Determination and evaluation of the nonadditivity in wetting of molecularly heterogeneous surfaces

The interface between water and folded proteins is very complex. Proteins have "patchy" solvent-accessible areas composed of domains of varying hydrophobicity. The textbook understanding is that these domains contribute additively to interfacial properties (Cassie's equation, CE). An ever-growing number of modeling papers question the validity of CE at molecular length scales, but there is no conclusive experiment to support this and no proposed new theoretical framework. Here, we study the wetting of model compounds with patchy surfaces differing solely in patchiness but not in composition. Were CE to be correct, these materials would have had the same solid-liquid work of adhesion (WSL) and time-averaged structure of interfacial water. We find considerable differences in WSL, and sum-frequency generation measurements of the interfacial water structure show distinctively different spectral features. Molecular-dynamics simulations of water on patchy surfaces capture the observed behaviors and point toward significant nonadditivity in water density and average orientation. They show that a description of the molecular arrangement on the surface is needed to predict its wetting properties. We propose a predictive model that considers, for every molecule, the contributions of its first-nearest neighbors as a descriptor to determine the wetting properties of the surface. The model is validated by measurements of WSL in multiple solvents, where large differences are observed for solvents whose effective diameter is smaller than similar to 6 angstrom. The experiments and theoretical model proposed here provide a starting point to develop a comprehensive understanding of complex biological interfaces as well as for the engineering of synthetic ones.

Moisture and energy dynamics in fields soils

Francesco Ciocca

Soil moisture is crucial to water-cycle as it affects infiltration, runoff and land-atmosphere interactions. In dry soils the role of water vapor fluxes becomes essential and causes a tight coupling between soil moisture dynamics and heat transfer, which makes measurements and modeling extremely difficult. Since advances in both theory and experimental techniques are required, we have focused on both aspects: we have performed theoretical-numerical investigations of soil moisture and energy dynamics at the land surface; tested novel measurements techniques, in laboratory and at field scale, to provide new insights into soil moisture dynamics and contribute to a comprehensive theory of coupled energy and moisture transport. As the dynamics of physical systems are dictated by conservation equations, it is common practice to estimate quantities that cannot be directly measured (e.g. vapor fluxes) from the residuals of balance equations. In laboratory as well as in field experiments, residuals are calculated from measurements that are sparse in space and discrete in time. We have theoretically and numerically shown how the use of discrete data may lead to large residuals even without measurements or model errors. We have demonstrated that residuals are very sensitive to the distance between the measurements and that they increase in presence of nonlinear processes, such as moisture transport. A careful error analysis is required to assess the reliability of residuals computed from discrete data and avoid misinterpretations of the underlying physical processes. Another crucial element dictating soil moisture dynamics and evaporation into the atmosphere is soil water-retention. Popular parametric models of the retention curve present a matric potential that becomes infinite at residual saturation, which is the water content below which liquid flow stops. Few extensions of the water-retention curves have been proposed, which remain rarely used in practice. We have studied the effects of different models on liquid and vapor transport. We have shown that parameterizations that allow vapor flux below residual saturation yield larger vapor fluxes that result in a drier soil surface, whereas deep soil remains relatively wet. When diurnal forcing is considered, potential evaporation is reduced in the energy-limited regime and enhanced in the moisture-limited regime. We have established that in this case extended water-retention models predict larger heat fluxes that might critically impact models at whole scales (possibly including global circulation models). Discrimination between different theoretical descriptions and parametric models requires experiments that monitor moisture and energy dynamics. This demands the use of accurate sensors and measurement techniques that are able to simultaneously monitor several quantities (e.g. soil moisture, temperature, soil thermal properties, solute concentration in soil water). As in general different probes are used, problems arise due to different footprints and sampled soil volumes. Multi functional heat pulse probes (MFHPPs) have been conceived to overcome these issues by allowing simultaneous measurements of all required variables. We have assessed MFHPPs ability to measure ground heat flux, which has to be accurately known to avoid significant imbalances in the surface energy budget. We have employed an experimental setup able to generate a non-uniform heat-flux field and we have demonstrated that MFHPPs can reliably capture the magnitudes and directions of the fluxes. This allows more complete information on ground heat flux than that obtained by the classic probes (e.g. ground heat flux plates). To monitor soil state in large-scale field applications we have investigated the possibility to infer distributed thermal conductivity and soil moisture profiles with an optical fiber. Information is obtained by monitoring soil response to an active heat pulse generated from the metal shield armoring the optic cable, and analyzing the temperature differences of the cooling phase, monitored each meter by a distributed temperature sensing system (DTS). We have shown that soil moisture measurements inferred from soil thermal response (recorded by the DTS) are in good agreement with independent measurements in wet and intermediately wet soils; however in dry soils moisture content is underestimated and further investigations are required to improve this technique for dry conditions. In this study we have significantly improved the knowledge of coupled energy and moisture transport on both theoretical-numerical and experimental sides. We have proven that reliable measurements can be obtained at different spatiotemporal scales, and provided both prognostic and diagnostic methods to optimize their use. Finally, we have suggested crucial improvements to numerical models which might have implications beyond field-soils dynamics.

