Two-point microrheology measurement from widely separated colloids can reveal the bulk rheological property of a fluid. We develop such a technique which measures the effective viscosity using two trapped particles in a dual trap optical tweezers by exploiting the motional resonance excited in the probe particle when the control particle is driven externally. We carry out the measurement both from the amplitude and the phase response of the resonance and show that the zero-crossing of the phase with respect to the drive signal at the resonance frequency gives more accurate results when the particles are separated widely. Later on, we compare our measured viscosity values with that measured using a commercial rheometer and obtain an agreement within ∼1 %. In future, this method can be extended to a linear viscoelastic fluid enabling high accuracy measurements.

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Shuvojit Paul, Randhir Kumar, and Ayan Banerjee, "Active microrheology using a two-particle system coupled by hydrodynamic interactions in optical tweezers," Proc. SPIE 10723, Optical Trapping and Optical Micromanipulation XV, 107231D (Presented at SPIE Nanoscience + Engineering: August 22, 2018; Published: 7 September 2018); https://doi.org/10.1117/12.2323191.