ABSTRACT
The petal effect is identified as a non-wetting state with high drop adhesion. The wetting behavior of petal surfaces is attributed to the papillose structure of their epidermis, which leads to a Cassie-Baxter regime combined with strong pinning sites. Under this scenario, sessile drops are pearl shaped and, unlike lotus-like surfaces, firmly attached to the surface. Petal surfaces are used as inspiration for the fabrication of functional parahydrophobic surfaces such as antibacterial or water-harvesting surfaces. In this work, two types of rose petals were replicated by using a templating technique based in Polydimethylsiloxane (PDMS) nanocasting. The topographic structure, the condensation mechanism under saturated environments and the wetting properties of the natural rose petal and their negative and positive replicas were analyzed. Finally, we performed prospective ice adhesion studies to elucidate whether petal-like surfaces may be used as deicing solutions.
Subject(s)
Fabaceae , Rosa , Surface Properties , Rosa/chemistry , Prospective Studies , WettabilityABSTRACT
A successful methodology for obtaining hybrid films which allow thermal triggering and continuous, irreversible, control of their hydrophilicity/hydrophobicity nature was developed. Two types of poly(dimethylsiloxane)-silica (PDMS@SiO2) films were prepared for that purpose: A hydrophilic film in which the thermal treatment causes an irreversible gradual increase of hydrophobicity; and a hydrophobic film that turns more hydrophilic upon thermal treatment. The opposite directionality of the change is dictated by the film substrate, on which the same hybrid is deposited. In both cases the thermal treatment induced a phase separation which caused the change in hydrophobicity. The magnitude of change in hydrophilicity/hydrophobicity is continuously controllable in both types of films by either the temperature or heating time. The films were characterized before and after heating by a variety of methods, including contact angle (CA) measurements with the sessile drop and the tilting plate methods, and by X-ray photoelectron spectroscopy (XPS) analysis. A thorough kinetic study was carried out, following the progress of the changes in the wettability property of the surfaces. The kinetics analyses proved that the changes in the wettability in all cases are due to phase separation processes, the directionality of which is determined by the treatment of the substrate on which the films are deposited. By monitoring the change of wettability (ΔCA) at various temperatures, an Arrhenius plot was obtained from which the activation energy and Arrhenius pre-exponential factor for the phase separation were derived, corroborating the proposed mechanism. To the best of our knowledge, this is the first use of phase separation behavior of a hybrid film in order to apply irreversible, thermally controllable change of surface wettability, tailored to proceed in opposite directions, and the first kinetic study of such a process.
ABSTRACT
Self-assembled monolayers of thiols on gold are employed to study the role of surface chemistry on adsorption of polyelectrolytes to solid substrates. The suitability of these substrates is demonstrated in optical reflectivity, which combines high sensitivity to the possibility to precisely control the hydrodynamic conditions at the solid/water interface. Therefore, this system allows the determination of both the adsorbed amount and the kinetics of adsorption. The behavior of two representative strong polyelectrolytes of opposite charge is discussed as a function of pH and of concentration of a monovalent electrolyte in aqueous solutions. The application of equivalent substrates with varying surface chemistry sheds light on the role of different energetic contributions driving polyelectrolyte adsorption.