Molecular Dynamics Study of Wetting of a Pillar Surface

Lundgren, Mathias; Allan, Neil L.; Cosgrove, Terence; George, Neil

doi:10.1021/la034224h

Cited by 89 publications

(78 citation statements)

References 11 publications

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“…To measure wettability of the model hydrophobic surfaces, we first computed the intrinsic contact angle of a water nanodroplet on the f lat surfaces (11,32,45). To this end, we used the following computational approach to determine the surface locus of a water nanodroplet.…”

Section: Resultsmentioning

confidence: 99%

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Coexistence and transition between Cassie and Wenzel state on pillared hydrophobic surface

Kondo

Yasuoka

Fujikawa³

et al. 2009

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

426

335

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Here, we show large-scale molecular dynamics simulation of transition between Wenzel state and Cassie state of water droplets on a periodic nanopillared hydrophobic surface. Physical conditions that can strongly affect the transition include the height of nanopillars, the spacing between pillars, the intrinsic contact angle, and the impinging velocity of water nanodroplet (''raining'' simulation). There exists a critical pillar height beyond which water droplets on the pillared surface can be either in the Wenzel state or in the Cassie state, depending on their initial location. The free-energy barrier separating the Wenzel and Cassie state was computed on the basis of a statistical-mechanics method and kinetic raining simulation. The barrier ranges from a few tenths of k BT0 (where kB is the Boltzmann constant, and T 0 is the ambient temperature) for a rugged surface at the critical pillar height to Ϸ8 kBT0 for the surface with pillar height greater than the length scale of water droplets. For a highly rugged surface, the barrier from the Wenzel-to-Cassie state is much higher than from Cassie-to-Wenzel state. Hence, once a droplet is trapped deeply inside the grooves, it would be much harder to relocate on top of high pillars. free-energy barrier ͉ molecular dynamics simulation ͉ nanodrop raining experiment ͉ Wenzel-to-Cassie state transition I t is well known that microtextured or nanotextured hydrophobic surfaces can become superhydrophobic (1-39). In fact, nature provides first examples of superhydrophobic surfaces, such as lotus leaves and water striders' nonwetting legs (40-42). Synthetic microtextured surface structures like the lotus leaves have been fabricated to achieve high water repellency such that on these surfaces, water droplets are typically in the Cassie state (43) rather than the Wenzel state (44). In general, water droplets adhere more strongly to the textured surface in the Wenzel state than in the Cassie state, causing stronger contact-angle hysteresis. Hence, in many practical applications such as self-cleaning surfaces (6, 17), the Cassie state is preferred over the Wenzel state. It is also known that as the degree of surface roughness increases, the Cassie state becomes increasingly favorable compared with the Wenzel state. Hence, at certain degree of roughness, the Wenzel state and Cassie state can become more or less equally favorable and may even coexist on the same surface. From a statistical-mechanics point of view, the 2 states can coexist when they are separated by a high free-energy barrier by which one state is still metastable (free-energy local minimum), and the other is thermodynamically stable (free-energy global minimum). In this article, we present computer simulation evidence of coexisting Wenzel/Cassie state (or the bistable state) for water droplets on pillared hydrophobic surface. We have studied 4 conditions that affect the transition between the Wenzel and Cassie state: (i) The height of nanopillars, (ii) the spacing between pillars, (iii) the impinging velocity of w...

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Section: Resultsmentioning

confidence: 99%

“…A larger spacing of 14.8/14.9 Å between pillars was also examined. This near-square-lattice pillar arrangement has been used previously by Lundgren et al (11,32). The height of nanopillars is adjustable, ranging from 2 graphite-interlayer distance (6.7 Å) to 30 interlayer distance (100.4 Å).…”

mentioning

confidence: 99%

Coexistence and transition between Cassie and Wenzel state on pillared hydrophobic surface

