From superhydrophobic to superhydrophilic surfaces tuned by surfactant solutions

Abstract

The wettability of hydrophobic surfaces is generally improved by surfactant solutions. The wetting behavior of superhydrophobic surfaces can be classified into two types, in terms of the variation of contact angle with surfactant concentration c s . Contact angle is controlled by surface tension for common linear surfactants and becomes independent of c s as c s Ͼ critical micelle concentration. Consequently, superhydrophobic surfaces remain in hydrophobic range, as reported. However, for branch-tailed surfactants such as sodium-bisethylhexylsulfosuccinate and didodecyldimethylammonium bromide, superhydrophobic surfaces can turn superhydrophilic by increasing c s owing to continuous reduction of solid-liquid interfacial tension. The superhydrophobicity is recoverable simply by water rinsing.Wetting phenomena of water droplets on substrates are of crucial concern in our daily life as well as in engineering and science. Wetting contact angle ͑CA͒ between the liquidgas ͑l-g͒ and liquid-solid ͑l-s͒ interfaces is used to characterize the nature of solid-fluid interactions. Conventionally, a surface is termed superhydrophobicity when water contact angle is greater than 150°. 1 The opposite effect is superhydrophilicity whereby a droplet is converted into a film. Superhydrophobic and superwetting surfaces have potential use in the microfluidic and sensor applications. 2 In capillarityrelated applications, however, surfaces with strong water repellency make wetting and spreading very difficult to control. In general, specific surfactants can be added to tune the wetting and spreading behavior of the liquid on hydrophobic interfaces. Nevertheless, it was reported that superhydrophobic surfaces remain in hydrophobic or superhydrophobic range for common surfactants, such as nonionic octaethylene glycolmono-n-decyl ether ͑C 10 E 8 ͒ and ionic sodium dodecyl sulfate ͑SDS͒. 3,4 As a result, the traditional approach ͑surfactant addition͒ seems unable to turn superhydrophobic surfaces into highly wettable surfaces ͑less than 20°͒, and the superhydrophobic surface cannot be easily cleaned.The wetting behavior is governed by two factors, the chemical composition and the roughness of the solid surfaces. The wettability of an ideal flat solid is depicted by Young equation, 5 cos = ͑␥ s-g − ␥ l-s ͒ / ␥ l-g , where ␥ s-g , ␥ l-s , and ␥ l-g represent the interfacial tensions of solid-gas, l-s, and l-g interfaces, respectively. The highest contact angle for a water droplet on a smooth substrate, such as-CF 3 groups with surface-free energy of 6.7 mJ/ m 2 , is generally about 120°. 6 Beyond this angle, the fine surface roughness becomes the dominant factor in increasing the contact angle. The mechanisms responsible for the effect of surface roughness were addressed by Wenzel 7 and later by Cassie and Baxter. 8 The superhydrophobic surface can be regarded as a surface composed of two types of homogeneous patches that have different solid-fluid interfacial tensions. Because the rough structure is mainly filled with air, the openings of th...

Superhydrophobicity: Cavity growth and wetting transition

“…A consequence of the rough surface features in combination with wetting agents is a strong adsorption to the surfaces, thus a non-complete removal of surfactant is likely to occur. However, restoration of the long-range (>1 µm) force after rinsing with water suggests that it is possible, in agreement with contact angle studies on superhydrophobic substrates, 10 to clean a fouled superhydrophobic surface using surfactant solutions, thus restoring e.g. a self-cleaning property.…”

“…The surfactant decreases the wetting tension of water on the superhydrophobic surfaces, hence facilitating wetting and inducing a transition from the Cassie-Baxter to the Wenzel state, in agreement with contact angle measurements for surfactants turning a superhydrophobic surface into hydrophobic. 10,56,57 The long-range (>1 µm) type of force measured in pure water was found to be restored after measurements in the SDS solutions after flushing with pure water, however not to the full extent. A consequence of the rough surface features in combination with wetting agents is a strong adsorption to the surfaces, thus a non-complete removal of surfactant is likely to occur.…”

“…[53][54][55] The role of wetting agents and specifically surfactants at superhydrophobic surfaces has been studied in detail. 10,[56][57][58] We report long-range (>1 µm) normal forces between superhydrophobic surfaces in aqueous solution measured by AFM colloidal probe microscopy and show that superhydrophobicity displays as a series of events which warrants discussion of each event together with its statistical occurrence. We hereby extend work by Singh et al 30 increase the attachment of the smaller particles to the larger probes.…”

“…ethanol or a surfactant. [8][9][10] The mechanism behind interactions between hydrophobic surfaces in aqueous solutions has received a lot of attention ever since the first measurements by Israelachvili and Pashley using the surface force apparatus (SFA). 11 Studies confirmed the interactions were too long-range to be a result solely of van der Waals forces.…”

Communications: Wall free capillarity and pendant drop removal

“…In fact, the pendant drops can be easily removed if the sessile drops are on a superhydrophilic surface. 7 According to Eq. ͑2͒, for a given droplet volume, the lower the contact angle, the larger the radius of curvature.…”