TWO-PARTICLE OPTICAL-TWEEZERS MICRORHEOLOGY: FROM INDEPENDENT FLUCTUATIONS TO HYDRODYNAMIC CROSS-TALK

Passive optical-tweezers microrheology has become an established tool for determining the linear viscoelastic properties of complex fluids from the thermal fluctuations of embedded probe particles. However, conventional single-particle approaches cannot directly quantify the hydrodynamic interactions between neighbouring probes, which become increasingly important as the interparticle distance decreases. Here, we present a novel analytical framework for two-particle passive microrheology that simultaneously characterises the viscoelastic response of the surrounding medium and quantifies the frequency-dependent hydrodynamic cross-talk between two optically trapped particles. The framework is based on the analysis of both the independent fluctuations of the probe particles and their correlated motion, providing a unified description of bulk rheology and interparticle hydrodynamic interactions within the same experiment. The methodology is first validated using Newtonian reference fluids, for which the measured hydrodynamic coupling is shown to agree with Faxén’s correction over the investigated range of particle separations. We then extend the analysis to polyacrylamide (PAM) solutions of different molecular weights, representative of shear-thinning viscoelastic fluids, and finally to salmon sperm DNA solutions prepared at different concentrations. By systematically varying the separation between the trapped particles, we investigate how hydrodynamic interactions evolve as a function of both interparticle distance and the viscoelastic nature of the surrounding medium. While measurements performed at large particle separations recover the intrinsic viscoelastic properties of the fluid with negligible hydrodynamic crosstalk, progressively smaller separations reveal a pronounced frequency-dependent coupling between the particles. Comparison between polymer solutions further demonstrates that increasing molecular weight and viscoelasticity significantly modify the hydrodynamic interaction spectrum. The proposed framework establishes two-particle optical-tweezers microrheology as a powerful experimental platform for stidying bulk viscoelasticity and hydrodynamic interactions.