Effect of a Cationic Surfactant on Droplet Wetting on Superhydrophobic Surfaces

Abstract

We experimentally and theoretically examine the influence of a double-chain cationic surfactant, didodecyldimethylammonium bromide (DDAB), on the wetting states and contact angles on superhydrophobic (SH) surfaces made of hydrophobic microcylinders. We use two types of micropatterns of different surface roughness, r, and packing fraction, ϕ, and vary nine dimensionless surfactant concentrations (C S), normalized by the critical micelle concentration (CMC), in the experiments. At low C S, some of the surfactant-laden droplets are in a gas-trapping, Cassie–Baxter (CB) state on the high-roughness microstructures. In contrast, some droplets are in a complete-wetting Wenzel (W) state on the low-roughness microtextures. We found that the contact angle of CB drops can be well predicted using a thermodynamic model considering surfactant adsorption at the liquid–vapor (LV) and solid–liquid (SL) interfaces. At high C S, however, all of the DDAB drops wet in a Wenzel mode. Based on a Gibbsian thermodynamic analysis, we find that for the two types of superhydrophobic surfaces used, the Wenzel state has the lowest thermodynamic energy and thus is more favorable theoretically. The CB state, however, is metastable at low C S due to a thermodynamic energy barrier. The metastable CB wetting state becomes more stable on the SH microtextures with greater ϕ and r, in agreement with our experimental observations. Finally, we generalize this surface-energy analysis to provide useful designs of surface parameters for a DDAB-laden surfactant droplet on the SH surface with a stable and robust CB state.

Evaporation Dynamics of Surfactant-Laden Droplets on a Superhydrophobic Surface: Influence of Surfactant Concentration

“…These useful surfaces allow water drop to sit on the top of the surface textures with gas trapped underneath forming heterogeneous wetting of partial gas–liquid and liquid−solid interfaces in the so-called Cassie–Baxter (CB) or “Fakir” wetting state. , The presence of a gas layer reduces the interfacial energy, thereby thermodynamically making a CB state more favorable by reducing the liquid–solid contact area. The thermodynamically favorable CB state, however, can easily be ruined by transitioning to a homogeneous wetting (Wenzel, W) state via various external influences, such as temperature variation, , surfactant additives, − or evaporation. , To date, the wetting and evaporation processes on SH substrates have been extensively studied experimentally and theoretically; however, most studies focus on pure liquids. ,− ,,− In contrast, many relevant and crucial areas remain unexplored; one of these is the influence of amphiphilic aqueous surfactant solutions on the evaporation process upon SH surfaces.…”

Dynamic Wetting of Ionic Liquid Drops on Hydrophobic Microstructures

“…All experiments were performed under ambient temperature (22 °C) at 1 atm and relative humidity of 16 ± 1%. Details regarding the experimental setup and image analysis can be found in our previous papers reporting the initial wetting states of water and surfactant-laden drops, − whereas we focus on the static and dynamic wetting of ionic liquids here.…”

Durable and transparent super anti-wetting coatings with excellent liquid repellency and anti-fouling performance based on fluorinated polysiloxane

“…[17][18][19][20] In general, superhydrophobic surfaces can be achieved by introducing micro/nanostructures and reducing the surface energy. 21,22 Superhydrophobic surfaces exhibit excellent self-cleaning, and anti-biofouling and anti-corrosion performances. [23][24][25] However, superhydrophobic surfaces can be easily wetted by liquids of low surface tension.…”