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Insight Into the Superlubricity and Self-Assembly of Liquid Crystals.

ABSTRACT: Liquid crystals are promising molecular materials in the application of lubrication. Herein, the microscale solid superlubricity is accomplished by the construction of uniform and ordered self-assembly of several liquid crystals. The self-assembly structures on a highly oriented pyrolytic graphite (HOPG) surface are explicitly revealed by using scanning tunneling microscopy (STM). Meanwhile, the nanotribological performance of the self-assemblies are measured by using atomic force microscopy (AFM), revealing ultralow friction coefficients lower than 0.01. The interaction energies are calculated by density functional theory (DFT) method, indicating the positive correlation between friction coefficients and interaction strength. The effort on the self-assembly and superlubricity of liquid crystals could enhance the understanding of the nanotribological mechanism and benefit the further application of liquid crystals as lubricants.

SUBMITTER: Tan S

PROVIDER: S-EPMC8226320 | biostudies-literature | 2021

REPOSITORIES: biostudies-literature

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Insight Into the Superlubricity and Self-Assembly of Liquid Crystals.

Tan Shanchao S Tao Jiayu J Luo Wendi W Shi Hongyu H Tu Bin B Jiang Hao H Liu Yuhong Y Xu Haijun H Zeng Qingdao Q

Frontiers in chemistry 20210611

Liquid crystals are promising molecular materials in the application of lubrication. Herein, the microscale solid superlubricity is accomplished by the construction of uniform and ordered self-assembly of several liquid crystals. The self-assembly structures on a highly oriented pyrolytic graphite (HOPG) surface are explicitly revealed by using scanning tunneling microscopy (STM). Meanwhile, the nanotribological performance of the self-assemblies are measured by using atomic force microscopy (AF ...[more]

PMID: 34178941

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Project description:The rheology of colloidal suspensions is of utmost importance in a wide 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 about 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 rodlike 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.

Project description:We report an investigation of the self-assembly of the monosialoganglioside (GM(1)) at interfaces formed between aqueous solutions of 10 microM GM(1) (at 25 degrees C) and micrometer-thick films of the nematic liquid crystal (LC) 4'-pentyl-4-cyanobiphenyl (5CB). We observe the process of spontaneous transfer of GM(1) onto the interfaces to be accompanied by continuous ordering transitions within the micrometer-thick films of the LC. At saturation coverage, the GM(1) orders the LC in an orientation that is perpendicular to the interface, an orientation that is similar to that caused by phospholipids such as dilauroylphosphatidylcholine (DLPC). This result suggests an interaction between the LC and GM(1) that is dominated by the hydrophobic tails of the GM(1). Relative to DLPC, however, we observe the dynamics of the LC ordering transition driven by GM(1) to be slow (2 h for DLPC versus 100 h for GM(1)). To provide insight into the origins of the slow dynamics of the GM(1)-induced ordering transition in the LC, we performed two additional measurements. First, we quantified the time-dependent adsorption of GM(1) at the LC interface by using fluorescently-labeled GM(1). Second, we used the Langmuir-Schaefer method to transfer preorganized monolayers of GM(1) from an air-water interface to the aqueous-LC interface. Results obtained from these two experiments are consistent with a physical picture in which the final stages of spontaneous adsorption/ordering of GM(1) at the aqueous-LC interface dictate the dynamics of the LC ordering transition. This rate limiting process underlying the ordering transition was substantially accelerated by heating the system above the phase transition temperature of GM(1)(26 degrees C), suggesting that the phase state of the GM(1) micellar aggregates in bulk solution strongly influences the kinetics of the final stages of ordering/adsorption of GM(1) at the LC interface. Overall, these results and others presented in this manuscript reveal that it is possible to decorate interfaces of a nematic LC with GM(1), and that the assembly of GM(1) at these interfaces impacts the dynamic and equilibrium ordering of the LC.

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Insight Into the Superlubricity and Self-Assembly of Liquid Crystals.

Publications

Insight Into the Superlubricity and Self-Assembly of Liquid Crystals.

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