlled, thermocapillary transport of microdroplets. In this approach, elemental fluidic operations such as transport, mixing, and metering can be rapidly and flexibly configured in a single device to perform a wide array of chemical assays and analyses.
To demonstrate this technology, a model problem of a small, thin, liquid droplet on a solid surface is examined. A thermal field is imposed on the solid such that a surface-tension gradient is produced on the free surface of the droplet. The resultant thermocapillary flow inside the droplet modifies the contact angles of the droplet and thus forces the motion of the contact line. Lubrication theory is used to describe this internal flow and the subsequent contact-line motion. Results for two-dimensional and three-dimensional droplets moving on horizontal and inclined surfaces will be presented.