Direct measurements of structural forces and twist transitions in cholesteric liquid crystal films with a surface force apparatus

Weichao, Zheng,; Perez-Martinez, Carla; Petriashvili, Gia; Perkin, Susan; Zappone, Bruno

doi:10.1039/c9sm00487d

Soft Matter

2019

DOI: 10.1039/c9sm00487d

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Direct measurements of structural forces and twist transitions in cholesteric liquid crystal films with a surface force apparatus

Zheng, Weichao

Carla Perez-Martinez

Gia Petriashvili

et al.

Abstract: Mechanical micro-confinement of a cholesteric liquid crystal between crossed cylinders produces dislocations, twist transitions and oscillatory structural forces.

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Cited by 4 publications

References 77 publications

(100 reference statements)

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Topological barriers to defect nucleation generate large mechanical forces in an ordered fluid

Zappone

Bartolino

2021

Proc. Natl. Acad. Sci. U.S.A.

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Common fluids cannot sustain static mechanical stresses at the macroscopic scale because they lack molecular order. Conversely, crystalline solids exhibit long-range order and mechanical strength at the macroscopic scale. Combining the properties of fluids and solids, liquid crystal films respond to mechanical confinement by both flowing and generating static forces. The elastic response, however, is very weak for film thicknesses exceeding 10 nm. In this study, the mechanical strength of a fluid film was enhanced by introducing topological defects in a cholesteric liquid crystal, producing unique viscoelastic and optomechanical properties. The cholesteric was confined under strong planar anchoring conditions between two curved surfaces with sphere–sphere contact geometry similar to that of large colloidal particles, creating concentric dislocation loops. During surface retraction, the loops shrank and periodically disappeared at the surface contact point, where the cholesteric helix underwent discontinuous twist transitions, producing weak oscillatory surface forces. On the other hand, new loop nucleation was frustrated by a topological barrier during fluid compression, creating a metastable state. This generated exceptionally large forces with a range exceeding 100 nm as well as extended blueshifts of the photonic bandgap. The metastable cholesteric helix eventually collapsed under a high compressive load, triggering a stick-slip–like cascade of defect nucleation and twist reconstruction events. These findings were explained using a simple theoretical model and suggest a general approach to enhance the mechanical strength of one-dimensional periodic materials, particularly cholesteric colloid mixtures.

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Topological barriers to defect nucleation generate large mechanical forces in an ordered fluid

Zappone

Bartolino

2021

Proc. Natl. Acad. Sci. U.S.A.

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Equilibrium surface force measurements in thin liquid crystal films

Zappone,

Ruths

2024

Liquid Crystals

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The surface force balance: direct measurement of interactions in fluids and soft matter

Hayler,

Groves,

Guerrini

et al. 2024

Rep. Prog. Phys.

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Over the last half-century, direct measurements of surface forces have been instrumental in the exploration of a multitude of phenomena in liquid, soft, and biological matter. Measurements of van der Waals interactions, electrostatic interactions, hydrophobic interactions, structural forces, depletion forces, and many other effects have checked and challenged theoretical predictions and motivated new models and understanding. The gold-standard instrument for these measurements is the \textit{surface force balance}, or \textit{surface forces apparatus}, where interferometry is used to detect the interaction force and distance between two atomically smooth planes, with 0.1~nm resolution, over separations from about 1~\unit{\um} down to contact. The measured interaction force \textit{vs.} distance gives access to the free energy of interaction across the fluid film; a fundamental quantity whose general form and subtle features reveal the underlying molecular and surface interactions and their variation. Motivated by new challenges in emerging fields of research, such as energy storage, biomaterials, non-equilibrium and driven systems, innovations to the apparatus are now clearing the way for new discoveries. It is now possible to measure interaction forces (and free energies) with control of electric field, surface potential, surface chemistry; to measure time-dependent effects; and to determine structure \textit{in situ}. Here, we provide an overview the operating principles and capabilities of the surface force balance with particular focus on the recent developments and future possibilities of this remarkable technique.

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Direct measurements of structural forces and twist transitions in cholesteric liquid crystal films with a surface force apparatus

Abstract: Mechanical micro-confinement of a cholesteric liquid crystal between crossed cylinders produces dislocations, twist transitions and oscillatory structural forces.

Cited by 4 publications

References 77 publications

Topological barriers to defect nucleation generate large mechanical forces in an ordered fluid

Topological barriers to defect nucleation generate large mechanical forces in an ordered fluid

Equilibrium surface force measurements in thin liquid crystal films

The surface force balance: direct measurement of interactions in fluids and soft matter

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