microchannels

Microchannels are fluidic passages with cross-sectional dimensions in the micrometer to sub-millimeter range, typically tens to hundreds of micrometers in hydraulic diameter. They enable confinement and precise handling of small fluid volumes, and their high surface-to-volume ratio enhances heat and mass transfer relative to macro channels. In most microchannels, flow is laminar (low Reynolds number) and driven by pressure differences or electrokinetic effects.

Common geometries include rectangular and circular cross-sections, with straight, serpentine, or folded layouts to maximize residence time or compactness. Materials range from silicon and glass to polymers such as polydimethylsiloxane (PDMS) and cyclic olefin copolymer (COC); metal channels are also used in some applications. Fabrication methods include soft lithography, MEMS-based etching, laser micromachining, micro-milling, and bonding/lamination techniques.

Fluid behavior in microchannels depends on fluid properties. Many applications use Newtonian liquids, while biofluids and polymers may be non-Newtonian. Wall effects and electrokinetic phenomena (electroosmosis and electrophoresis) can drive flow without mechanical pumps. The high surface area fosters rapid heat transfer, but mixing is limited under purely laminar conditions and often requires passive or active strategies.

Applications span microfluidic labs-on-a-chip, chemical synthesis in microreactors, analytical devices, and cooling of microelectronics and electronics. In cooling, microchannels increase heat transfer coefficients and enable compact liquid cooling systems. In biology and chemistry, they enable controlled reactions, cell manipulation, and sensor integration.

Key design considerations include pressure drop, thermal performance, chemical compatibility, and clogging or fouling due to deposits. Bubble formation and removal are practical concerns. Tolerances, surface roughness, and bonding integrity influence reliability, especially in modular microfluidic systems.

Performance metrics commonly used are pressure drop, Nusselt number for heat transfer, and a mixing or reaction yield index for microreactors. Ongoing research seeks to improve flow control, mixing efficiency, and integration with sensors and actuators.