title: Crowding-induced anisotropic transport modulates reaction kinetics in nanoscale porous media.
abstract: We quantify the emergence of persistent anisotropy in the diffusion of spherical tracer particles through a nanoscale porous medium composed of a uniform distribution of purely symmetric crowding particles. We focus on the interior of a biological cell as an example of such a medium and find that diffusion is highly directional for distances comparable to the size of some organelles. We use a geometrical procedure that avoids the standard orientational averaging to quantify the anisotropy of diffusive paths and show that the point source distributions are predominantly of prolate ellipsoidal shape as a result of local volume exclusion. This geometrical symmetry breaking strongly skews the distribution of kinetic rates of diffusion-limited reactions toward small values, leading to the result that, for short to intermediate times, almost 80% of the rates measured in an ensemble of heterogeneous media are smaller than the expected rate in an ideal homogeneous medium of similar excluded volume fraction. This crowding-induced modulation may have implications for our understanding and measurement of diffusion-controlled intracellular reaction kinetics and for experimental nanotechnology applications, such as nanoparticle-based bioimaging and drug delivery, where diffusion plays an important role.
authors.affiliation: Institute for Mathematical Sciences, Imperial College London, United Kingdom. Ramon.Grima@ed.ac.uk
authors.forename: R
authors.initials: R
authors.lastname: Grima
authors.forename: S N
authors.initials: SN
authors.lastname: Yaliraki
authors.forename: M
authors.initials: M
authors.lastname: Barahona
mesh_terms: D016880:Anisotropy; D004058:Diffusion; D007700:Kinetics; D049329:Nanostructures / Q000737:chemistry*; D016062:Porosity; D012399:Rotation