title: Periodic dynamics, localization metastability, and elastic interaction of colloidal particles with confining surfaces and helicoidal structure of cholesteric liquid crystals.
abstract: Nematic and cholesteric liquid crystals are three-dimensional fluids that possess long-range orientational ordering and can support both topological defects and chiral superstructures. Implications of this ordering remain unexplored even for simple dynamic processes such as the ones found in so-called "fall experiments," or motion of a spherical inclusion under the effects of gravity. Here we show that elastic and surface anchoring interactions prompt periodic dynamics of colloidal microparticles in confined cholesterics when gravity acts along the helical axis. We explore elastic interactions between colloidal microparticles and confining surfaces as well as with an aligned ground-state helical structure of cholesterics for different sizes of spheres relative to the cholesteric pitch, demonstrating unexpected departures from Stokes-like behavior at very low Reynolds numbers. We characterize metastable localization of microspheres under the effects of elastic and surface anchoring periodic potential landscapes seen by moving spheres, demonstrating the important roles played by anchoring memory, confinement, and topological defect transformation. These experimental findings are consistent with the results of numerical modeling performed through minimizing the total free energy due to colloidal inclusions at different locations along the helical axis and with respect to the confining substrates. A potential application emerging from this work is colloidal sorting based on particle shapes and sizes.
authors.affiliation: Department of Physics, University of Colorado at Boulder, Boulder, Colorado 80309, USA.
authors.forename: Michael C M
authors.initials: MC
authors.lastname: Varney
authors.affiliation: Department of Physics, University of Colorado at Boulder, Boulder, Colorado 80309, USA and Materials Science and Engineering Program, University of Colorado at Boulder, Boulder, Colorado 80309, USA.
authors.forename: Qiaoxuan
authors.initials: Q
authors.lastname: Zhang
authors.affiliation: Max-Planck-Institut für Intelligente Systeme, D-70569 Stuttgart, Germany and Institut für Theoretische Physik IV, Universität Stuttgart, D-70569 Stuttgart, Germany.
authors.forename: Mykola
authors.initials: M
authors.lastname: Tasinkevych
authors.affiliation: Centro de Física Teórica e Computacional and Faculdade de Ciências da Universidade de Lisboa, Av. Prof. Gama Pinto 2, 1649-003 Lisboa, Portugal.
authors.forename: Nuno M
authors.initials: NM
authors.lastname: Silvestre
authors.affiliation: National Institute of Standards and Technology, Boulder, Colorado 80305, USA.
authors.forename: Kris A
authors.initials: KA
authors.lastname: Bertness
authors.affiliation: Department of Physics, University of Colorado at Boulder, Boulder, Colorado 80309, USA and Materials Science and Engineering Program, University of Colorado at Boulder, Boulder, Colorado 80309, USA and Department of Electrical, Computer, and Energy Engineering and Liquid Crystals Materials Research Center, University of Colorado at Boulder, Boulder, Colorado 80309, USA and Renewable and Sustainable Energy Institute, National Renewable Energy Laboratory and University of Colorado at Boulder, Boulder, Colorado 80309, USA.
authors.forename: Ivan I
authors.initials: II
authors.lastname: Smalyukh