Measuring and overcoming limits of the Saffman-Delbrück model for soap film viscosities

Spatial Cross-Correlated Diffusion of Colloids under Shear Flow

The spatial cross-correlated diffusion of colloidal particles is often used as an essential tool to study the dynamic properties of a fluid because it directly describes the hydrodynamic interaction between two particles in a fluid. However, the experimental measurement of cross-correlated diffusion can be substantially modified by even a weak shear flow. In this work, the effect of a shear flow on spatial cross-correlated diffusion is demonstrated using experimental measurements that show a clear dependence on pair angles. An analytical solution is proposed to explain the experimental observations. A numerical simulation is performed to systemically demonstrate the influence of shear flow on spatial cross-correlated diffusion. The results of the experiment, theoretical analysis, and numerical simulation agree well with each other. Therefore, this research provides a sensitive experimental method to determine the weak shear flow in any quasi-two-dimensional fluid systems.

Energy dissipation in flows through curved spaces

Fluid dynamics in intrinsically curved geometries is encountered in many physical systems in nature, ranging from microscopic bio-membranes all the way up to general relativity at cosmological scales. Despite the diversity of applications, all of these systems share a common feature: the free motion of particles is affected by inertial forces originating from the curvature of the embedding space. Here we reveal a fundamental process underlying fluid dynamics in curved spaces: the free motion of fluids, in the complete absence of solid walls or obstacles, exhibits loss of energy due exclusively to the intrinsic curvature of space. We find that local sources of curvature generate viscous stresses as a result of the inertial forces. The curvature- induced viscous forces are shown to cause hitherto unnoticed and yet appreciable energy dissipation, which might play a significant role for a variety of physical systems involving fluid dynamics in curved spaces.

Visualizing the Dynamics of Nanoparticles in Liquids by Scanning Electron Microscopy

Taking advantage of ionic liquid nonvolatility, the Brownian motions of nanospheres and nanorods in free-standing liquid films were visualized in situ by scanning electron microscopy. Despite the imaging environment's high vacuum, a liquid cell was not needed. For suspensions that are dilute and films that are thick compared to the particle diameter, the translational and rotational diffusion coefficients determined by single-particle tracking agree with theoretical predictions. In thinner films, a striking dynamical pairing of nanospheres was observed, manifesting a balance of capillary and hydrodynamic interactions, the latter strongly accentuated by the two-dimensional film geometry. Nanospheres at high concentration displayed subdiffusive caged motion. Concentrated nanorods in the thinner films transiently assembled into finite stacks but did not achieve high tetratic order. The illustrated imaging protocol will broadly apply to the study of soft matter structure and dynamics with great potential impact.