Interfacial jamming and buckling of particle-coated bubbles can result in the arrest of foam coarsening.

The hydrophobicity of a particle surface can be tuned by the addition of surfactants that change the surface free energy for their attachment to a liquid interface. In this work, we report an experimental study where the wettability properties of silica nanoparticles are modified by the adsorption of alkyltrimethylammonium surfactants (C_nTAB, n = 12, 16) on the surface of the particles. We have pointed out that the wettability of the complexes is controlled by an intricate balance of electrostatic and hydrophobic interactions between the particle surface and the surfactant. These interactions play an important role in the structure of the surfactant–particle nanocomposite interfacial layer.

The relative wettability of oil and water on solid surfaces is generally governed by a complex competition of molecular interaction forces acting in such three-phase systems. Herein, we experimentally demonstrate how the adsorption of in nature abundant divalent Ca²⁺ cations to solid-liquid interfaces induces a macroscopic wetting transition from finite contact angles (≈10°) with to near-zero contact angles without divalent cations. We developed a quantitative model based on DLVO theory to demonstrate that this transition, which is observed on model clay surfaces, mica, but not on silica surfaces nor for monovalent K⁺ and Na⁺ cations is driven by charge reversal of the solid-liquid interface. Small amounts of a polar hydrocarbon, stearic acid, added to the ambient decane synergistically enhance the effect and lead to water contact angles up to 70° in the presence of Ca²⁺. Our results imply that it is the removal of divalent cations that makes reservoir rocks more hydrophilic, suggesting a generalizable strategy to control wettability and an explanation for the success of so-called low salinity water flooding, a recent enhanced oil recovery technology.

Many physical properties of concentrated dispersions of immiscible fluids are captured by the concept of an osmotic pressure, which measures how much energy is required to deform the bubbles or drops upon compaction. This pressure has a strong impact on the flow and drainage behavior of dispersions. Nevertheless, theoretical models describing its variation with the volume fraction ϕ of the continuous phase are so far available only in the limits of low or high ϕ and experimental data are scarce. We report an experimental study of osmotic pressure in foams and emulsions, showing how the effects of ϕ, disorder, grain size, polydispersity and interfacial tension can all be captured by a single law which satisfies previously established theoretical constraints. Building on this result, we propose the first equation which accurately describes the variation of the volume fraction with the height of a fluid dispersion under gravity.

The interfacial properties of aqueous dispersions are strictly connected to the transfer/accumulation of particles into the surface layer, driven by the particle hydrophobicity. The addition of surfactants adsorbing on the particles and tuning their hydrophobicity represents therefore an attractive route to control the properties of these liquid interfaces. These mixed systems present however rather complex behaviours, which are still difficult to understand, deserving both theoretical developments and experimental investigations. Here the results of a wide study on the interfacial properties of a silica nanoparticle dispersion added with CTAB are presented, which allows to elucidate some basic aspects. A set of different techniques and methodologies have been utilised to measure the dilational viscoelasticity in a wide frequency range: from 10⁻³ to 10³ Hz. The study concerned both particle layers obtained after the spontaneous accumulation/segregation at the dispersion interface, and layers obtained after surface compression. The analysis of these data in the framework of available models provides qualitative and quantitative information about kinetic and structural features of these complex mixed layers. The results evidence different relaxation mechanisms effective on different timescales, which involves both the particles and the surfactant adsorbed on them. Besides the specific results concerned with the particular investigated system, the study confirms the surface dilational rheology investigation as a powerful method for surface science, in particular, as far different techniques are utilised to explore dilational properties in a broad frequency window.