Microfluidics 101: Understanding the Basics

Laminar vs. Turbulent Flow: One of the first things to understand about microscale fluid behavior is that flow is laminar under typical microfluidic conditions. The flow regime is characterized by the dimensionless Reynolds number (Re), which is the ratio of inertial forces to viscous forces in the fluid. In microfluidic channels, Re is usually very low – often well below 100 and even <1.0 in many cases (Basic Microfluidic Concepts). (For comparison, turbulence in pipes usually requires Re > ~2000.) Such low Reynolds numbers mean viscous forces dominate, and the fluid flows in smooth, orderly layers with no chaotic eddies. Two liquids introduced into a microchannel will flow in parallel laminar streams without instant mixing. Any mixing between streams occurs only by molecular diffusion across the interface (Microsoft PowerPoint – LECTURE [Compatibility Mode]). This has profound implications for microfluidic design and experiments:

• Because microflows are laminar, predictable flow patterns can be established and maintained. For example, one stream can envelop another to focus particles or cells, or multiple reagents can be brought together with controlled interfaces. The lack of turbulence allows precise spatial control of fluids within the chip.
• The downside is that mixing relies on diffusion, which is slow over macroscopic distances. In a microchannel, two fluids may flow side by side for centimeters with only a narrow diffusion zone blending them. Special micromixers (discussed later) are often employed to enhance mixing since the flow itself won’t turbulently mix the streams (Microsoft PowerPoint – LECTURE [Compatibility Mode]). This is reflected in a high Péclet number (Pe) in many microfluidic flows – convection dominates over diffusion, so without intervention, mixing is diffusion-limited (Understanding Flow Control Microfluidics in Nanoscale Structures).

Scaling Laws and Dominant Forces: The microscale regime also means that other forces scale differently than we are used to at the macroscale. A classic example is the square-cube law: as you miniaturize a system, volume (which relates to weight/inertia) decreases faster than surface area. Thus, surface-related forces become very important, while body forces like gravity become negligible. In microfluidic channels, surface tension and viscous forces usually outweigh gravity. A water drop in a 100 µm channel will stick to walls or move by capillary action rather than falling due to weight (Frontiers | Capillary microfluidics for diagnostic applications: fundamentals, mechanisms, and capillarics). This has several consequences:

• Capillary Action: Microfluidic devices can leverage capillary forces to move fluids. For instance, if a channel’s surface is made hydrophilic, capillary pressure can spontaneously draw a liquid through the channel without any pump. (At this scale, the Bond number – ratio of gravity to surface tension – is very low, so liquids defy gravity and can even flow upward (Frontiers | Capillary microfluidics for diagnostic applications: fundamentals, mechanisms, and capillarics).) This enables passive pumping in point-of-care diagnostics and paper-based microfluidics.
• No Slip at Walls: Viscous forces and the small dimensions mean fluid velocity is effectively zero at channel walls (the no-slip condition), creating a parabolic velocity profile across a channel cross-section. This laminar velocity profile means fluid in the center moves faster than fluid near the walls. Molecules near the walls rely mostly on diffusion to spread, which again underscores why mixing or reagent transport can be slow without design considerations.
• Molecular Diffusion Times: Length scales are small, so diffusion over, say, 50 µm is fast (on the order of milliseconds). But if a channel is long and two streams flow side by side, complete mixing may still require milliseconds to seconds of contact time depending on diffusion coefficients. Microfluidic designers use scaling analysis to ensure channels are long enough (or incorporate mixing structures) such that diffusion can do its job given the flow speed.

In summary, microfluidic flows are highly predictable and stable due to low Reynolds numbers and laminar behavior (Basic Microfluidic Concepts). However, the dominance of viscous and surface forces means engineers must account for slow mixing and capillary effects. Designing effective microfluidic experiments involves exploiting these characteristics (for precise control) while overcoming their challenges (e.g., using mixers to counter slow diffusion).