Capillary Condensation of a Nematic Liquid Crystal Observed by Force Spectroscopy

Kočevar, K.; Borštnik, A.; Muševič, Igor; Žumer, S.

doi:10.1103/physrevlett.86.5914

Cited by 70 publications

(39 citation statements)

References 21 publications

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“…This effect, long ago observed by Miyano [6], was also recently detected by a surface force apparatus measuring the related capillary condensation [7]. Although nematics are uniaxial in the bulk, the wetting layer may exhibit biaxiality due to the lower symmetry near the surface [8].…”

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confidence: 87%

Modeling planar degenerate wetting and anchoring in nematic liquid crystals

2005

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Abstract. -We propose a simple surface potential describing the planar degenerate anchoring of nematic liquid crystals, i.e., the tendency of the molecules to align parallel to one another along any direction parallel to the surface. We show that, at lowest order in the tensorial Landau-de Gennes order-parameter, fourth-order terms must be included. We analyze the anchoring and wetting properties of this surface potential. In the nematic phase, we find the desired degenerate planar anchoring, with positive scalar order-parameter and some surface biaxiality. In the isotropic phase, we find, in agreement with experiments, that the wetting layer may exhibit a uniaxial ordering with negative scalar order-parameter. For large enough anchoring strength, this negative ordering transits towards the planar degenerate state.Introduction. -Nematic liquid crystals are non-polar liquids consisting of elongated molecules aligned, on the average, parallel to a common direction ±n [1]. The unit vector n is called the director. Exceptionally, in the case of "negative order," the molecules may be distributed perpendicularly to n. The nematic order is quantified by a symmetric traceless tensor Q, of Cartesian components Q ij (i, j = 1, 2, 3). Microscopically, calling m a unit vector parallel to the long axis of the molecules, the tensor order-parameter can be defined as Q ij = m i m j −

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confidence: 87%

Modeling planar degenerate wetting and anchoring in nematic liquid crystals

2005

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“…The capillary pressure inside the liquid bridge is fixed by the relative humidity of the vapor phase, and the volume of the liquid bridge depends on the size of the gap. Capillary condensation also occurs in liquid crystals [34,35], aqueous mixtures of polymers [36,37], and model fluid mixtures [38] when confined in small gaps.…”

Section: Introductionmentioning

confidence: 99%

Capillary adhesion at the nanometer scale

Cheng

Robbins

2014

Phys. Rev. E

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Molecular dynamics simulations are used to study the capillary adhesion from a nonvolatile liquid meniscus between a spherical tip and a flat substrate. The atomic structure of the tip, the tip radius, the contact angles of the liquid on the two surfaces, and the volume of the liquid bridge are varied. The capillary force between the tip and substrate is calculated as a function of their separation h. The force agrees with continuum predictions based on macroscopic theory for h down to ∼5 to 10 nm. At smaller h, the force tends to be less attractive than predicted and has strong oscillations. This oscillatory component of the capillary force is completely missed in the macroscopic theory, which only includes contributions from the surface tension around the circumference of the meniscus and the pressure difference over the cross section of the meniscus. The oscillation is found to be due to molecular layering of the liquid confined in the narrow gap between the tip and substrate. This effect is most pronounced for large tip radii and/or smooth surfaces. The other two components considered by the macroscopic theory are also identified. The surface tension term, as well as the meniscus shape, is accurately described by the macroscopic theory for h down to ∼1 nm, but the capillary pressure term is always more positive than the corresponding continuum result. This shift in the capillary pressure reduces the average adhesion by a factor as large as 2 from its continuum value and is found to be due to an anisotropy in the pressure tensor. The component in the plane of the substrate is consistent with the capillary pressure predicted by the macroscopic theory (i.e., the Young-Laplace equation), but the normal pressure that determines the capillary force is always more positive than the continuum counterpart.

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“…nematization transitions [34] and, more recently, even to obtain the pitch in cholesteric materials with high accuracy [35]. The control parameter is h/p, where h is the pore width and p the cholesteric pitch; this parameter dictates the number of cholesteric periods or helix index in the cell, and its variation will induce transitions between cholesteric slabs of different numbers of half-turns of the director (winding transitions).…”

Section: Introductionmentioning

confidence: 99%

Capillary and winding transitions in a confined cholesteric liquid crystal

2015

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We consider a Lebwohl-Lasher model of chiral particles confined in a planar cell (slit pore) with different boundary conditions, and solve it using mean-field theory. The phase behaviour of the system with respect to temperature and pore width is studied. Two phenomena are observed: (i) an isotropic-cholesteric transition which exhibits an oscillatory structure with respect to pore width, and (ii) an infinite set of winding transitions caused by commensuration effects between cholesteric pitch and pore width. The latter transitions have been predicted and analysed by other authors for cholesterics confined in a fixed pore and subject to an external field promoting the uniaxial nematic phase; here we induce winding transitions solely from geometry by changing the pore width at zero external field (a setup recently explored in Atomic-Force Microscopy experiments). In contrast with previous studies, we obtain the phase diagram in the temperature vs pore width plane, including the isotropic-cholesteric transition, the winding transitions and their complex relationship. In particular, the structure of winding transitions terminates at the capillary isotropic-cholesteric transition via triple points where the confined isotropic phase coexists with two cholesterics with different helix indices. For symmetric and asymmetric monostable plate anchorings the phase diagram are qualitatively similar.

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Capillary Condensation of a Nematic Liquid Crystal Observed by Force Spectroscopy

Cited by 70 publications

References 21 publications

Modeling planar degenerate wetting and anchoring in nematic liquid crystals

Modeling planar degenerate wetting and anchoring in nematic liquid crystals

Capillary adhesion at the nanometer scale

Capillary and winding transitions in a confined cholesteric liquid crystal

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