An Insight into the Viscoelasticity of Self-Assembling Smectic Liquid Crystals of Colloidal Rods from Active Microrheology Simulations
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Liquid CrystalsColloidsDynamic Monte Carlo simulationsMicrorheologyBrownian motion
Journal of Chemical Theory and Computation
SponsorshipF.A.G.D. was funded by the NextGenerationEU program of the European Union, the Plan de Recuperación, Transformación y Resiliencia, and the Ministerio de Universidades, as part of the “Maria Zambrano” grants for the requalification of the Spanish university system 2021-2023 called by the Pablo de Olavide University.; F.A.G.D., A.M.P and A.P. acknowledge the International Exchanges Grant IES\R1\191066, awarded by The Royal Society; A.P. is supported by a “Maria Zambrano Senior” researcher fellowship, financed by the European Union within the NextGenerationEU program and the Spanish Ministry of Universities.; A.M.P. acknowledges financial support from project PID2021-127836NB-I00 (funded by MCIN/AEI/10.13039/501100011033/ FEDER “A way to make Europe”).; A. C. acknowledges support from Consejería de Transformación Económica, Industria, Conocimiento y Universidades de la Junta de Andalucía/FEDER (project grant P20-00816), and from the Spanish Ministerio de Ciencia, Innovación y Universidades, and FEDER (project grant PPID2021-126121NB-I00).; The authors acknowledge the C3UPO of the Pablo de Olavide University for the support with HPC facilities and the use of the Computational Shared Facility at The University of Manchester
The rheology of colloidal suspensions is of utmost importance in an ample variety of interdisciplinary applications in formulation technology, determining equally interesting questions in fundamental science. This is especially intriguing when colloids exhibit a degree of long-range positional or orientational ordering, as in liquid crystals (LCs) of elongated particles. Along with standard methods, microrheology (MR) has emerged in recent years as a tool to assess the mechanical properties of materials at the microscopic level. In particular, by active MR one can infer the viscoelastic response of a soft material from the dynamics of a tracer particle being dragged through it by external forces. Although considerable efforts have been made to study the diffusion of guest particles in LCs, little is known on the combined effect of tracer size and directionality of the dragging force on the system’s viscoelastic response. By dynamic Monte Carlo simulations, we apply active MR to investigate the viscoelasticity of self-assembling smectic (Sm) LCs consisting of rod-like particles. In particular, we track the motion of a spherical tracer whose size is varied within a range of values matching the system’s characteristic length scales and being dragged by constant forces that are parallel, perpendicular or at 45◦ to the nematic director. Our results reveal a uniform value of the effective friction coefficient as probed by the tracer at small and large forces, whereas a nonlinear, force-thinning regime is observed at intermediate forces. However, at relatively weak forces the effective friction is strongly determined by correlations between the tracer size and the structure of the host fluid. Moreover, we also show that external forces forming an angle with the nematic director provide additional details that cannot be simply inferred from the mere analysis of parallel and perpendicular forces. Our results highlight the fundamental interplay between tracer size and force direction in assessing the MR of Sm LC fluids.