Ultrasensitive rotating photonic probes for complex biological systems

Shu Zhang, Lachlan J. Gibson, Alexander B. Stilgoe, Itia A. Favre-Bulle, Timo A. Nieminen, and Halina Rubinsztein-Dunlop

We use rotational photonic tweezers to access local viscoelastic properties of complex fluids over a wide frequency range. This is done by monitoring both passive rotational Brownian motion and also actively driven transient rotation between two angular trapping states of a birefringent microsphere. These enable measurement of high- and low-frequency properties, respectively. Complex fluids arise frequently in microscopic biological systems, typically with length scales at the cellular level. Thus, high spatial resolution as provided by rotational photonic tweezers is important. We measure the properties of tear film on a contact lens and demonstrate variations in these properties between two subjects over time. We also show excellent agreement between our theoretical model and experimental results. We believe that this is the first time that active microrheology using rotating tweezers has been used for biologically relevant questions. Our method demonstrates potential for future applications to determine the spatial-temporal properties of biologically relevant and complex fluids that are only available in very small volumes.

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