The collapse transition on superhydrophobic surfaces

Kusumaatmaja, Halim; Blow, Matthew L.; Dupuis, Alexandre; Yeomans, J. M.

doi:10.1209/0295-5075/81/36003

Cited by 144 publications

(172 citation statements)

References 22 publications

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“…In other words, at low pillar height, the Wenzel state is the only stable state for the droplet. But when the height is beyond a critical value, e.g., 13.4 Å, the Cassie state is metastable, separated from the stable Wenzel state by a free-energy barrier (18,23,37). As such, coexistence of Wenzel and Cassie state for water droplets is possible, depending on the initial location of the droplets (18,19,23,(35)(36)(37).…”

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.

412

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%

“…To gain more insights into relative stability of the Cassie and Wenzel states, it is important to have quantitative values of the free-energy barrier that separates the Wenzel and Cassie state, as schematically shown in Fig. 3 (23,37). To this end, we used 2 computer simulation methods: 1 kinetic and 1 equilibrium.…”

Section: Resultsmentioning

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.

412

335

View full text Add to dashboard Cite

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“…Whilst this is only an analogy, the idea of skin effect due to surface tension and the existence of a natural length scale for objects to be able to bridge asperities are useful in considering superhydrophobic surfaces. Whether a liquid penetrates or not is determined by the cost in surface free energy for wetting down the surface structure [23,24,25,26]. …”

Section: How the Suspended State Stays Suspendedmentioning

confidence: 99%

An introduction to superhydrophobicity

Shirtcliffe

McHale

Atherton

et al. 2010

Advances in Colloid and Interface Science

549

373

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“…If a liquid flows past a rough hydrophobic (i.e. superhydrophobic) surface, roughness may favor the formation of trapped gas bubbles, resulting in a large slip length [9][10][11][12][13][14]. For rough hydrophilic surfaces the situation is much less clear, and opposite experimental conclusions have been made: one is that roughness generates extremely large slip [15], and one is that it decreases the degree of slippage [16,17].…”

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confidence: 99%

Random-Roughness Hydrodynamic Boundary Conditions

2010

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We report results of lattice Boltzmann simulations of a high-speed drainage of liquid films squeezed between a smooth sphere and a randomly rough plane. A significant decrease in the hydrodynamic resistance force as compared with that predicted for two smooth surfaces is observed. However, this force reduction does not represent slippage. The computed force is exactly the same as that between equivalent smooth surfaces obeying no-slip boundary conditions, but located at an intermediate position between peaks and valleys of asperities. The shift in hydrodynamic thickness is shown to depend on the height and density of roughness elements. Our results do not support some previous experimental conclusions on very large and shear-dependent boundary slip for similar systems.

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The collapse transition on superhydrophobic surfaces

Cited by 144 publications

References 22 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

An introduction to superhydrophobicity

Random-Roughness Hydrodynamic Boundary Conditions

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