Measurements of surface elastic torques in liquid crystals : a method to measure elastic constants and anchoring energies

Faetti, Sandro; Gatti, M.; Palleschi, V.

doi:10.1051/rphysap:01986002107045100

Cited by 14 publications

(4 citation statements)

References 26 publications

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“…We fit eq to the experimental measurements of P (h) vs d LC using a methodology that minimized the least-squares error (red line in Figure ). When performing the fit, we used K = 10 pN, which is consistent with prior measurements of the bend elastic constant of 5CB. , From arbitrary initial guesses, each of the parameters P ∞ , W S,0 , and W F were fit to the experimental data (using the method of least-squares) one at a time while holding the other two constant until convergence of all three values was obtained. This progressive parameter estimation helped ensure that the fitted parameters remained bounded and unique.…”

Section: Resultsmentioning

confidence: 99%

Surface-Controlled Orientational Transitions in Elastically Strained Films of Liquid Crystal That Are Triggered by Vapors of Toluene

Pantoja

Abbott

2016

ACS Appl. Mater. Interfaces

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We report the fabrication of chemically patterned microwells that enable the rapid and facile preparation (by spin coating and patterned dewetting) of thin films of liquid crystals (LCs) that have precise thicknesses (0.7-30 μm), are supported on chemically defined substrates, and have free upper surfaces. We use these microwells to prepare elastically strained nematic LC films supported on silica glass, gold, or polystyrene substrates and thereby characterize the response of the strained LC films to vapors of toluene. We report that low concentrations of toluene vapor (<500 ppm) can partition into the LC to lower the anchoring energy of the LC on these substrates, thus allowing the elastic energy of the strained LC film to drive the LC films through an orientational transition. The central role of the toluene-induced change in surface anchoring energy is supported by additional experiments in which the response of the nematic LC to changes in film thickness and substrate identity are quantified. A simple thermodynamic model captures these trends and yielded estimates of anchoring energies (8-22 μJ/m(2)). Significantly, the orientational transitions observed in these strained LC thin films occur at concentrations of toluene vapor that are almost 1 order of magnitude below those which lead to bulk phase transitions, and they are not triggered by exposure to water vapor. Overall, these results hint at principles for the design of responsive LC-based materials that can be triggered by concentrations of aromatic, volatile organic compounds that are relevant to human health.

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Section: Resultsmentioning

confidence: 99%

Surface-Controlled Orientational Transitions in Elastically Strained Films of Liquid Crystal That Are Triggered by Vapors of Toluene

Pantoja

Abbott

2016

ACS Appl. Mater. Interfaces

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“…Several methods exist for measuring properties of a nematic liquid crystal using optical techniques − and force sensing techniques . The surface forces apparatus (SFA) is a technique that accurately uses both techniques to measure thin films in general and liquid crystal in particular. , Recently the SFA has been used to study surface forces and concurrently the thin film patterns induced by an external electric field .…”

Section: Introductionmentioning

confidence: 99%

“…Several methods exist for measuring properties of a nematic liquid crystal using optical techniques 10−12 and force sensing techniques. 13 The surface forces apparatus (SFA) is a technique that accurately uses both techniques to measure thin films in general 14 and liquid crystal in particular. 7,15 Recently the SFA has been used to study surface forces and concurrently the thin film patterns induced by an external electric field.…”

Section: ■ Introductionmentioning

confidence: 99%

Simultaneous Measurements of Molecular Forces and Electro-Optical Properties of a Confined 5CB Liquid Crystal Film Using a Surface Forces Apparatus

Kristiansen¹,

Zeng

Zappone³

et al. 2015

Langmuir

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Using a surface forces apparatus (SFA), we studied the forces associated with the reorientation of molecules of a common nematic thermotropic liquid crystal, 4′-n-pentyl-4-cyanobiphenyl (5CB), confined between two conducting (silver) surfaces and its optical behavior under the influence of electric fields with varying magnitudes and field directions. A transient attractive force was observed due to partial reorientations of the liquid crystal molecules and the flow of free ions, in addition to a stronger constant capacitance attraction between the silver surfaces. At the same time, the optical properties of the liquid crystals were observed perpendicular to the silver surfaces. Observations of shifts and fluctuations of the extraordinary wave of the (multiple beam) interference fringes measure the refractive index of the director component parallel to the surface, which is sensitive to tilt motion (or reorientation) of the liquid crystal molecules that provided details of the anisotropic orientations of the molecules and domains. Any lateral differential refractive index change is easily observed by optical microscopy. The optical microscope imaging showed that the changes in the optical properties are due to convective flow at domain boundaries of the liquid crystal molecules (and possible free ions) between the two charged surfaces. At low electric fields, propagation of domain boundaries was observed, while at higher electric fields, hexagonal patterns of flowing molecules were observed. The interplay of the force measurements and optical observations reveal a complex dynamic behavior of liquid crystals subjected to varying electric fields in confined spaces.

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“…Thus we consider only isolated aggregates. In the limit of strong anchoring (anchoring energy ¿ H p K 22 Dx ϳ 10 26 J m 22 ), the elastic torque exerted on each aggregate by the solvent has a particularly simple form: t 2NaH p K 22 Dx sin͑f 0 2 f͒, where a is the area per molecule in the crystalline film [13]. If the condition for strong anchoring is not satisfied, the director rotates at the interface, somewhat reducing the torque exerted on the aggregate.…”

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confidence: 99%

Formation of Uniaxial Molecular Films by Liquid-Crystal Imprinting in a Magnetic Field

et al. 2000

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Scanning tunneling microscopy was used to study molecular order in monolayer organic films formed by solution-phase growth from thermotropic liquid crystal solvents. The films develop macroscopically uniaxial alignment, with adlayer orientation controlled by an external magnetic field through interactions mediated by the liquid crystal. Results are presented for two films deposited from nematic and smectic- A solvents, along with a discussion of the alignment mechanism.

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Measurements of surface elastic torques in liquid crystals : a method to measure elastic constants and anchoring energies

Cited by 14 publications

References 26 publications

Surface-Controlled Orientational Transitions in Elastically Strained Films of Liquid Crystal That Are Triggered by Vapors of Toluene

Surface-Controlled Orientational Transitions in Elastically Strained Films of Liquid Crystal That Are Triggered by Vapors of Toluene

Simultaneous Measurements of Molecular Forces and Electro-Optical Properties of a Confined 5CB Liquid Crystal Film Using a Surface Forces Apparatus

Formation of Uniaxial Molecular Films by Liquid-Crystal Imprinting in a Magnetic Field

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