Morphological wetting transitions at chemically structured surfaces

Lipowsky, Reinhard

doi:10.1016/s1359-0294(00)00086-8

Cited by 87 publications

(88 citation statements)

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“…There is also a significant body of literature on the wetting of chemically patterned surfaces (for reviews see [20,21]). One further question that naturally arises is how spreading might depend on surface topography.…”

Section: Introductionmentioning

confidence: 99%

Dynamic wetting and spreading and the role of topography

McHale¹,

Newton

Shirtcliffe

2009

J. Phys.: Condens. Matter

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The spreading of a droplet of a liquid on a smooth solid surface is often described by the Hoffman-de Gennes law, which relates the edge speed, v e , to the dynamic and equilibrium contact angles θ and θ e by v e ∝θ(θ 2 -θ e 2 ). When the liquid wets the surface completely and the equilibrium contact angle vanishes, the edge speed is proportional to the cube of the dynamic contact angle. When the droplets are non-volatile this law gives rise to simple power laws with time for the contact angle and other parameters in both the capillary and gravity dominated regimes. On a textured surface the equilibrium state of a droplet is strongly modified due to the amplification of the surface chemistry induced tendencies by the topography. The most common example is the conversion of hydrophobicity into superhydrophobicity. However, when the surface chemistry favors partial wetting, topography can result in a droplet spreading completely. A further, frequently over-looked consequence of topography is that the rate at which an out-of-equilibrium droplet spreads should also be modified. In this report, we review ideas related to the idea of topography induced wetting and consider how this may relate to dynamic wetting and the rate of droplet spreading. We consider the effect of the Wenzel and Cassie-Baxter equations on the driving forces and discuss how these may modify power-laws for spreading. We relate the ideas to both the hydrodynamic viscous dissipation model and the molecular-kinetic theory of spreading. This suggests roughness and solid surface fraction modified Hoffman-de Gennes laws relating the edge speed to the dynamic and equilibrium contact angle. We also consider the spreading of small droplets and stripes of non-volatile liquids in the capillary regime and large droplets in the gravity regime. In the case of small non-volatile droplets spreading completely, a roughness modified Tanner's law giving the dependence of dynamic contact angle on time is presented. We review existing data for the spreading of small droplets of polydimethylsiloxane oil on surfaces decorated with micro-posts. On these surfaces, the initial droplet spreads with an approximately constant volume and the edge speed-dynamic contact angle relationship follows a power law v e ∝θ p . As the surface texture becomes stronger the exponent from p=3 towards p=1 in agreement with a Wenzel roughness driven spreading and a roughness modified Hoffman-de Genne's power law. Finally, we suggest that when a droplet spreads to a final partial wetting state on a rough surface, it approaches its Wenzel equilibrium contact angle in an exponential manner with a time constant dependent on roughness.

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Section: Introductionmentioning

confidence: 99%

Dynamic wetting and spreading and the role of topography

McHale¹,

Newton

Shirtcliffe

2009

J. Phys.: Condens. Matter

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“…The dynamics of the drops will be affected by any chemical heterogeneities on the surface [1,2,3]. Until recently such disorder was usually regarded as undesirable.…”

