Inscribing Wettability Gradients Onto Superhydrophobic Carbon Nanotube Surfaces

Abstract

Superhydrophobic surfaces with ultra-low contact angle hysteresis fi nd a wide range of applications in the fi elds of selfcleaning, anti-icing, anti-fogging, or microfl uidics. [ 1,2 ] Here, the feasibility of a novel type of superhydrophobic surface for guided droplet transport, which is a key issue for microfl uidic systems, is demonstrated. Among such applications, open-surface digital microfl uidics (OSDM) offers specifi c advantages in terms of remote controllability and programmability of droplet transport. [ 3,4 ] One of the major concerns while manipulating aqueous droplets at solid surfaces is the large contact angle hysteresis which arises due to chemical or physical heterogeneities of the surface. [ 5 ] Moreover, it is usually desirable to locally tune the surface properties in a well-controlled and reversible manner. Although several examples have been reported demonstrating the fabrication of chemically or physically modifi ed superhydrophobic surfaces, [ 6,7 ] tunability and fast control over the surface properties remains a challenge. Among the different materials used for the fabrication of superhydrophobic surfaces, carbon nanotubes (CNTs) are interesting because of their chemically well-defi ned nature and response in wettability to different stimuli (using plasma [ 8 ] or UV light). [ 9,10 ] Vertically aligned CNT structures gain additional signifi cance since transport properties of corresponding surfaces are strongly anisotropic, [ 11 ] which facilitates localized changes in temperature or electric fi eld. Several studies have been made in the past to tune the hydrophobic nature of aligned carbon nanotubes to produce superhydrophobic surfaces. In general, two methods have been developed to increase the hydrophobicity of CNT surfaces a) coating with a low surface energy material [ 12,13 ] and b) micropatterning of the CNT surface structure [ 14 ] so as to increase their surface roughness. Desirable characteristics of CNTs like, for example, thermal conductivity are compromised on coating with a foreign material. CNTs thus act as mere template structure for providing the desired nanoscale roughness. Micropatterning, on the other hand, is able to induce controlled surface roughness but relies on complex and expensive masking/lithography techniques involving multiple process steps. Superhydrophobic materials, including CNTs, exhibit two types of wetting behavior at high contact angles: a) very low contact angle hysteresis (like on lotus leaves) where the water droplet easily rolls off [ 10,15,16 ] or b) very high hysteresis (like on rose petals) where the water droplets pins to the surface. [ 17,18 ] While most of the surface modifi cation processes result in either one of these states, it is highly desirable to achieve a reversible switching between these two states on the same material. A reversible and localized switching between states with different contact angles and different levels of contact angle hysteresis has an enormous potential, for instance, for microfl uidic applications. By cre...

Superhydrophobic hierarchically structured surfaces in biology: evolution, structural principles and biomimetic applications

“…The most successful product economically on the market, the facade paint Lotusan , is based on various inorganic particles, including TiO 2 as pigment in a hydrophobic silicone resin resulting in a perfect hierarchically structured surface. On a much smaller scale, carbon nanotubules provide an example of superhydrophobic surfaces [179][180][181][182].…”

Carbon nanotubes in microfluidic lab-on-a-chip technology: current trends and future perspectives

“…Besides, as-grown CNTs are susceptible to oil fouling as a result of their high surface energy, and also the mechanically fragile nature of the grown CNTs causes them to collapse upon light friction or penetration of water and to lose their superhydrophobic effect (Zhu et al 2014; Li et al 2010). A method that has been suggested by many researchers to increase the superhydrophobicity of CNTs is to modify the as-pre-pared surface with low surface energy materials (Babu 2014). In this regard, Meng and Park (2010), carried out surface modifications of MWCNTs through dispersal in a fluoropolymer (FP) solution thus forming FP-grafted MWCNTs.…”

“…Also, another study by the same team showed that irradiation of the obtained super-hydrophilic surface with a CO 2 laser led to evaporation of the surface of the VACNT containing oxygen polar groups and thus, the superhydrophobic effect of the surface could be restored (Lobo et al 2010). Another study with the same purpose was carried out by Babu (2014), who fabricated superhydrophobic 3D-aligned CNT arrays, possessing a wettability gradient, without any coating or micropatterning of the CNT surfaces, and only using “water-assisted chemical vapor deposition” (WACVD). The required surface roughness was achieved by a CVD regrowth technique and then the surface was passivated by polydimethylsiloxane functionalization.…”

Polysiloxane/multiwalled carbon nanotubes nanocomposites and their applications as ultrastable, healable and superhydrophobic coatings

“…We have prepared a series of superhydrophobic and superoleophobic surfaces with excellent properties using organosilanes [35][36][37][38]. Carbon nanotubes are frequently used for fabricating superhydrophobic surfaces owing to their high aspect ratio [39][40][41][42][43], but the durability of the surfaces remains to be improved. In addition, the effects of diameter and acid activation of carbon nanotubes have not yet studied.…”