Microscopic wetting phenomena

Hautman, Joseph; Klein, Michael L.

doi:10.1103/physrevlett.67.1763

Cited by 201 publications

(166 citation statements)

References 29 publications

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“…To estimate the deviations from linear additivity, we study wetting free energies, quantified in terms of microscopic analogues of water contact angle (26). We determine nanodroplet contact angles from averaged droplet contours following the approach of refs.…”

Section: Models and Observationsmentioning

confidence: 99%

“…The overall surface energetics will only approximately follow the sum of all component contributions (19,38). As with molecularly mixed surfaces, we examine the (non)additivity of wetting free energy on heterogeneous protein surfaces by calculating microscopic analogues of water contact angle (26,33) as function of surface composition. The repeating units of the surfaces are two different surface patches of melittin dimer, a well-characterized protein (39-41) with regions of contrasting polarities.…”

Section: Models and Observationsmentioning

confidence: 99%

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Probing surface tension additivity on chemically heterogeneous surfaces by a molecular approach

Wang

Bratko

Luzar

2011

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

105

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Surface free energy of a chemically heterogeneous surface is often treated as an approximately additive quantity through the Cassie equation [Cassie ABD (1948) Discuss Faraday Soc 3:11-16]. However, deviations from additivity are common, and molecular interpretations are still lacking. We use molecular simulations to measure the microscopic analogue of contact angle, θ c , of aqueous nanodrops on heterogeneous synthetic and natural surfaces as a function of surface composition. The synthetic surfaces are layers of graphene functionalized with prototypical nonpolar and polar head group: methyl, amino, and nitrile. We demonstrate positive as well as negative deviations from the linear additivity. We show the deviations reflect the uneven exposure of mixture components to the solvent and the linear relation is recovered if fractions of solvent-accessible surface are used as the measure of composition. As the spatial variations in polarity become of larger amplitude, the linear relation can no longer be obtained. Protein surfaces represent such natural patterned surfaces, also characterized by larger patches and roughness. Our calculations reveal strong deviations from linear additivity on a prototypical surface comprising surface fragments of melittin dimer. The deviations reflect the disproportionately strong influence of isolated polar patches, preferential wetting, and changes in the position of the liquid interface above hydrophobic patches. Because solvent-induced contribution to the free energy of surface association grows as cos θ c , deviations of cos θ c from the linear relation directly reflect nonadditive adhesive energies of biosurfaces. wetting free energy | surface functionalization | nanopatterning | Cassie relation | biointeractions W etting phenomena on chemically heterogeneous surfaces are important in material sciences and biology, in examples ranging from inkjet printing to protein hydration (1, 2). Conventional metrics of surface interactions, designed for homogeneous systems, can often be applied to mixed surfaces characterized by averaged properties of multiple ingredients. The design of composite surface materials and characterization of biosurfaces benefit from combining rules predicting the interfacial free-energy change of wetting, Δγ, from the knowledge about individual constituents and surface composition. In view of the Young equation, Δγ ¼ −γ cos θ c , the strength of intersolute adhesion, W a ∼ −2ðΔγ þ γ sg Þ, relates to contact angle θ c ; here, γ and γ sg denote surface tensions of the solvent and dry solute, respectively (3). Contact angles on macroscopic heterogeneous surfaces are commonly estimated by the Cassie equation (4, 5),[1]developed by assuming linear additivity of the wetting free energies, Δγ. Here, f α is the projected fractional area occupied by component α, θ α is the contact angle on a homogeneous surface of type α, and r α is the Wenzel roughness factor (6), which can be defined as the ratio of solvent-exposed areas of patch of type α in the mixture and that of a ...

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Section: Models and Observationsmentioning

confidence: 99%

Section: Models and Observationsmentioning

confidence: 99%

Probing surface tension additivity on chemically heterogeneous surfaces by a molecular approach

Wang

Bratko

Luzar

2011

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

105

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“…29) partially wetting the substrate. The contact angle, which can be defined following reference [165], is roughly independent of the cluster size (characterized by its number of atoms n, for 50 < n < 500. This angle can be tuned by changing the cluster-substrate interaction.…”

Section: D Island Diffusion Mechanismsmentioning

confidence: 99%

Growth of nanostructures by cluster deposition: Experiments and simple models

1999

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This paper presents a comprehensive analysis of simple models useful to analyze the growth of nanostructures obtained by cluster deposition. After detailing the potential interest of nanostructures, I extensively study the first stages of growth (the submonolayer regime) by kinetic MonteCarlo simulations. These simulations are performed in a wide variety of experimental situations : complete condensation, growth with reevaporation, nucleation on defects, total or null clustercluster coalescence . . . . The main scope of the paper is to help experimentalists analyzing their data to deduce which of those processes are important and to quantify them. A software including all these simulation programs is available at no cost on request to the author. I carefully discuss experiments of growth from cluster beams and show how the mobility of the clusters on the surface can be measured : surprisingly high values are found. An important issue for future technological applications of cluster deposition is the relation between the size of the incident clusters and the size of the islands obtained on the substrate. An approximate formula which gives the ratio of the two sizes as a function of the melting temperature of the material deposited is given. Finally, I study the atomic mechanisms which can explain the diffusion of the clusters on a substrate and the result of their mutual interaction (simple juxtaposition, partial or total coalescence . . . ).

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“…Such an approach, although appealing from the simplicity of the method, provides inconsistent results, particularly for small droplets [11]. Understandingly, different methods of increasing complexity have been devised for this purpose [15][16][17]. Figure 3 illustrates the difficulties associated with defining the contact angle using two-dimensional slices of molecular snapshots, especially for small droplets.…”

Section: Introductionmentioning

confidence: 99%

On the Calculation of Solid-Fluid Contact Angles from Molecular Dynamics

Santiso

Herdes

Müller

2013

Entropy

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Abstract:A methodology for the determination of the solid-fluid contact angle, to be employed within molecular dynamics (MD) simulations, is developed and systematically applied. The calculation of the contact angle of a fluid drop on a given surface, averaged over an equilibrated MD trajectory, is divided in three main steps: (i) the determination of the fluid molecules that constitute the interface, (ii) the treatment of the interfacial molecules as a point cloud data set to define a geometric surface, using surface meshing techniques to compute the surface normals from the mesh, (iii) the collection and averaging of the interface normals collected from the post-processing of the MD trajectory. The average vector thus found is used to calculate the Cassie contact angle (i.e., the arccosine of the averaged normal z-component). As an example we explore the effect of the size of a drop of water on the observed solid-fluid contact angle. A single coarsegrained bead representing two water molecules and parameterized using the SAFT-γ Mie equation of state (EoS) is employed, meanwhile the solid surfaces are mimicked using integrated potentials. The contact angle is seen to be a strong function of the system size for small nano-droplets. The thermodynamic limit, corresponding to the infinite size (macroscopic) drop is only truly recovered when using an excess of half a million water coarse-grained beads and/or a drop radius of over 26 nm.

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Microscopic wetting phenomena

Cited by 201 publications

References 29 publications

Probing surface tension additivity on chemically heterogeneous surfaces by a molecular approach

Probing surface tension additivity on chemically heterogeneous surfaces by a molecular approach

Growth of nanostructures by cluster deposition: Experiments and simple models

On the Calculation of Solid-Fluid Contact Angles from Molecular Dynamics

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