Adsorption on a periodically corrugated substrate

Rejmer, K.; Napiórkowski, Marcin

doi:10.1103/physreve.62.588

Cited by 24 publications

(26 citation statements)

References 38 publications

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“…For triangular surfaces there is only a wetting transition [20], in agreement with the phenomenology reported for simple fluids [26]. While for simple fluids the wetting transition occurs at a lower temperature than the transition at planar surfaces, for nematics the wetting field may exceed the planar surface transition field.…”

Section: Results For Nematic Wetting Of Triangular and Sinusoidalsupporting

confidence: 87%

Nematic wetting and filling of crenellated surfaces

et al. 2012

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We investigate nematic wetting and filling transitions of crenellated surfaces (rectangular gratings) by numerical minimization of the Landau-de Gennes free energy as a function of the anchoring strength, for a wide range of the surface geometrical parameters: depth, width, and separation of the crenels. We have found a rich phase behavior that depends in detail on the combination of the surface parameters. By comparison to simple fluids, which undergo a continuous filling or unbending transition, where the surface changes from a dry to a filled state, followed by a wetting or unbinding transition, where the thickness of the adsorbed fluid becomes macroscopic and the interface unbinds from the surface, nematics at crenellated surfaces reveal an intriguingly rich behavior: in shallow crenels only wetting is observed, while in deep crenels, only filling transitions occur; for intermediate surface geometrical parameters, a new class of filled states is found, characterized by bent isotropic-nematic interfaces, which persist for surfaces structured on large scales, compared to the nematic correlation length. The global phase diagram displays two wet and four filled states, all separated by first-order transitions. For crenels in the intermediate regime re-entrant filling transitions driven by the anchoring strength are observed.

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Section: Results For Nematic Wetting Of Triangular and Sinusoidalsupporting

confidence: 87%

Nematic wetting and filling of crenellated surfaces

et al. 2012

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“…Wetting on rough substrates presents a richer phenomenology than for flat substrates. Interfacial unbinding may occur via a sequence of different phase transitions like unbending (or filling) and unbinding [6,7]. However, a simpler picture arises if we assume wetting as a one-step transition.…”

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confidence: 99%

Wetting transition of a nematic liquid crystal on a periodic wedge-structured substrate

2008

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In this article, we generalize Wenzel law, which assigns an effective contact angle for a droplet on a rough substrate, when the wetting layer has an ordered phase, like a nematic. We estimate the conditions for which the wetting behavior of an ordered fluid can be qualitatively different from the one usually found in a simple fluid. To particularize our general considerations, we will use the Landau-de Gennes mean field approach to investigate theoretically and numerically the complete wetting transition between a nematic liquid crystal and a saw-shaped structured substrate.PACS numbers: 61.30. Hn, 61.30.Dk It is known that the wetting behavior of a fluid is deeply altered in the presence of rough or structured substrates. Leaves for instance have often developed a patterned texture, with micro reliefs, in order to adapt themselves to a particular humid environment [1]. Recently, technological advances allowed the controlled manufacturing of artificial micro structured substrates, which were used to show spectacular results concerning waterrepellency, switchable wettability, and other practical applications [2].In this article, we will investigate the wetting behavior of an ordered fluid (a nematic liquid crystal), on a rough substrate. We will first review some simple considerations about isotropic fluids and rough substrates, and then we will generalize these ideas for the case of ordered fluids. We will particularize our study by considering the complete wetting of a nematic on a saw-shaped substrate. Quantitative results will be obtained by solving analytically and numerically the Landau-de Gennes free energy.Consider two isotropic phases at coexistence (let us call them A and B phases), their bulk free energy densities being the same f A = f B = 0. Suppose now the B-phase is the one preferred in the far field, and our system is in the presence of a flat substrate or wall. The substrate interacts with the fluid through a local surface energy with strength w, which favors the A-phase. In this situation, an A-phase layer may appear close to the wall, because the decrease we have in the surface energy is already sufficient to compensate the creation of an interface between the two phases. Let us define the wettability function g(w) as g(w) = σ BW − σ AW where σ αβ is the surface tension associated to a flat α-β interface. For fixed bulk coexistence conditions, the wettability coefficient will depend on the strength of the surface energy, and usually is an increasing function of w. The Young equation yields g(w) = σ AB cos θ π where θ π is the contact angle of the sessile drop. Thus, as w increases, the contact angle θ π decreases. Eventually, θ π = 0 at the wetting transition, when the A-B interface unbinds from the substrate. In this case g(w = w t π ) = σ AB , where w t π is the transition value. For larger values of w, the interface remains unbounded as a thick A-phase layer is formed between the substrate and the bulk B-phase (complete wetting). The specific effective interactions between the wall and t...

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“…However, this prediction can be changed as interactions between the substrate and the liquid-vapor interface are taken into account. So, if there is a first-order wetting transition for the planar substrate, the wetting transition is still first order but shifted to larger values of θ π (smaller temperatures) [10]. On the other hand, for planar continuous wetting its location and nature are unchanged [9].…”

Section: (I)] Ismentioning

confidence: 96%

“…Thus the elastic energy of the deformations may be large enough to prevent any filling transition, even if it would normally occur in the case of a simple fluid. It should also be pointed out that the interfacial free energies of the filled and wet states are normally very close to each other [10], and a very tiny perturbation can exchange their relative stability.…”

Section: B Thermodynamic Description Of Filling and Wetting Transitimentioning

confidence: 99%

“…If these laws are justified in some situations [7], they may also fail in general theoretical or experimental conditions [8]. In addition to the usual wetting transition, simple fluids at bulk coexistence may present other surface transitions as the filling transition [9,10]. From a thermodynamical point of view, the filling transition in simple fluids always precedes a complete wetting transition.…”

Section: Introductionmentioning

confidence: 99%

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Filling and wetting transitions of nematic liquid crystals on sinusoidal substrates

Patrício¹,

Silvestre²,

Pham³

et al. 2011

Phys. Rev. E

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Close to sinusoidal substrates, simple fluids may undergo a filling transition, in which the fluid passes from a dry to a filled state, where the interface remains unbent but bound to the substrate. Increasing the surface field, the interface unbinds and a wetting transition occurs. We show that this double-transition sequence may be strongly modified in the case of ordered fluids, such as nematic liquid crystals. Depending on the preferred orientation of the nematic molecules at the structured substrate and at the isotropic-nematic interface, the filling transition may not exist, and the fluid passes directly from a dry to a complete-wet state, with the interface far from the substrate. More interestingly, in other situations, the complete wetting transition may be prevented, and the fluid passes from a dry to a filled state, and remains in this configuration, with the interface always attached to the substrate, even for very large surface fields. Both transitions are observed only for a same substrate in a narrow range of amplitudes.

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Adsorption on a periodically corrugated substrate

Cited by 24 publications

References 38 publications

Nematic wetting and filling of crenellated surfaces

Nematic wetting and filling of crenellated surfaces

Wetting transition of a nematic liquid crystal on a periodic wedge-structured substrate

Filling and wetting transitions of nematic liquid crystals on sinusoidal substrates

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