Physical nature of particle transport in elastically responsive hydrogels: one and two particle microrheology

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Designs for modern clinical diagnostic tools utilise the diffusion ofpathogens such as viruses into hydrogels, in which pathogens get in contactwith functionalised units and give rise to detectable optical or mechanicalsignals. Nature effectively protects various tissues from bacterial andviral invasion using gel-like mucus films, which are almost impenetrableto specific pathogens. This project combines extensive simulations andanalytical arguments to answer the physical question on how thermallydriven tracer particles, which are larger than the typical network mesh,diffuse in elastic networks. In particular, we address the connectionbetween the size and shape of the tracer particle and its mobility in thegel network and follow up on questions such as: What is the maximal sizeof the particle such that it can still penetrate into and diffuse throughthe gel? Which role does the tracer shape play? What are the correlationsbetween the motion of the tracer and the surrounding gel network? Whicheffect does spatial disorder of the gel have on the tracer mobility? And finally,which role is played by the degree of deformability of the tracer, thatis, can we detect significant differences between motion of hard-coreversus soft-core tracer particles? Similar questions will be asked for thecorrelated motion of two vicinal tracers in the gel, a setup typically usedin two-particle microrheology. The diffusive characteristics of the tracerwill be quantified in terms of the ensemble and time averaged particledisplacements, the non-Gaussianity and non-ergodicity parameters, as wellas the correlation and probability distribution functions of the diffusingparticles. The results of this project will provide an important input forthe analysis of microrheology experiments and allow novel insights intothe physico-chemical properties of particle diffusion in heterogeneouslystructured and elastically responsive environments, such as those insideliving biological cells, in multicellular organisations, in mucus films,or gels used for clinical diagnostic tools.