Discuss different types of microfluidic channels and explain their operating principles.

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## Types of microfluidic channels and their operating principles

Microchannels are classified by **geometry** and by **how the fluid is driven**.

### By geometry and function
| Type | Description | Use |
|---|---|---|
| Straight channel | Simple uniform path | Reaction, detection, separation |
| Serpentine (meander) | Long folded path in a small area | Mixing, long residence time, PCR |
| T- and Y-junction | Two inlets merge into one outlet | Laminar mixing, droplet generation |
| Flow-focusing | Sheath flows squeeze the core stream | Droplet generation, cell focusing |
| Spiral | Curved path creates Dean flow | Inertial cell separation |
| Branched / tree | Splits flow into parallel channels | Distribution, gradient generators |

### By driving principle
| Type | Operating principle |
|---|---|
| Pressure-driven | A pump or syringe applies ΔP; parabolic (Poiseuille) profile, Q = ΔP/R_h |
| Electrokinetic (electroosmotic) | An electric field moves the ionic layer along charged walls; nearly flat (plug) profile |
| Capillary-driven | Surface tension pulls liquid into hydrophilic channels; no external pump |
| Centrifugal | Rotation of a disc pushes fluid outward; valves set by spin speed |

### By cross-section
Rectangular (common from soft lithography), trapezoidal or V-shaped (anisotropic KOH etching of silicon), semicircular (isotropic etching of glass), circular (capillary tubes).

```mermaid
flowchart TD
    A[Microfluidic channels] --> B[Geometry]
    A --> C[Driving force]
    B --> B1[Straight]
    B --> B2[Serpentine]
    B --> B3[T and Y junction]
    B --> B4[Flow focusing]
    C --> C1[Pressure-driven]
    C --> C2[Electroosmotic]
    C --> C3[Capillary]
    C --> C4[Centrifugal]
```

### Exam tip
Two tables (geometry and driving force) plus the hydraulic-resistance idea give full marks.

Types of microfluidic channels and their operating principles

By geometry and function

By driving principle

By cross-section

Exam tip

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