One bead, one fingerprint
Microfluidic Hall Simulator · SoftwareX
When a magnetic bead is carried past a Hall sensor in a microfluidic channel, it leaves a specific bipolar voltage pulse. Read that pulse and you can tell how big the bead was, how fast it moved, and how close it passed.
FIG. 1 — The pulse, live · interactive
Watch the fingerprint form
The bead sweeps the channel; the lower trace is the Hall voltage it induces, position-for-position. Change the flow speed, bead size, and distance and watch the pulse reshape.
FIG. 2 — Why the pulse is bipolar
Why the pulse is bipolar
The bead is a tiny magnetic dipole. Directly over the sensor its field points straight down through the graphene — a strong positive reading. But as it approaches and departs, the field lines curl and the perpendicular component actually flips sign, giving the two negative side-lobes. The math is Bz ∝ (2h² − x²)/(x² + h²)^(5/2), which crosses zero at x = √2·h. That shape is the fingerprint: its height scales with bead volume and field, its width with how fast the bead moves and how far it sits from the sensor.
Try it: shrink the bead–sensor distance and the pulse gets tall and sharp; speed up the flow and it narrows in time; grow the bead and the whole thing scales up. A detector just has to recognize this template against noise.
Building flow cytometry, magnetic bead assays, or lab-on-chip readout? Let’s talk.
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