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confidence: 99%

Control of drop positioning using chemical patterning

Dupuis

Léopoldès

Bucknall

et al. 2005

Applied Physics Letters

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We explore how chemical patterning on surfaces can be used to control drop wetting. Both numerical and experimental results are presented to show how the dynamic pathway and equilibrium shape of the drops are altered by a hydrophobic grid. The grid proves a successful way of confining drops and we show that it can be used to alleviate mottle, a degradation in image quality which results from uneven drop coalescence due to randomness in the positions of the drops within the jetted array.From microfluidic technology to detergent design and ink-jet printing it is important to investigate the way in which drops move across surfaces. The dynamics of the drops will be affected by any chemical heterogeneities on the surface [1,2,3]. Until recently such disorder was usually regarded as undesirable. However with the advent of microfabrication techniques it has become possible to control the chemical patterning of a substrate down to nanoscale, leading to the possibility of exploring how such patterning can control, rather than disturb, drop motion.A practical example where such an approach might be of use is in ink-jet printing. Although ink-jet printers are widely available for domestic use the quality of the images is still not sufficiently robust to allow widespread industrial applications. The possibility of replacing the traditional contact techniques with electronically controlled template design, particularly for small print runs, is highly desirable for both efficiency and cost.In the printed image a patch of colour is produced by jetting drops in a regular, square array. The closer the drops the more intense the colour of the patch appears to the eye. To achieve a solid colour the aim is that drops jetted at a distance apart comparable to their diameter should coalesce and form a uniform covering of ink. However, in practice, randomness in the positions at which the drops land, combined with surface imperfections, often lead to local coalescence and the formation of large, irregular drops with areas of bare substrate between them as shown in fig. 3(a) and the upper part of fig. 4. Such configurations are likely to lead to poor image quality, called mottle [4].In an attempt to overcome this problem we demonstrate how using a two-dimensional array of hydrophobic chemical stripes can be used to control the equilibrium shape, the position and the dynamic pathway of spreading drops. The hydrophobic stripes form barriers controlling the drops and allowing their relative positions to be tuned. The behaviour of single drops, and then an array of drops on the patterned surfaces, is explored both by solving the hydrodynamics equations of motion by means of a lattice Boltzmann algorithm and by performing suitable experiments.In the numerical modelling we consider a liquid-gas system of density n(r) and volume V . The surface of the substrate is denoted by S. The equilibrium properties of the drop are described by the free energywhere Einstein notation is understood for the Cartesian label α and where ψ b (n) is the free en...

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“…2, the slender capillary bridge consists of two plates parallel to each other, both of which have slender rectangular structured surfaces. The structured surface is produced by depositing hydrophilic strips (region I) on a substrate with a hydrophobic surface (region II) through different techniques (Gau et al, 1999;Lipowsky, 2001;Broesch and Frechette, 2012;Broesch et al, 2013;Valencia et al, 2001) (see Fig. 2a).…”

Section: Characterizationsmentioning

confidence: 99%

“…The widths (l dx ) of the liquid-solid interfaces are confined by the hydrophobic region II and are constant, but the contact angle and the curvature of the lateral liquid-gas interface along the length of the hydrophilic region I will vary with the decline in the capillary bridge height, termed the "hinge movement" of the bilateral liquid-gas interfaces at the location where the wettability varies (Broesch and Frechette, 2012;Broesch et al, 2013;Valencia et al, 2001;Yaneva et al, 2005). As the spacing between the plates decreases further, the triple contact line at the two ends of the capillary bridge slips and touches region II, at which point the triple contact line is pinned and the all-around liquid-gas interface of the capillary bridge hinges, which inevitably leads to variations such as the forces of the capillary bridge, internal pressure, surface curvature, and contact angles (Lipowsky, 2001;Broesch and Frechette, 2012;Broesch et al, 2013).…”

Section: Characterizationsmentioning

confidence: 99%

“…By using micro-contact printing, vapor deposition, photolithography, and other techniques, a hydrophobic substrate surface can be patterned with hydrophilic strips to produce a structured surface, so as to control the morphology of the capillary bridge and achieve the desired mechanical properties. This has a high application value in fields such as microelectronics, semiconductors, MEMS, and microchannels (Bowden, 1997;Gau et al, 1999;Bush et al, 2010;Lipowsky, 2001;Broesch and Frechette, 2012;Broesch et al, 2013).…”

Section: Introductionmentioning

confidence: 99%

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Shape and force analysis of capillary bridge between two slender structured surfaces

Zhu

Jia

et al. 2015

Mech. Sci.

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Abstract. When a capillary bridge of a constant volume is formed between two surfaces, the shape of the liquid bridge will change as the separation between those surfaces is varied. To investigate the variable forces and Laplace pressure of the capillary bridge, as the shape the bridge evolves, a pseudo-three-dimensional force model of the capillary bridge is developed. Based on the characteristics of the slender structured surface, an efficient method is employed to directly solve the differential equations defining the shape of the capillary bridge. The spacing between the plates satisfying the liquid confined within the hydrophobic region of the structured surface is calculated. The method described in this paper can prevent meshing liquid surfaces such that, compared with Surface Evolver simulations, the computing speed is greatly improved. Finally, by comparing the results of the finite element simulations performed with Surface Evolver with those of the method employed in this paper, the practicality of the method is demonstrated.

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Morphological wetting transitions at chemically structured surfaces

Cited by 87 publications

References 50 publications

Dynamic wetting and spreading and the role of topography

Dynamic wetting and spreading and the role of topography

Control of drop positioning using chemical patterning

Shape and force analysis of capillary bridge between two slender structured surfaces

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