Kondo

Yasuoka

Fujikawa³

et al. 2009

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

426

335

View full text Add to dashboard Cite

show abstract

“…Lundgren et al [41] carried out molecular dynamics simulation studies to understand the effect of certain factors, such as topography, composition of the solid surface and CA variation with protrusion height on the hydrophobic behavior of a material [42]. Their simulation experiments have observed a transition state between Wenzel's and Cassie's due to an increase in the pillar (protrusion) height.…”

Section: Mechanics and Nature Of Wettingmentioning

confidence: 99%

Superhydrophobic Surfaces

et al. 2014

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The development of biomaterials for specific application with the desired function requires a careful combination of surface roughness, surface energy (and their polar and dispersion component), the presence of functional group, and the response of materials to the body fluid. The main constituent of body fluid is water; therefore, material response to water can be used to predict the behavior of the material implanted in the human body. Protein adsorption, cell and bacteria proliferation, alkaline phosphate activity, and so on are some of the assessments that are conducted during the in vitro studies on the materials, which depend on the hydrophilic or hydrophobic nature of the material in order to provide first insight into the material response. These responses of the material further depend on surface roughness, surface energy and the presence of functional group on the surface. Hence, learning the wetting behavior of material is necessary for its engineering. Thus, to nurture the material behavior in the body environment, understanding of contact angle (CA), its dependence on various factors and fabrication of desired surface is extremely necessary. In this chapter, the basics of CA in contrast to natural phenomena present in the surrounding environment such as surfaces of lotus, rice and ramie leaves, correlations between the morphological

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“…(2) is called the Wenzel equation [3], and Eq. (3) is called the Cassie-Baxter equation [4,5], which is a modified form of the Wenzel equation. Chow [6] and Patankar [7] further developed the theoretical model to consider the wetting phenomena of a droplet on more complicated rough surfaces.…”

Section: Introductionmentioning

confidence: 99%

“…They varied the condition of solid surface such as root mean square roughness, and then calculated the contact angle hysteresis for hydrophilic and hydrophobic surfaces. Lundgren et al measured the contact angle of a water droplet on a surface with pillars using molecular dynamics simulation [5,13]. They calculated parameters such as the contact angle between a solid wall and a water droplet for different droplet sizes, the surface energies, and the shapes of the pillars, and compared computational results with theoretical equations.…”

Section: Introductionmentioning

confidence: 99%

Study of the wetting characteristics of water droplet on a heterogeneous pillared surface

Kwon

Ambrosia

et al. 2015

J Mech Sci Technol

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We investigated the wetting characteristics of a water droplet on a heterogeneous pillared surface at the nano-scale including contact angle, molecule inflow percentage and density fields and compared them with the wetting characteristics of a water droplet on a homogeneous pillared surface. Molecular dynamics simulations were employed to analyze the wetting behavior of water droplets on surfaces with pillar structures by considering different potential energies including bond, angle, Lennard-Jones and Coulomb to calculate the interacting forces between water molecules and the surface. The heterogeneous surfaces considered had pillars with a different surface energy than the base surface. It was found that the difference in surface energy between the base surface and pillar had little effect on the hydrophobicity of the surface at low pillar heights. However cases with pillar heights over H = 4.24 Å, the pillar surface energy has a larger effect on the molecule inflow percentage with the maximum differences in the range from 33.8% to 47.2% depending on the base surface energy. At a pillar height of H = 16.96 Å, the pillar surface energy has a large effect on the contact angle of the water droplet with the maximum differences in the range from -26.1% to -40.62% depending on the base surface energy. There was a large variation in the contact angle of the droplet as the pillar height increased when there was a large difference in the surface energies between the base and the pillars.

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Molecular Dynamics Study of Wetting of a Pillar Surface

Cited by 89 publications

References 11 publications

Coexistence and transition between Cassie and Wenzel state on pillared hydrophobic surface

Coexistence and transition between Cassie and Wenzel state on pillared hydrophobic surface

Superhydrophobic Surfaces

Study of the wetting characteristics of water droplet on a heterogeneous pillared surface

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