Testing the performance of superhydrophobic aluminum surfaces.

The analysis of wetting properties of superhydrophobic surfaces may be a difficult task due to the restless behavior of drops on this type of surfaces and the limitations of goniometry for high contact angles. A method to validate the performance of superhydrophobic surfaces, rather than standard goniometry, is required. In this work, we used bouncing drop dynamics as a useful tool to predict the water repellency of different superhydrophobic surfaces. From bouncing drop experiments conducted over a wide range of superhydrophobic surfaces, we found that those surfaces with a proper roughness degree and homogeneous chemical composition showed higher water-repellency. We also conducted a drop condensation study at saturating conditions aimed to determine whether there is direct correlation between water repellency and condensation delay. We found that the drop condensation process is strongly related to the surface topography, as well as the intrinsic wettability. The condensation is promoted on rough surfaces but it is delayed on intrinsically hydrophobic surfaces. However, the differences found in condensation delay between the superhydrophobic surfaces explored in this study cannot be justified by their chemical homogeneity nor their efficiency as water repellent surfaces, separately.

Forces between different latex particles in aqueous electrolyte solutions measured with the colloidal probe technique.

In this article, a compilation of results on direct force measurements between colloidal particles in monovalent salts carried out with the colloidal probe technique based on Atomic Force Microscopy was presented. The interaction forces between similar and dissimilar particles was studied and it was concluded that, in general, these force profiles may be satisfactorily quantified by the DLVO theory down to distances of few nanometers. However, in the specific case where the charge of one of the involved particle is close to neutral, it was found that the surface potential of this particle may change its sign depending on the sign of charge of the opposite particle. In this respect, the assumption that the surface potential of a particle is a property only related to the particle surface features and the bulk properties is called into question. Microsc. Res. Tech. 80:144-152, 2017. © 2016 Wiley Periodicals, Inc.

Interaction forces, heteroaggregation, and deposition involving charged colloidal particles.

Force profiles as well as aggregation and deposition rates are studied for asymmetrically charged particles and surfaces in aqueous electrolytes theoretically. Interactions are calculated within the Derjaguin, Landau, Verwey, and Overbeek (DLVO) theory, whereby the electrostatic part is modeled at Poisson-Boltzmann (PB) level. Unequally charged surfaces are examined, from the symmetric system, where both surfaces are equally charged, to fully asymmetric systems, where the surfaces are oppositely charged. Charged-neutral systems, where one surface is charged and the other is neutral, emerge as an essential scenario. In this case, the choice of boundary conditions used for solving the PB equation is crucial, whereby constant charge and constant potential boundary conditions lead to either fully repulsive or fully attractive forces. Consequently, charge regulation has a major influence on particle aggregation and deposition rates too. In the charge-neutral case, substantial shifts in the critical coagulation concentration (CCC) are observed when the regulation properties are changed. In the presence of multivalent ions, these systems behave similarly to the symmetrically charged ones. The CCC decreases with the square of the valence in weakly charged systems, while unrealistically high charge densities are needed to recover the classical Schulze-Hardy limit, which predicts a sixth power dependence on valence.

Accurate predictions of forces in the presence of multivalent ions by Poisson-Boltzmann theory.

Forces between positively and negatively charged colloidal particles across aqueous salt solutions containing multivalent ions are measured directly with the atomic force microscope (AFM). The measurements are interpreted quantitatively with Poisson-Boltzmann (PB) theory. Thereby, the surface potentials and regulation properties of the particle surfaces are extracted from symmetric measurements between the same types of particles. This information is used to predict force profiles in the asymmetric situations involving different types of particles without any adjustable parameters. These predictions turn out to be very accurate, which demonstrates that the mean-field PB theory is reliable down to distances of about 5 nm. While various reports in the literature indicate that this theory should fail due to neglect of ion correlations, such effects seem important only at higher concentrations and smaller distances.

Predicting aggregation rates of colloidal particles from direct force measurements.

Direct force measurements between negatively charged colloidal latex particles of a diameter of 1 µm were carried out in aqueous solutions of various inorganic monovalent and multivalent cations with the multiparticle colloidal probe technique based on the atomic force microscope (AFM). The observed force profiles were rationalized within the theory of Derjaguin, Landau, Verwey, and Overbeek (DLVO). In the presence of monovalent and divalent cations, this theory was capable to describe the force profiles correctly down to distances of a few nm. At shorter distances, however, a strong non-DLVO attraction was identified. For more highly charged cations, an additional and more long-ranged non-DLVO attractive force is observed, and it was interpreted by surface charge heterogeneities. On the basis of these force profiles, the aggregation rates, which were independently measured by light scattering, can be predicted relatively well. The main conclusion of this study is that, in the present system, direct force measurements do capture the principal interactions driving aggregation in colloidal suspensions.