Effects of Hydrodynamic Interactions on the Near-SurfaceDiffusion of Spheroidal Molecules

摘要

We investigated diffusion of spheroidal molecules near a planar surface, accounting for spatially dependent translational and rotational mobilities of molecules resulting from their hydrodynamic interactions with the plane. Rigid-body Brownian dynamics simulations of prolate ellipsoids of revolution of an axial ratio in the range of 1.5 to 3.0, suspended in a viscous fluid, with a no-slip flat boundary confining the suspension were employed. Mobility tensor matrices of molecules were evaluated as functions of spheroids’ distance and orientation with respect to the plane. Hydrodynamic interactions with the surface lead to substantial changes of spheroids’ translational diffusion coefficients both in the direction perpendicular and parallel to the plane when compared with the values characterizing the bulk diffusion. Moreover, the short-time translational diffusion of molecules, measured in the laboratory frame, both in an unbounded fluid and under the confinement, is non-Gaussian, with much larger deviations from Gaussianity observed in the latter case. In an unbounded fluid, distributionsof translational displacements of molecules deviate from those expectedfor a simple Brownian motion as a result of shape anisotropy. In thepresence of the plane, spheroids experience an additional anisotropicdrag, and consequently, their mobilities depend on their positionsand orientations. Therefore, anomalies in the short-time dynamicsobserved under confinement can be explained in terms of the so-calleddiffusing-diffusivity mechanism. Our findings have implications forunderstanding of a wide range of biological and technological processesthat involve diffusion of anisotropic molecules near surfaces of naturaland model cell membranes, biosensors and nanosensors, and electrodes.