Theory of microrheology in complex fluids

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GRUBER, Markus, 2019. Theory of microrheology in complex fluids [Dissertation]. Konstanz: University of Konstanz

@phdthesis{Gruber2019Theor-46666, title={Theory of microrheology in complex fluids}, year={2019}, author={Gruber, Markus}, address={Konstanz}, school={Universität Konstanz} }

eng Gruber, Markus Theory of microrheology in complex fluids Attribution-NonCommercial-ShareAlike 4.0 International 2019 2019-08-09T09:35:57Z 2019-08-09T09:35:57Z The investigation of local viscoelastic properties of biological samples is of interest for both fundamental research on mechanical rigidity of cells [SSV+10] as well as the understanding of cancer [SVF+16; BNA09]. One method for these investigations is microrheology, where one puts the rheometer into the sample rather than the sample into the rheometer as in classical rheology. The measurement device for microrheology is a set of microscopic beads, which are immersed in the sample. The measurement itself consists of the observation of these probe particles under thermal motion only (passive microrheology) or under additional external fields like magnets or optical tweezers (active microrheology). There are, however, still open questions on how to relate the motion of the probe to the viscoelastic properties in complex liquids [Zia18], which consist of a simple liquid, the solvent, with suspended particles or polymers.<br /><br />In this thesis a theoretical description of active microrheology in two model systems for complex fluids is studied: a colloidal glass and a dilute suspension of active, self-propelled particles. For the colloidal glass, mode-coupling theory (MCT) is employed to describe constant- force microrheology [GAPF16] and the resulting equations of motion are solved numerically. This allows the investigation of the delocalization transition, which separates the regime of an elastic response of the probe from the regime with a viscous response above a critical force. The critical behavior is studied analytically and numerically, supported by the analysis of schematic models and simulations. A comparison with an experiment reveals dynamic heterogeneities in the motion of the probe, which can be connected to a bimodal structure of the van Hove function. For the dilute suspension of active particles, an expansion of the pair correlation function is used to describe constant velocity active microrheology with a spheroidal probe. In this case, an increase of the friction coefficient of the probe is found for oblate spheroids at large activities, while for prolate spheroids the friction coefficient is reduced beyond the value in a pure solvent. Gruber, Markus

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