Microscopic flow measurements using optically trapped microprobes

Summary from only given. In previous work, we used a confocal detection modification of a method to measure fluid viscosity by recording the response of a microsphere in a viscous medium under the action of rapidly oscillating photonic force. We showed that the viscosity of the fluid can be accurately and rapidly measured by detecting the phase lag of the motion. In this paper, we extend this technique to measure local fluid flow. A spherical micron-sized silica bead is trapped in a flow cell on an inverted microscope by a strongly focused beam from a Ti-sapphire laser. An acousto-optic deflector nudges the particle periodically by moving the laser tweezers back and forth.