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Paramagnetic particles as sensitive force detectors in liquids

We study how paramagnetic colloids in external magnetic fields can be used as sensitive force detectors in liquid systems. An experimental setup has been constructed which uses optical tweezers to calibrate the force between paramagnetic dipoles. In the operative mode the optical tweezers are switched off and the unknown force is measured by tuning a magnetic field and therefore the dipolar interaction between the two paramagnetic dipoles. The setup can be used to measure the force versus distance as well as for probing critical bond forces in the range between 5 fN and 1 pN. We apply the system to measure capillary forces at a water–air interface and for probing polyelectrolyte bridges between colloids.