Why Pressure Drop Matters
Every microfluidic pump has a maximum pressure rating. If the ΔP your rig demands exceeds what the pump can deliver, the actual flow rate will be lower than the setpoint — sometimes dramatically lower, sometimes zero. This is the single most common source of "my flow rate is wrong" problems in microfluidics.
Syringe pumps typically deliver 100–200 mbar of driving pressure before stalling. Pressure-driven controllers like Fluigent and Elveflow go to 1–2 bar routinely. Specialty high-pressure systems (HPLC-style) reach 10+ bar.
Before ordering tubing, compute ΔP. If it lands above your pump's rating, shorten the tubing, go to a larger ID, or switch fluids.
The Fourth-Power Trap
Hagen-Poiseuille resistance scales as 1/r4. This is the single most counterintuitive fact in microfluidics.
Cutting your tubing ID in half multiplies ΔP by 16. Going from 1 mm ID to 0.5 mm ID turns a 50 mbar system into an 800 mbar system at the same flow rate. Many labs discover this the hard way when they "upgrade" to narrower tubing to reduce dead volume and find the pump can no longer drive the flow.
Rule of thumb: use the widest ID you can tolerate for your dead-volume budget. Narrow tubing is a last resort, not a first choice.
Viscosity Dominates at Low Re
In the laminar regime (Re < 2000), ΔP scales linearly with viscosity and linearly with flow rate. Switching from water (1 mPa·s) to 50% glycerol (6 mPa·s) multiplies ΔP by 6× — same tubing, same flow rate.
Temperature also matters: water at 37 °C is 31% less viscous than at 20 °C, so running a cell-culture perfusion at body temperature cuts pump load meaningfully compared to room-temperature benchmarks.
For viscous fluids (glycerol, blood, polymer solutions, protein stocks), pick a wider tubing ID or reduce flow rate. For watery buffers, you have more margin.
