Fluid-Mediated Rotational Coupling and Hydrodynamic Interactions in Colloidal Particles Using Optical Trapping

In fluid systems, the forces on solid bodies are mediated by hydrodynamic interactions, which significantly influence the collective dynamics of physical and biological systems. Understanding these interactions is essential for designing complex fluids. In this study, we introduce a novel methodology using optical trapping to measure hydrodynamic interactions across a wide range of pair separations, extending the analysis to viscoelastic fluids. By analyzing the hydrodynamic coupling of particle rotation induced by another particle's motion, we successfully measured lubrication fields—previously difficult to observe experimentally—and these results were validated through theoretical models and simulations. This innovative approach enabled precise measurements of hydrodynamic effects, revealing new insights into non-linear fluid responses.

Furthermore, we explored time-dependent interactions in viscoelastic fluids, where the memory effects of the medium play a crucial role, linking these to the medium’s rheological properties. The experimental hydrodynamic interaction data can be applied to develop advanced simulation tools, improve constitutive models for complex systems such as gels and slurries, and provide deeper understanding for designing materials with tailored mechanical properties.