EPFL2013

In-silico zein/tannic acid colloidal nanoparticles and their activity at oil and water interface of Pickering emulsion using molecular dynamics simulation

Niloufar Sharif

The computational method was used to explain the activity of zein-based colloidal nanoparticles at inter-face between water and oil phases. The alpha-zein/tannic acid colloidal nanoparticles (ZTPs) were made using three steps including dissolving alpha-zein (Z19) in aqueous ethanol (ZE), alpha-zein/tannic acid interactions and antisolvent precipitation. The ZTP showed decreased Rg and SASA as expected as well as hydrogen bonds between zein and tannic acids. Moreover, the acceleration, to simulate ultrasonication, indicated the breaking up and recoalescence after applying on the oil droplet. Subsequently, after the combination of ZTPs and acceleration at interface between oil and water phases, results confirmed the capability of the ZTPs to surround the oil phase, the increased density of oil molecules around the hydrophobic surface of ZTPs and more van der Waals interactions. These findings explain the activity of ZTPs at the interface between oil and water from molecular view. (C) 2022 Elsevier B.V.

ELSEVIER2022

Wetting properties of flat-top periodically structured superhydrophobic surfaces

Laura Barbieri

In recent years, superhydrophobic surfaces have attracted a considerable amount of attention from the academic and industrial communities. This growing interest is mainly caused by the fundamental physico-chemical theoretical aspects that remain obscure and the number of promising practical applications emerging in a wide range of fields, such as textiles, self-cleaning coatings, and micro-fluidic systems. Superhydrophobic surfaces exhibit simultaneously high static water contact angles and low resistance to liquid motion on the surface (i.e. low contact angle hysteresis). These properties result from the combination of the chemical hydrophobicity of the topmost layers of the surface and its roughness, the latter being the dominant factor. In this PhD work, advantage has been taken of recent advances in micro-/nano-processing technologies to fabricate microstructured surfaces with specific and controlled roughness. This has enabled systematic experimental investigations to be carried out to address some of the still unanswered questions relating to superhydrophobic phenomena. Silicon wafers were microstructured by photolithography in order to obtain periodical distributions of well-defined flat-top obstacles. A gas-phase silanization process was used to cover the prepared microstructured surfaces with a hydrophobic dense mono-layer of perfluorodecyltrichlorosilane. Several series of samples in which each roughness parameter (distance between obstacles, obstacle height, obstacle size, obstacle shape, etc.) was individually varied were fabricated, and static and dynamic contact angle variations as a function of each parameter were studied. The results obtained by water contact angle measurements were compared to the classic Wenzel and Cassie models. The first assumes that the liquid wets the asperities of the rough substrate completely (referred to as wetted state), whilst the second describes the liquid as sitting on a mixture of air and solid (referred to as composite state). By systematically varying a given parameter, a transition between the composite and the wetted regime was observed. Simple thermodynamic considerations based on the energy minimization of the drop-substrate system showed that Cassie and Wenzel contact angles correspond to two energy minima of the system. Experimentally, the Cassie regime is observed when the Cassie angle is the absolute energy minimum, while when the Wenzel angle is the absolute energy minimum, either the Wenzel regime or a metastable Cassie state is observed. The existence of these metastable states is explained theoretically by the energy barrier that the system has to overcome to reach the Wenzel state when a drop is gently deposited on the surface, and good agreement with experimental data is demonstrated. Water and n-hexadecane dynamic contact angle measurements performed in two different configurations, i.e. with a "negligible" and a "NON-negligible" extra-pressure exerted on the drop-substrate system, permitted the validity of the theoretical model proposed to be confirmed. They clearly evidenced the region of stability of both Wenzel and Cassie regimes, and particularly the region where the Cassie regime is metastable, which is the really interesting one for all the potential applications of rough superhydrophobic surfaces as self-cleaning, and non-sticking surfaces, since only the metastable composite states are those characterized by a very high contact angle and a very low contact angle hysteresis, the two main requirements for a good superhydrophobic surface. With systematic reduction of the absolute size of the obstacles in the micrometer range, an increase in the extent and robustness of the composite states was observed. Drop contact line fragmentation seems to be the most important factor in determining the extent and robustness of the observed composite states. Results obtained by varying the obstacle top-surface shape demonstrate that contact line length and corrugations are only secondary factors. Studies on the influence of asperity absolute size performed in the micrometer range, together with preliminary but fundamental results obtained on nanostructured substrates, allow the proposal of interesting conclusions concerning determination of roughness size where macroscopic superhydrophobicity appears and vanishes. A typical size of ∼ 50 µm (where "typical size" refers to the width of the obstacle top-surface) can be considered the upper limit of the superhydrophobicity scale range, with a length scale of some tens of nanometers as the lower limit. In this range, the length scale comprised between several hundred nanometers and one micrometer can be assumed the most suitable size for obtaining superhydrophobic properties of larger extent and robustness. The related appropriate asperity height must be sufficiently great to prevent the drop meniscus from touching the bottom of the asperities by vibration, inducing a transition to the wetted Wenzel state.