Papers by Keyword: Wetting Dynamics

Abstract: Accurate characterization of the underwater stability of superhydrophobic surfaces is crucial for the design of durable anti-fouling materials and advanced microfluidic concepts. Although superhydrophobic breakdown is a major issue that hampers full exploitation of superhydrophobic functional materials, suitable characterization methods are lacking and relatively little is known about the wetting dynamics. In this work we explore a novel method based on attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR) for large-area in-situ analysis of wetting states and wetting transitions on nanostructured surfaces. Spontaneous wetting is induced on superhydrophobic silicon nanopillars through in-situ modulation of the liquid composition and surface tension. The high surface sensitivity of ATR-FTIR enables quantitative evaluation of the instantaneous liquid composition and wetted area. Critical transition criteria for superhydrophobic breakdown are assessed using both ATR-FTIR and goniometric measurements. Significant deviations from classical wetting models are revealed, emphasizing the need for more accurate transition criteria and careful experimental validation. Breakdown kinetics near the critical transition are found to be significantly slowed down on nanostructured surfaces, which underlines the necessity for accurate characterization of wetting dynamics at the nanoscale. The proposed ATR-FTIR method can be promising for dynamic studies of wetting transitions on more advanced surfaces, as hierarchical structures or oleophobic designs.

Abstract: The excellent spreading and wetting behaviour of superspreader solutions has been known and extensively studied over recent years. However, explanations for spreading dynamics and accompanying mathematical models have not yet proved completely successful. Many attempts have been made to quantify the spreading exponents, but none of the models so far was able successfully to describe the whole wetting process of trisiloxane solutions, especially on hydrophobic surfaces. We have investigated the partial wetting of Silwet L-77® superspreader solutions of high concentrations (well above CMC) on polymer coated substrates of varying hydrophobicity. Results obtained can be explained in terms of the Marangoni effect as the major driving force for trisiloxane enhanced spreading. A simple theory, which involves surface tension gradients governing the spreading process, was developed in order to explain the specific evolution of the drop radius and consequent decrease in the contact angle. The proposed model was found to be in excellent agreement with the experimental results. Determined equation coefficients were shown to be dependent on both surfactant concentration and the hydrophobicity of the substrate.